CCAPP Seminars

Early Stages of Ultra High Energy Cosmic Ray Air Showers as a Diagnostic of Exotic Primaries

10/06/06
Eun-Joo Ahn (Bristol Institute/U.Delaware)

The nature of ultra high energy cosmic rays (UHECRs) remains an enigma. UHECR detection rate is increasing with new generation detectors which will speed up the process of understanding these energetic particles. The field of UHECRs is briefly reviewed with a focus on air shower characterisation of the primaries. I show that the study of the first (few) interaction(s) can substitute the full scale Monte Carlo in analysing the air shower characteristics. This method is advantageous for testing new models with many parameters. Exotic primaries can be compared with well studied primaries such as protons and iron nuclei. One such exotic case is the TeV black hole creation which can happen in models of large extra dimensions. High energy neutrinos interacting with air molecules may form these objects in the Earth's atmosphere, and a good way of discriminating them from other backgrounds is through air shower studies.

Dark Matter vs. Modified Gravity

10/17/06
Scott Dodelson (Fermilab/Chicago)
Download Seminar PPT

There is abundant evidence for dark matter in the universe. Why even consider the alternative of modifying gravity? Despite its role as an underdog, modified gravity has scored a number of successes. Several recent advances, both theoretical and observational, have given new life to this old idea. I will try to convince you that the game is not over, and that the struggle between the two approaches is as exciting as ever.

Concepts and Challenges of the Accelerating Universe

10/27/06
Eric Linder (UC Berkeley/Berkeley Lab)

Recent developments in understanding the influence of dark energy dynamics on cosmological observables have led to several insights in how to reveal the nature of dark energy. This includes the categorization of many physics models for the dark energy into either freezing or thawing behavior, recognition of differences from the inflation scenario, and methods for robustly distinguishing a physical dark energy from a modification of gravitational physics. These have definite consequences for experiment design, such as prescription of the relative precision needed for dynamics measurements, the need for probes of both cosmological expansion and large scale structure growth, and how dark energy microphysics can contribute a theory-induced systematics limit on many techniques.

Exploring the Dark Energy Domain

11/03/06
Dragan Huterer (Chicago)

One of the great mysteries of modern cosmology is the origin and nature of dark energy - a smooth component that contributes about 70% of the total energy density in the universe and causes its accelerated expansion. In this talk I describe and critically evaluate a variety of methods, from simple parametrizations to non-parametric methods, to model the background expansion history in the presence of dark energy. Then I present results from a comprehensive study of a class of dark energy models, commenting on current and expected future constraints, insights into the dynamics of dark energy, figures of merit, and a classification of theoretical models.

Galaxy groups in DEEP2: Implications for cosmic evolution

11/07/06
Brian Gerke (UC-Berkeley)

Groups and clusters of galaxies, as the largest, most recently formed objects in the universe, carry much information about the recent history of the cosmos. By studying these systems at a variety of epochs, it is possible to reconstruct both the evolution of clusters and the history of large-scale structure formation. Such studies provide important constraints on theories of galaxy formation and on cosmological parameters. With the recent completion of the DEEP2 Galaxy Redshift Survey at z~1, it is now possible to perform detailed studies of galaxy groups and clusters over a wider redshift range than ever before. In this talk I will present recent results suggesting that, at the DEEP2 epoch, galaxy groups had *only recently* become suitable environments for shutting off star formation in galaxies. I will also present evidence that DEEP2 groups are underluminous in the X-ray band, when compared with local systems. Finally, I will describe an ongoing project to compare the DEEP2 group population to the local sample detected in the 2dFGRS. This work will allow new tests of galaxy-formation theory by probing evolution in cluster mass-to-light ratios. It will also permit new constraints on cosmological parameters by measuring the evolution of the group abundance between z~1 and the present day; in particular, this study should provide the first-ever constraint from cluster counts on the dark energy equation of state parameter.

Light and Shadows from Dark Matter

11/17/06
Stefano Profumo (Caltech)

Even though the Dark Matter is dark - and therefore features very suppressed electro-magnetic interactions - photons can be, in principle, very sensitive probes of this as yet undetected and unknown (in its fundamental particle physics nature) component of the Universe. Dark matter can pair annihilate into Standard Model particles that yield photons in their subsequent decays, or it can directly pair annihilate into monochromatic photons, or decay into photons. In certain scenarios, photons can also resonantly scatter off Dark Matter, depleting the photon flux from sources located behind (or at the center) of high density Dark Matter concentrations. In this talk, I will review and present new results on photons as a probe of the fundamental nature of Dark Matter.

From Outer Space to Inner Space: Particle Physics in the Age of Precision Cosmology

12/1/06
Will Kinney (U. of Buffalo)

I will give an overview of exciting new developments at the interface between astrophysics and particle physics, focusing on the physics of inflation in the very early universe. New cosmological observations such as that from the WMAP satellite and the Sloan Digital Sky Survey have achieved unprecedented precision: Uncertainties in cosmological parameters such as the curvature of space and the density of matter have shifted from order unity to of order a few percent. As a result, it is possible for the first time to place meaningful constraints on the physics of the universe during the epoch of inflation, when the universe is believed to have expanded exponentially and quantum processes created the seeds for structure in the universe. This epoch is of great interest for fundamental physics, and cosmology is giving us the first observational hints of physics at ultra-high energy, where Grand Unification and perhaps even quantum gravity may be relevant.

The Radial Distribution of Galactic Satellites

12/12/06
Jacqueline Chen (Bonn)
Download Seminar PDF

The spatial distribution of satellite galaxies around host galaxies can illuminate the relationship between satellites and dark matter subhalos and aid in developing and testing galaxy formation models. The projected cross-correlation of bright and faint galaxies offers a promising avenue to putting constraints on the radial distribution of satellite galaxies. Previous efforts to constrain the distribution attempted to eliminate interlopers from the measured projected number density of satellites and found that the distribution is generally consistent with the expected dark matter halo profile of the parent hosts. The measured projected cross-correlation can be used to analyze contributions from satellites and interlopers together, using a halo occupation distribution (HOD) based analytic model for galaxy clustering. Tests on mock catalogs constructed from simulations show promise in this approach. Analysis of Sloan Digital Sky Survey (SDSS) data shows results generally consistent with interloper subtraction methods, although the radial distribution is poorly constrained with the current dataset and larger samples are required.

STARCaL: Precision Astrophysics & Cosmology Enabled by a Tunable Laser in Space

01/12/07
Justin Albert (U. of Victoria)
Download Powerpoint

We propose a tunable laser-based satellite-mounted spectroscopic, spectrophotometric, and absolute flux calibration system, to be utilized by ground- and space-based telescopes. As spectrophotometric calibration plays a significant role in the accuracy of photometric redshift measurement, and photometric redshift accuracy is important for measuring dark energy using SNIa, weak gravitational lensing, and baryon oscillations, a method for reducing such uncertainties is needed. We propose to improve spectrophotometric calibration, currently obtained using standard stars, by placing a tunable laser and a wide-angle light source on a satellite by early next decade (perhaps included in the upgrade to the GPS satellite network) to improve absolute flux calibration and relative spectrophotometric calibration across the visible and near-infrared spectrum. For spectroscopic measurements, the precision calibration of wavelength scale that is enabled can reduce uncertainties on measurements of fundamental constants using, e.g., quasar absoption lines. In addition to fundamental astrophysical applications, the system has broad utility for atmospheric & climate science, defense and national security applications, and space communication.

Mass Profiles and Mass-to-light Ratios of SDSS Clusters from Lensing in the SDSS

1/19/07
Erin Sheldon (NYU)
Download Seminar PDF

The Maxbcg catalog of galaxy clusters, created from 7500 square degrees of Sloan Digital Sky Survey (SDSS) imaging data, is the largest yet assembled. These objects, ranging from small groups to massive clusters, provide an excellent laboratory to study the formation of structures in our universe. I will present measurements of the mean radial mass profile measured from weak gravitational lensing as a function of cluster richness and luminosity. The wide area of the SDSS allows measurements ranging from the inner halo (25 kpc) well into the surrounding large scale structure (30 Mpc). As predicted by the cold dark matter model, these mass profiles have a distinctive non-power law shape. They are well described by a universal NFW profile in the inner halo and linear correlations on large scales. The virial mass scales strongly with cluster richness. We also measure the total light of the galaxies in and around the clusters. The light is distributed in the cluster differently than the mass, with the light being more centrally concentrated due to the presence of the brightest cluster galaxy. We find that the mass to light ratio is scale dependent and asymptotically approaches the same global value on large scales, independent of cluster mass.

Infalling Satellites and Structure of Galactic Disks in CDM Models

1/23/07
Stelios Kazantzidis (KIPAC/Stanford)
Download Seminar Powerpoint

The Cold Dark Matter (CDM) model of hierarchical structure formation has emerged as the dominant paradigm in galaxy formation theory owing to its remarkable ability to explain a plethora of observations on large scales. Yet, on galactic and sub-galactic scales the CDM model has been neither convincingly verified nor disproved, and several outstanding issues remain unresolved. Using a set of high-resolution numerical simulations I investigate whether the abundance of substructure predicted by CDM models is in conflict with the existence of thin, dynamically fragile galactic stellar disks. I show that encounters of massive subhalos with the center of the host potential where the disk resides at z < 1 are quite common and yield significant damage to the disk. However, these violent interactions are not absolutely ruinous to the survival of disks. I demonstrate that infalling satellites produce several distinct observational signatures including flaring, long-lived, low-surface, ring-like and filamentary structures, and a complex vertical morphology that resembles the commonly adopted thin-thick disk profiles used in the analysis of disk galaxies. These results imply that substructure plays a significant role in setting the structure of disks. Upcoming galactic surveys and astrometric satellites offer a unique opportunity to distinguish between competing cosmological models and constrain the nature of dark matter on non-linear scales through detailed observations of galactic structure.

Ultraviolet Pumping of the 21 cm Line in the High Redshift Universe

2/13/07
Leonid Chuzhoy (U.Texas)
Download Seminar PDF

The next generation of radio telescopes (LOFAR, MWA, SKA, 21CMA) promises to open a new observational window into the epoch preceding the end of reionization. By measuring the redshifted 21 cm signal from neutral hydrogen, the new telescopes can provide us with information on the history of reionization, the nature of the first radiation sources, the spectrum of the primordial density perturbation field, the physical properties of dark matter particles and so on. Besides the technical challenge, the correct extraction and interpretation of the measured signal requires accurate modeling of the physical processes that affect it. Unlike the collisionally pumped 21 cm signal from the nearby sources, the signal from high redshift intergalactic medium is pumped primarily by ultraviolet (UV) resonance photons. In this talk I will describe new calculations of UV pumping, which take into account several previously neglected physical processes, including the backreaction of induced hyperfine transitions on the incident UV photons and conversion of X-rays into the UV photons. I will show that neglecting these processes generally results in completely erroneous interpretation of the observed 21 cm signal.

Quasars Probing Quasars: Shedding (Quasar) Light on High Redshift Galaxies

2/20/07
Joseph Hennawi (Berkeley)
Download Seminar Powerpoint

With close pairs of quasars at different redshifts, a background quasar sightline can be used to study a foreground quasar in Ly-alpha absorption. This novel experiment allows us to probe the foreground quasar environment on scales as small as a galactic disk where the ionizing flux from the quasar could be as large as ~ 10,000 times the extragalactic UV background. I will discuss the manifold cosmological applications of these rare projected sightlines: they provide new laboratories for studying the faint fluorescent recombination radiation from the high redshift Universe, they constrain the environments, emission geometry, and radiative histories of quasars, and they shed light on the distribution and kinematics of the gas in high redshift proto-galaxies.

A New Look at the Galactic Diffuse GeV Excess

03/06/07
Brian Baughman (UCSC/SCIPP)
Download Seminar PDF

The EGRET experiment onboard the Compton Gamma-ray Observatory have provided the most precise measurements of the gamma-ray sky to date. EGRET measurements of diffuse emission across the sky show an excess above 1 GeV. This “GeV excess” has been a topic of great debate and interest since its original discovery by Hunter et al. in 1997. While various attempts have been made to explain the measurement as new phenomena the possibility remains that it may be due to unknown instrumental effects. To examine this, I have modified the GLAST simulation and reconstruction software to model the EGRET instrument. This detailed modeling has allowed me to explore the parameters of the EGRET instrument, in both its beam-test configuration and in-orbit on CGRO, in greater detail than has previously been published. While it was our intention to examine the possibility that the GeV excess was the result of some hereto yet unknown instrumental effect, I have instead found that the GeV excess is significantly increased when previously unaccounted for instrumental effects are considered. I will present a new measurement of diffuse gamma-ray emission in the inner Galaxy, as well as the methodology used in our measurement.

Cosmology in the Era of Large Surveys

3/13/07
Ryan Scranton (U. of Pittsburgh)
Download Seminar PDF

The past decade has seen an unprecedented improvement in our understanding of the basic picture of the universe. We have gone from factors of 2 uncertainties in the age and matter density of the universe to better than 10% precision thanks to a vast increase in the available survey information. Along the way, the depth and breadth of these surveys have made previously impossible measurements a reality. I will discuss two such cases: detection of cosmic magnification and the integrated Sachs-Wolfe effect. Another unexpected benefit of these surveys has been the discovery of dark energy. While the current generation has been sufficient to demonstrate its existence, we will have to wait until the next round of surveys to fully explore the details of dark energy behavior throughout the history of the universe. I will finish with a discussion of some of the tools we will need to develop over the course of the next several years to fully exploit the power of future surveys like the LSST, SPT and JDEM.

Neutrino Astronomy and Particle Physics with the IceCube Detector

4/5/07
Doug Cowen (Pennsylvania State U.)
Download Seminar Powerpoint

IceCube is a neutrino detector under construction at the South Pole. It is designed to search primarily for energetic neutrinos from cosmological sources, but is also sensitive to many other signals from neutrinos and other particles. In this talk we will make the case for neutrinos as astronomical messengers, describe how the IceCube detector will be able to detect them, discuss results from both the first year of full-scale running of the (partially-constructed) detector and from IceCube's progenitor AMANDA, and conjecture about when IceCube will make discoveries.

The Least Luminous Galaxies in the Universe

4/10/07
Beth Willman (Harvard-Smithsonian Center for Astrophysics)
Download Seminar Powerpoint

In the last few years, a combination of observational and computational advances have ignited the field of near-field cosmology - using galaxies in the nearby Universe as tracers of dark matter on small-scales and using their detailed properties as fossil records of the process of galaxy formation from the earliest times until now. For example, since 2005, a dozen dwarf galaxies have been discovered around the Milky Way and M31 that are less luminous than previously thought possible to exist. These discoveries will both revolutionize our understanding of galaxy formation at the lowest luminosities and will shed new light on the properties of dark matter on galaxy scales. I present the results of these searches and discuss them in a cosmological context.

Closing in on Ultra-High Energy Cosmogenic Neutrinos with the Radio Detection Technique

4/16/07
Amy Connolly

No diffuse cosmic neutrino flux has yet been observed, but the highest energy cosmic rays imply an associated flux of neutrinos. These neutrinos, with energies that exceed 10^18 eV, will point back to their source, are nearly unattenuated over cosmic distances, and in any detection medium, will induce interactions at center-of-mass energies beyond those seen at any accelerator on earth. I will describe current and future experiments that seek ultra-high energy cosmic neutrinos, which are so evasive they require detection volumes beyond 100's of km^3. Volumes of such size are achievable using the radio Cerenkov technique, and I will discuss current and future projects that utilize this detection method, including the ANITA balloon experiment which just completed its first full physics flight in January of this year.

Probing Hydrogen Reionization

4/17/07
Adam Lidz (Harvard-Smithsonian CfA)

Detailed observations of the Epoch of Reionization (EoR) will characterize the nature of the first luminous sources in the Universe, describe their impact on the surrounding IGM, and fill in a significant gap in our knowledge of the history of the Universe. I will describe recent efforts to theoretically model the EoR. Then I will discuss the theoretical interpretation of quasar absorption spectra at z~6, and comment on future 21 cm probes of reionization.

A New Twist on Galaxy Scaling Relations

4/24/07
Dennis Zaritsky (Univ. of Arizona)
Download Seminar PDF

Galaxy evolution has proven to be a difficult problem, partly because we appear to be unable to separate various parts of the problem. I will discuss new results on galaxy scaling relations that suggest that galaxy structure may be much more scalable than previously appreciated. Our extended Fundamental Plane formalism has implications for the nature of spheroids on all scales, the physical processes that might affect the smallest galaxies, the distribution of baryons within dark matter halos, and the evolutionary state of spiral galaxies.

Galactic Cosmic Rays and Diffuse Gamma-Ray Emission

5/1/07
Igor Moskalenko (Stanford)
Download Seminar PPT

Practically all our knowledge of cosmic ray (CR) propagation comes from studies of the composition and spectra of CR species. Therefore, astrophysics of cosmic rays and gamma rays depends very much on the quality of the data and their proper interpretation. Combining the data of different experiments into a single interpretive model of the Galaxy gives us a better chance to understand the mechanisms of particle acceleration, the role of CR in the dynamics and evolution of the Galaxy, and to provide a common background model upon which further progress in related areas can be made. The new generation gamma-ray observatory GLAST is to be launched in December of 2007; it covers the energy range from MeV to TeV energies and for the first time will close the gap between the spacecraft instruments and ACTs. GLAST will advance our knowledge of the detected sources, discover thousands of new sources, and provide invaluable insight into the propagation of CR in the Galaxy. In my talk, I will summarize the current status of astrophysics of CR and speculate on what we can learn from GLAST and other space missions.

Voids of dark energy

5/8/07
Irit Maor (Case Western)
Download Seminar PPT

I discuss the clustering properties of a dynamical dark energy component. Modelling the dark energy as a light scalar field, The linear evolution of perturbations is numerically explored. The regime where the mass scale of the field is comparable to the Hubble scale gives non trivial dynamics, and the scalar field tends to form underdensities in response to the gravitationally collapsing matter. I shall discuss in detail the physics behind the formation of such voids, and the generality of these results. Detection of dark energy voids will clearly rule out the cosmological constant as the main source of the present acceleration.

The Growth of Massive Galaxies

05/15/07
Andrew Benson (Caltech)
Download Seminar PPT
Download Seminar PDF

I will present new results from ongoing work aimed at understanding how massive (mostly elliptical) galaxies grow. Evidence is accumulating that this process isn't as simple as was previously thought - mergers between galaxies might not be the only (or even the main) driver of this growth. I will demonstrate that the perceived difficulty of forming massive galaxies at high redshifts in cold dark matter models is not a problem at all. The real problem is preventing them from becoming too massive! I'll show the latest results from model calculations along with some current observational measures and prospects for the future.

Galaxy and halo evolution and environment

5/22/07
Ravi Sheth (U. Penn)
Download Seminar PPT

I will show a selection of measurements showing how galaxy properties and star formation histories correlate with their environment. I will then describe halo model interpretations of such measurements. These include a rather simple description of what appear to otherwise be complex correlations between galaxies and their environments; a number of ways in which the halo model makes a connection to observables which were previously the domain of SPH or semi-analytic galaxy formation models (e.g., the different formation histories and mass-to-light ratios of central and satellite galaxies, and the intercluster light component); and a complete description of the no-merger passive evolution model which can provide the basis for understanding the assembly of stellar mass in the most massive galaxies.

Angular signatures of dark matter in the diffuse gamma ray background

5/29/07
Pasquale Serpico (Fermilab)
Download Seminar PDF

Dark matter annihilating in the halo of our Galaxy and elsewhere in the universe is expected to generate a diffuse flux of gamma rays, potentially observable with next generation satellite-based experiments, such as GLAST. We present the expected signatures of dark matter, in particular the deterministic features in the angular distribution of this radiation at large scales, both pertaining the galactic and the extragalactic contribution. If at least a few percent of the diffuse gamma ray background observed by EGRET is the result of dark matter annihilations, then GLAST should be able to detect many of the signatures discussed in this talk.

The nature of cosmic explosions

6/27/07
Avishay Gal-Yam
Download Seminar PPT

I will try to review what we know about the various classes of cosmic explosions, how we came by that knowledge, and what we do not yet know. I will focus on some areas of recent progress, and will present some prospects for the near and mid-term future.

The Search for Neutrinos from Gamma-Ray Bursts with AMANDA

9/7/07
Kyler Kuehn (UC Irvine)
Download Seminar PPT

The Antarctic Muon and Neutrino Detector Array (AMANDA) is a neutrino telescope located beneath the ice at the geographic South Pole. AMANDA searched for high energy neutrinos from both discrete and diffuse astrophysical sources from 1997 to 2004. Neutrino telescopes like AMANDA provide a unique window into the nature of various astrophysical phenomena, complementing what can be learned from other ground- or space-based observatories. We present the results of AMANDA's neutrino observations correlated with more than 400 gamma-ray bursts (GRBs) in the Northern Hemisphere during the first seven years of AMANDA operation. During this time period, AMANDA's effective collection area for muon neutrinos was larger than that of any other existing detector. Based on our observations, we set the most stringent upper limit on muon neutrino emission correlated with gamma-ray bursts to date. The impact of this limit on several theoretical models of GRBs is discussed, as well as the future potential for detection of GRBs by AMANDA's successor, IceCube.

Reverse Engineering Galaxy Formation

9/11/07
Jeremy Tinker (University of Chicago)

I will show new results from halo occupation analyses of clustering measurements that provide insight into the processes that determine galaxy properties. From simultaneous analysis of galaxy correlation functions and galaxy void statistics, I show that the occupation of galaxies in halos at fixed mass is independent of large-scale environment. This is true for galaxies selected on luminosity, color, and morphology, implying that these galaxy properties are determined by the mass of the halo in which they sit, irrespective of the formation history of that halo.

The Distribution of Baryons in Galaxy Clusters and Groups

9/20/07
Anthony Gonzalez (University of Florida)
Download Seminar Powerpoint

If galaxy clusters are indeed fair samples of the universe, then a basic expectation is that the baryon fraction in clusters and groups should reflect the universal value. Observed shortfalls have therefore led to proposals of missing baryons in a warm gas component, as well as other more controversial interpretations. I will present the results of a program to understand the distribution of baryons in nearby galaxy clusters and groups, as well as the properties of their central galaxies. A main focus of this work is to quantify the total stellar baryon fraction, including stars in both galaxies and the intracluster light, and combine these data with published measurements of the hot baryon fraction in the intracluster medium (ICM). We find that the total baryon fraction is independent of cluster mass, with no compelling evidence for missing baryons. I will also present related results from this program pertaining to cluster galaxy evolution, galaxy structure, and chemical enrichment of the ICM.

Gamma-Ray Bursts: Recent Progress

9/26/07
Pawan Kumar (University of Texas, Austin)
Download Semiar PPT

I will describe recent progress in our understanding of gamma-ray bursts. Using early x-ray data from the Swift/XRT we are able to determine the distance from the center of explosion where gamma-ray emission is generated. I will also discuss what we have learned about the mechanism by which gamma-ray photons are generated, and the composition of the relativistic outflow in this explosions.

Two-filter Cluster Finding in the Millennium Simulation (...and beyond)

10/09/07
Joanne Cohn (UC - Berkeley)
Download Seminar PDF

The era of large scale galaxy surveys is now upon us. These surveys make possible large samples of galaxy clusters, which can be used for a variety of astrophysical and cosmological purposes. One of the most promising methods for selecting such samples of galaxy clusters is known as the red sequence method. It requires only two filters and has been shown to be successful in low redshift pilot studies. In order to better understand what sorts of objects are found by these methods at higher redshift, we used two filter cluster searches on outputs of the Millennium Simulation. We found a higher fraction of blends as redshift increased. We expect that the properties causing this blending are generic. We also explored ways to reduce or compensate for the blending in the analysis, which highlighted the crucial role of extremely accurate mock catalogues.

Galaxy Content of Clusters and Groups in the Local Universe

10/23/07
Sarah Hansen (University of Chicago)

I will present recent analysis of SDSS imaging data quantifying the population of galaxies in MaxBCG-identified clusters and groups. I will discuss the distributions of satellite galaxy luminosity and satellite color and the dependence of these on cluster properties. I will also show the relationship of Brightest Cluster Galaxy luminosity to cluster mass and to satellite galaxy luminosity. These measurements of cluster light, in combination with lensing results, also allow measurement of ensemble cluster mass-to-light profiles. This study demonstrates the power of cross-correlation background correction techniques for measuring galaxy populations in purely photometric data, and provides a baseline for the study of galaxy evolution in higher redshift samples.

The race to detect dark matter with noble liquids, XENON10 results, and the LUX experiment

11/6/07
Tom Shutt (Case Western)
Download Seminar PPT

Detectors based on liquid noble gasses have the potential to revolutionize the direct search search for WIMP dark matter. The XENON10 experiment, of which I am a member, has recently announced the results from it's first data run and is now the leading WIMP search experiment. LUX, a large-scale follow-up to XENON10 and other experiments using xenon, argon and neon have the potential to rapidly move from the current kg-scale target mass to the ton scale and well beyond. This should allow a (nearly) definitive test or discovery of dark matter if it is in the form of weakly interacting massive particles.

On Scatter in Galaxy Cluster Mass-Observable Relations

11/20/07
Douglas Rudd (Institute for Advanced Study, Princeton)

In this talk I will discuss issues relevant to the use of galaxy cluster abundances to constrain the properties of dark energy. In particular I will focus on the use of self-calibration to jointly constrain cosmological and cluster model parameters using the large cluster samples provided by SZ surveys. These surveys should have sufficient statistics and sensitivity to dark energy to remain competitive with other dark energy probes, provided the connection between cluster observables and mass is describable in a small number of extra nuisance parameters. I will discuss the distribution of cluster SZ observables extracted from a large sample of simulated clusters and the link between halo assembly history and scatter in the mass-observable relations.

Beyond WMAP: the CMB at large and small scales

12/04/07
Joanna Dunkley (Princeton)

I will discuss some current and future measurements of the Cosmic Microwave Background anisotropy: its small-scale intensity and large-scale polarization. I will describe some of the physics that we will be able to test at small-scales with the Atacama Cosmology Telescope, including better determining properties of neutrinos, and testing for non-standard inflation. At large-scales the next goal is to obtain evidence for inflation by observing gravitational waves, but we face challenges due to the significant level of polarized radiation from the Milky Way. I will discuss current efforts to better understand and characterize this emission, paving the way for future experiments.

Cosmic Strings from Supersymmetric Flat Directions

1/22/08
David Morrissey (U. of Michigan)
Download Seminar PDF

Cosmic strings are non-trivial configurations of scalar (and vector) fields that are stable on account of a topological conservation law. They can be formed in the early universe as it cools after the Big Bang. The scalar fields required to form cosmic strings arise naturally if Nature is supersymmetric at high energies. A common feature of supersymmetric theories are directions in the scalar potential that are extremely flat. Combining these two ingredients, the cosmic strings associated with supersymmetric flat directions are qualitatively different from ordinary cosmic strings. In particular, flat-direction strings have very stable higher-winding modes, and are very wide relative to the scale of their energy density. These novel features have important implications for the formation and evolution of a network of flat-direction cosmic strings in the early universe. They also affect the observational signatures of the strings, which include gravity waves, dark matter, and modifications to the nuclear abundances and the blackbody spectrum of the microwave background radiation.

Prospects For Detecting Dark Matter In Light Of The WMAP Haze

1/29/08
Gabrijela Zaharijas
Download Seminar PDF

Observations by the WMAP experiment have identified an excess of microwave emission from the center of our Galaxy, dubbed the "WMAP Haze". It has previously been shown that the origin of the haze could be linked to synchrotron emission from relativistic electrons and positrons produced in the annihilations of dark matter particles - implying a possible detection of dark matter. If dark matter annihilations are in fact responsible for this phenomenon, then other annihilation products will also be produced, including gamma rays. In this talk, I will present the prospects of detecting gamma rays from dark matter annihilations in the Galactic Center region which could reject or confirm this scenario in the near future.

Observing the First Galaxies and the Reionization Epoch

2/5/08
Steven Furlanetto (Yale)
Download Seminar Powerpoint

Finding and understanding the earliest generations of galaxies is one of the frontiers of modern cosmology. Although enormous strides have been made in the past decade, the current observational evidence is ambiguous at best. I will describe two routes toward improving the situation. First, searches for high-redshift galaxies through their Lyman-alpha emission lines can teach us not only about the galaxies themselves but also about the intergalactic medium (IGM). While current measurements constrain their abundance, the clustering of these objects promises to reveal even more information. Second, three-dimensional "tomography" 21 cm emission (or absorption) by the neutral IGM has the potential to unlock the detailed distribution of baryons between recombination and reionization. I will describe how this cosmic background can teach us about the eras of the first stars, first black holes, and reionization itself. I will also describe some of the challenges facing these measurements.

First Results from the SDSS-II Supernova Survey

2/8/08
Ben Dilday (University of Chicago)

The SDSS-II Supernova (SN) Survey was undertaken during the Fall months of 2005-2007, with the primary goal of discovering and observing several hundred type Ia supernovae (SNe) in order to improve constraints on dark energy. Additional goals of the survey include determining SN rates and properties and exploring systematics in the use of type Ia SNe as cosmological distance indicators. I will provide an overview of the survey, which resulted in the discovery and spectroscopic confirmation of ~500 type Ia SNe, and discuss cosmological results based on 95 SNe from the first (2005) season. I will describe in detail our studies of the type-Ia SN rate, including (i) the most precise measurement of the rate at low redshift (z< 0.12) from the first season, (ii) extension of the rate measurement to z~0.25, (iii) study of the SN Ia rate as a function of host galaxy properties, e.g., star formation rate, and (iv) study of the rate in galaxy clusters. These rate measurements can provide improved observational constraints on the progenitor systems of type Ia SNe and can therefore improve the utility of SNe Ia as cosmological distance indicators.

GRB Science with GLAST: From Onboard Detection to the Search for Quantum Gravity

2/12/08
Frederick Kuehn

The Gamma Ray Large Area Space Telescope (GLAST) is the next generation high energy gamma ray observatory. Set to launch in Mid 2008, it is charged with the broad scientific mission of studying a diverse range of phenomena such as gamma ray bursts (GRBs), active galactic nuclei, dark matter, the Sun, pulsars, micro-quasars, as well as mapping the entire gamma ray sky. I will discuss GLAST's ability to trigger and localize on GRBs with high energy emission. This capability is essential for multi-wavelength followup observations used to determine redshifts as well as constrain and inspire GRB models. It has been suggested that quantum gravity may modify the speed of light at high energies, such that it is no longer constant. GRBs are short, bright pulses of gamma rays at cosmological distances, spanning many orders of magnitude in energy. Due to the astronomical distance scales, slight speed differences between photons of different energies lead to measurable time delays. I will discuss GLAST's ability to constrain, or produce evidence for such a scenario.

Exploring GRB Astrophysics via a Correlated Broad-Band and Multi-Messenger Paradigm

2/13/08
Michael Stamatikos

I will present results based upon a synergistic methodology whose primary objective encompasses probing discrete gamma-ray burst (GRB) high-energy particle astrophysics via a broad-band, multi-messenger paradigm. The interface between leptonic and electromagnetic emission will be explored using the theoretical interpretation and correlative observations of high energy telescopes such as (i) Swift's Burst Alert Telescope (BAT), (ii) the Gamma-Ray Large Area Space Telescope (GLAST) Burst Monitor (GBM) and (iii) the Antarctic Muon and Neutrino Detector Array (AMANDA)/IceCube. Multi-wavelength analysis results include temporal studies of Swift GRBs featuring GRB 060218 in the context of the lag-luminosity relation, and simulations of joint photon energy spectra using Swift-BAT and GLAST-GBM. Probes for multi-messenger leptonic emission signatures via neutrino astronomy include modeling the correlated (TeV-PeV) muon neutrino flux from discrete GRBs featuring GRB 030329 in the context of canonical fireball phenomenology.

Why Measure the Mean Curvature Even Better and How to Do It

2/19/08
Lloyd Knox (UC Davis)

This will be a two-part talk. First I will discuss precision measurements of the mean curvature of the Universe: their motivation as powerful tests of inflation and the string theory landscape, and how well surveys motivated by dark energy can be used for detecting small amounts of mean curvature. The second part will be about some recent developments in tools for making cosmological parameter inferences from such surveys.

Observations of the Crab Nebula and Pulsar with VERITAS

2/26/08
Ozlem Celik (UCLA)

VERITAS, an array of four 12m diameter Cherenkov telescopes, is a ground based observatory designed to explore the very high energy gamma ray sky in the energy band between 100 GeV and 50 TeV. Observations of the Crab Nebula, which is accepted as the standard candle in gamma ray astronomy, have proven to be the best tool to calibrate and to characterize the performance of a Cherenkov telescope. Scientifically, it is interesting to measure its energy spectrum to confirm its power law nature across the VHE region and to search for pulsed emission from the Crab Pulsar at energies beyond the 10 GeV upper limit of the EGRET pulsar detection. With these motivations, we have observed the Crab extensively in the 2006 2007 season during the VERITAS 2 and 3 telescope commissioning phases. Using this data set I have reconstructed the energy spectrum of the steady emission from the Nebula. I have also measured the optical pulsed signal from the pulsar and have obtained an upper limit for the pulsed emission at gamma ray energies. On my talk, I will present the results of these studies.

Cosmic Rays and MINOS: Physics in the Background

3/3/08
Eric Grashorn

The Main Injector Neutrino Oscillation Search (MINOS) is a long baseline neutrino oscillation experiment designed to make a precision measurement of \delta m^2_{23}. Cosmic ray muons are a source of background for such an experiment, but they are an isotropic data source with many calibration and scientific uses. MINOS has measured the atmospheric muon charge ratio to very high precision, as well as the seasonal variation in underground muon rate. New models describing both physical effects have been developed, and both are shedding new light on K/\pi in cosmic ray airshowers. The shadow of the moon is an important analysis to establish the resolution and absolute pointing capability of a cosmic ray detector, and it can also be used to put limits on the anti-matter content of cosmic ray primaries.

A New Channel for Detecting Dark Matter Substructure in Galaxies

3/18/08
Charles Keeton (Rutgers University)
Download Seminar PDF

The Cold Dark Matter paradigm predicts that galaxy dark matter halos contain hundreds of bound subhalos left over from the hierarchical galaxy formation process. Testing this prediction provides unique access to the astrophysics of galaxy formation on small scales, and perhaps even the fundamental nature of dark matter. Gravitational lens flux ratios have been used to place the first constraints on dark matter substructure in galaxies out to redshift z~1. Now I propose to open a new frontier in substructure studies with gravitational lens time delays. Time delays offer several distinct advantages. The theory of "time delay millilensing" is rich and tractable. Time delays provide access not only to the total amount of substructure, but also to the distribution of subhalo masses. Good data are attainable now, and future large samples will allow us to measure substructure as a function of galaxy mass, redshift, and environment.

Searches for Neutrino Oscillations and Dark Matter with IceCube

3/25/08
Carsten Rott
Download Seminar PowerPoint

The IceCube Neutrino Observatory currently under construction at the South Pole, just finished a phenomenal season and has now half of the detector completed. It is a multi-purpose ice-Cherenkov detector, which has been taking data since the deployment of its first string in January 2005. After a brief introduction to the IceCube experiment and a summary of the main results, this talk will especially focus on the search for dark matter, neutrino oscillation and other analyses in IceCube?s low-energy regime (~30GeV-1TeV range). The talk will conclude with an outlook into possible detector extensions and discovery prospects.

Standard Candle, Standard Yardstick and Non-standard Gravity

4/1/08
Lam Hui

I will discuss four topics: (1) how correlated peculiar flows constitute a surprisingly important source of error for supernova cosmology, (2) how gravitational lensing introduces an observable anisotropy to the galaxy correlation function, and how it impacts baryon acoustic oscillation measurements, (3) how large scale structure data already put interesting constraints on theories of modified gravity, in particular ruling out the popular DGP model at the 3 sigma level, (4) how viable gravity models can be constructed which exhibit a see-saw behavior: a large cosmological constant yielding a small Hubble constant.

Exploring the High-z Frontier --- Galaxies at z~6 and beyond

4/8/08
Haojing Yan
Download Seminar PowerPoint

The current status of the study at z~6 will be reviewed from an observer's point of view, with the emphasis on the implications for the reionization. A couple of key unknown questions at z~6 will also be discussed. The progress in searching for galaxies at z>7 will be reported.

From quasars to dark energy : Adventures with the clustering of luminous red galaxies

4/15/08
Nikhil Padmanabhan
Download Seminar PDF

I will discuss some of the cosmological applications of a survey of luminous red galaxies (LRGs), from constraining the clustering and properties of low redshift quasars to a new survey to measure the expansion rate of the Universe with baryon oscillations. Starting on small scales, I will discuss the clustering of LRGs around z< 0.6 quasars in the SDSS, and constraints this places on the environments of quasars. I will then switch to scales two orders of magnitudes larger, and discuss the Baryon Oscillation Spectroscopic Survey -- a next generation survey to measure baryon oscillations, yield 1% distance measures to z=0.35 and z=0.6.

Probing Small-Scale Structure in Galaxies with Strong Gravitational Lensing

4/29/08
Arthur Congdon
Download Seminar PowerPoint

We use gravitational lensing to study the small-scale distribution of matter in galaxies. Roughly half of all observed four-image quasar lenses have image flux ratios that differ from the values predicted by simple lens potentials. We show that smooth departures from elliptical symmetry fail to explain anomalous radio fluxes, regardless of the assumed multipole truncation order. Our work strengthens the case for dark matter substructure, which is predicted by numerical simulations to constitute a few percent of a galaxy's mass. Our results have important implications for the "missing satellites" problem, i.e., the discrepancy between the predicted and observed numbers of dwarf satellites in galaxy halos. To complement flux-ratio studies, we consider how time delays between lensed images can be used to identify lens galaxies that contain small-scale structure. We derive an analytic relation for the time delay between the close pair of images in a "fold" lens, and perform Monte Carlo simulations to investigate the utility of time delays for studying small-scale structure in realistic lens populations. We compare our numerical predictions with systems that have measured time delays and discover two anomalous lenses. We conclude that both flux ratios and time delays in lens systems provide powerful complimentary probes of cosmological theory.

Cosmology from gravitational-wave standard sirens

5/06/08
Daniel Holz

We discuss the use of gravitational wave sources as probes of cosmology. The inspiral and merger of a binary system, such as a pair of black holes or neutron stars, is extraordinarily bright in gravitational waves. By observing such systems it is possible to directly measure an absolute distance to these sources out to very high redshift. When coupled with independent measures of the redshift, these "standard sirens" enable precision estimates of cosmological parameters. We review proposed GW standard sirens for the LIGO and LISA gravitational-wave observatories. Percent-level measurements of the Hubble constant and the dark energy equation-of-state may be feasible with these instruments.

How Baryonic Physics Influences Efforts to Exploit Weak Lensing as a Dark Energy Probe

5/17/08
Andrew Zentner

Cosmologists are faced with several profound puzzles. I will discuss two of them, namely the mystery of the dark energy and the process of galaxy formation. The expansion rate of the Universe is accelerating. The causative agent of this expansion is commonly referred to as the Dark Energy. Though it is ten years since accelerated expansion became firmly established as a feature of our Universe, we know little about the dark energy. At present, observations indicate that dark energy is consistent with Einstein's cosmological constant. Any deviations from the phenomenology of a cosmological constant are subtle and difficult to measure. However, large ongoing and future projects such as the Dark Energy Survey, the Large Synoptic Survey Telescope, and a Joint Dark Energy Mission should allow us to constrain the properties dark energy more than an order of magnitude more stringently than current observations can. These efforts may limit strongly any deviations from a cosmological constant, constrain models of acceleration due to deviations from General Relativity, or indicate the presence of dynamical dark energy. The information we receive about the contemporary Universe comes from galaxies and the stars and stellar explosions that occur within them, yet the process of galaxy formation within the standard cosmology is poorly understood. I will review some of the methods that will be used to constrain dark energy, but I will focus on weak gravitational lensing as a dark energy probe. Although weak lensing measurements are notoriously difficult, this method has the greatest potential statistical leverage on dark energy (though systematics remain a concern). Many recent studies have suggested that the fact that galaxy formation is poorly understood theoretically will thwart forthcoming efforts to constrain dark energy through weak lensing. I will show how it is possible both to constrain dark energy properties and to learn about the process of galaxy formation simultaneously through weak lensing measurements. This possibility is interesting and may expand the scientific reach of several current and future projects.

The Eddington Limit in Cosmic Rays: An Explanation for the Observed Faintness of Starbursting Galaxies

5/27/08
Aristotle Socrates

In terms of their energetics, interstellar cosmic rays are an insignificant by-product of star formation. However, due to their small mean free path, their coupling with interstellar gas is absolute in that they are the dominant source of momentum deposition on galactic scales. By defining an Eddington Limit in cosmic rays, we show that the maximum luminosity of bright starbursting galaxies is capped by the production and subsequent expulsion of cosmic rays. This simple argument may explain why galaxies are faint in comparison to quasars.

An Alternative Origin for Hypervelocity Stars

9/09/08
Mario Abadi (Universidad Nacional de Cordoba, Argentina)

Hypervelocity stars (HVS) are usually assumed to originate from the gravitational interaction of stellar systems with the supermassive black hole at the center of the Galaxy. We examine the latest HVS compilation and find peculiarities that are unexpected in this black hole-ejection scenario. We use numerical simulations to show that disrupting dwarf galaxies may contribute halo stars with velocities up to and sometimes exceeding the nominal escape speed of the system. These stars are arranged in a thinly-collimated outgoing ``tidal tail'' stripped from the dwarf during its latest pericentric passage. We speculate that some HVS may therefore be tidal debris from a dwarf recently disrupted near the center of the Galaxy.

The Star Formation History and the Neutrino Background

9/25/08
Shunsaku Horiuchi
University of Tokyo

The emission of neutrinos from a core-collapse supernova was dramatically confirmed in 1987, when neutrinos were detected from SN1987A. However, in the 20 years since this event, there have been no close enough supernovae to directly detect in neutrinos. The diffuse supernova neutrino background on the other hand provides an immediate opportunity to study supernova neutrinos. A critical input in this pursuit has been astronomical - the cosmological rate of core-collapse supernovae, which is directly related to the star formation rate. Recently, our understanding of the star formation rate and its evolution has greatly improved, and it is timely to address the prospects the neutrino background provides to study stellar physics. In this talk I will discuss the latest astronomical inputs and present results of checks on its reliability and impacts it has on the neutrino background.

The Evolution and Outflows of Hyper-Accreting Disks: Implications for Compact Object Mergers, Short Gamma-Ray Bursts, and Heavy Element Nucleosynthesis

9/30/08
Brian Metzger
Download Seminar PowerPoint

Massive, compact accretion disks are thought to form in a number of astrophysical events, including the merger of two neutron stars (NSs), the merger of a NS with a black hole, and following the accretion-induced collapse (AIC) of a white dwarf to a NS. These disks, termed "hyper-accreting" due to their large accretion rates of up to several solar masses per second, may power the relativistic jets which produce gamma-ray bursts (GRBs). In particular, accretion following the merger of two compact objects (NS-NS or NS-BH) is a popular model for the production of short-duration GRBs, an idea which has received recent support due to the localization of some short GRBs in host galaxies with little ongoing star formation. I will discuss calculations of the evolution of viscously-spreading, hyper-accreting disks, emphasizing important transitions in the disk's thermodynamic properties and their implications for the late-time X-ray activity observed following some short GRBs. I will also focus on the properties of slower outflows from the disk and their ability to synthesize heavy radioactive elements, the decay of which may power an optical or infrared transient ~ 1 day following the merger. I shall further argue that late-time outflows from the disk synthesize neutron-rich isotopes which are rare in our solar system, from which one can place interesting constraints on the short GRB beaming fraction and the rate of compact object mergers in our galaxy. In addition, I will show that late-time outflows from accretion disks produced by the AIC of a white dwarf may have important observational consequences for these thus far unidentified events.

The Merger Histories of LCDM Galaxies: disk survivability and the deposition of cold baryons via mergers.

10/14/08
Kyle Stewart
Download Seminar PowerPoint

We employ a high resolution LCDM N-body simulation to study the merger histories of galaxy-halos and the evolution of the merger rate with redshift. We confirm the existence of a 'universal' halo merger rate, and provide a slightly modified fitting formula as a function of halo mass, redshift, and merger mass ratio. We find that the majority of Milky Way-size halos have experienced at least one major merger (defined either as mass ratio > 1:3, or in terms of absolute mass m > 10^11 Msun/h), which raises concerns about the survivability of disk dominated galaxies in a LCDM universe. We go on to explore the baryonic content of these mergers using direct empirical constraints to assign statistically likely stellar and gas masses to the central galaxies within the halos of our simulations. We find that the vast majority of mergers into Milky-Way size halos at z>1 are very gas rich (gas fraction > 50%). If we presume that gas rich-mergers such as these may result in disk dominated galaxies (as has been suggested based on direct numerical simulations), we find that only 20% of Milky Way-size galaxies have experienced a "destructive" gas poor major merger since z=2, suggesting a possible explanation to the problem of disk survivability. We also measure the total deposition of cold baryons into galaxies via mergers and find that Milky Way-size galaxies have accreted approximately 30% of their current cold baryonic mass directly from major mergers since z=2, the majority of which is gaseous. Whether this deposited material is labeled to be a "cold flow" is ! subject to definition, but it seems almost empirically inevitable that direct cold gas deposition of this kind must occur.

Dark Matter and the Highest Redshift Galaxies: Revealing the Invisible with 2 Cosmic Supercolliders the "Bullet Cluster" 1E0657-56 and MACSJ0025-1222

10/28/08
Marusa Bradac

The cluster of galaxies 1E0657-56 has been the subject of intense research in the last few years. This system is remarkably well-suited to addressing outstanding issues in both cosmology and fundamental physics. It is one of the hottest and most luminous X-ray clusters known, and is unique in being a major supersonic cluster merger occurring nearly in the plane of the sky, earning it the nickname "the Bullet Cluster". Recently we have discovered a new Bullet-like cluster, MACSJ0025-1222. Allthough it does not contain a low-entropy, high density hydrodynamical `bullet,' this cluster exhibits many similar properties to the Bullet Cluster, and so we also use it to study dark matter. In this talk I will present our measurements of the composition of both systems (using gravitational lensing), show the (independent) evidence for the existence of dark matter, and describe limits that can be placed on the intrinsic properties of dark matter particles. In doing so, I will explain how these clusters offer a serious challenge to MOdified Newtonian Dynamics (MOND) theories. Finally I will conclude with some preliminary results we have on using the Bullet cluster as a `cosmic telescope' to explore the Universe in its infancy.

Collective Oscillations of Supernova Neutrinos

11/4/08
Basudeb Dasgupta
Tata Institute

Neutrinos oscillate in very unusual and interesting ways when their number densities are large, as in the case of neutrinos emitted from a core-collapse supernova. We present a formalism in a general three-flavor framework, that describes the peculiar flavor dynamics due to collective effects. We show how the flavor evolution may be ``factorized'' into two-flavor oscillations with hierarchical frequencies. We apply these ideas to a typical SN, where we show the interplay between collective and MSW effects, and predict some interesting signatures observable at large neutrino detectors, e.g hierarchy determination at extremely small theta13.

Connecting Galaxies, Halos, and Star Formation Rates Across Cosmic Time

11/11/08
Risa Wechsler
Stanford University

Recent observational and theoretical studies have indicated that galaxy luminosities and stellar masses are tightly correlated with the masses of their dark matter halo hosts. I will describe a powerful approach to understanding galaxy clustering which uses this tight correlation to connect galaxies to their host dark matter halos. This model is able to explain a variety of statistics of the galaxy distribution, including the luminosity, scale dependence, and redshift dependence of galaxy clustering. Based on this insight, I will present a new observationally-motivated model for understanding how halo masses, galaxy stellar masses, and star formation rates are related, and how these relations evolve with time. This model indicates that a wide variety of galaxy properties, including trends with environment, are set primarily by the masses of their dark halos.

The Challenge to Unveil the Microscopic Nature of Dark Matter

11/25/08
Scott Watson
University of Michigan
Download Seminar PDF

Despite the success of modern precision cosmology to measure the macroscopic properties of dark matter, its microscopic nature still remains elusive. LHC is expected to probe energies relevant for testing theories of electroweak symmetry breaking, and as a result may allow us to produce dark matter for the first time. Other indirect experiments, such as PAMELA, offer additional ways to probe the microscopic nature of dark matter through observations of cosmic rays. Results from a number of indirect detection experiments, along with hints from fundamental particle theories seem to suggest that our old views of the creat ed revisited. I will discuss both theoretical and experimental motivations for a new theory microscopic dark matter -- and the implications for future experiments such as LHC.

The angular power spectrum of the diffuse gamma-ray background as a probe of galactic dark matter substructure

12/9/08
Jennifer Siegal-Gaskins
CCAPP Ohio State University
Download Seminar PDF

Recent work has shown that dark matter annihilation in galactic substructure will produce diffuse gamma-ray emission of remarkably constant intensity across the sky, and in general this signal will dominate over the smooth halo signal at angles greater than a few tens of degrees from the galactic center. The large-scale isotropy of the emission from substructure suggests that it may be difficult to extract this galactic dark matter signal from the extragalactic gamma-ray background. I will show that dark matter substructure induces characteristic small-scale anisotropies in the diffuse emission which may provide a robust means of distinguishing this component. I will present the angular power spectrum of the emission from galactic dark matter substructure for several models of the subhalo population, and show that features in the power spectrum can be used to infer the presence of substructure. The anisotropy from substructure is substantially larger than that predicted for the extragalactic gamma-ray background, and consequently the substructure signal can dominate the measured angular power spectrum even if the extragalactic background emission is a factor of 10 or more greater than the emission from dark matter. I will show that for many scenarios a measurement of the angular power spectrum by Fermi will be able to constrain the abundance of substructure in the halo.

The Milky Way with SDSS: Decoding The Rosetta Stone of Galaxy Formation

1/6/09
Mario Juric
Princeton
Download Seminar PPT

The distribution, abundancies, and kinematics of stars in the Galaxy carry information about the hierarchical assembly, evolution, and structure of its luminous and dark components. However, small, biased and mostly local samples have traditionally hindered the mining and use of this rich resource. The situation has changed dramatically with large, precise, multiband, photometric and spectroscopic surveys such as the Sloan Digital Sky Survey (SDSS). With SDSS, we can directly and accurately map the stellar number density, metallicity, and kinematics, measure the scales of Galactic components, observe the relationships between their kinematic and physical properties, as well as identify and characterize disrupted remnants and dwarf satellites in the Galactic halo. All of these provide new and valuable insights on the process of assembly as well as the present day state of the Galaxy. This talk will review the results of photometric Milky Way studies with the SDSS, touching on future prospects and inferences about cosmology and galaxy formation in general. Discussion of the latter two is especially timely, given the imminent arrival of 2nd generation surveys such PanSTARRS, SkyMapper, DES, SDSS-III, GAIA and LSST. These surveys, covering 10-100x larger volumes with significantly improved accuracy, have the potential to revolutionize the theory of galaxy formation and near field cosmology in the next decade.

The Destruction of Thin Stellar Disks Via Cosmologically Common Mergers

1/8/09
Chris Purcell
U. of California, Irvine
Download Seminar PDF

Analytic and numerical investigations strongly indicate that the predominant mode of mass delivery into CDM halos involves subhalos roughly one-tenth as massive as their host. Cosmological simulations suggest that around 70% of Galactic-scale halos (M_host ~ 1e12 M_sun) have undergone a 1:10 merger event in the last 10 Gyr involving a large satellite galaxy (M_sat ~ 1e11 M_sun) several times as massive as the galactic disk itself. The survival and stability of thin stellar disks against these common and potentially destructive mergers has therefore been of prime interest in the field of galaxy formation and evolution. I present results from the highest-resolution suite of collisionless simulations performed to date involving 1:10 mass-ratio mergers motivated by cosmological conditions, quantifying the morphological and dynamical transformation of cold, thin galactic disks into hot and extremely thick stellar systems. I discuss the ramifications of this disk destruction for LCDM models of galaxy formation, and assess the future endeavors for analysis and simulation which may alleviate this concern.

Cosmology with the South Pole Telescope

1/13/09
Jeff McMahon
Univ. of Chicago

The South Pole Telescope is a 10-meter, millimeter wave telescope optimized for observation of the cosmic microwave background at arcminute resolution. In the 2006/2007 austral summer we assembled this telescope at the geographic South Pole and commissioned a sensitive 1000 element three-frequency camera. This instrument has now collected two years of survey data rep ive high resolution maps of the CMB to date. Using these data we published the first detection of new galaxy clusters selected with the Sunyaev-Zel'dovich (SZ) effect. Analysis of the completed survey will yield a large catalogue of SZ selected clusters which will be used to constrain the equation of state of dark energy. The SPT data will further constrain cosmological parameters through a measurement of the high-l CMB power spectrum. In addition, we are currently building a new polarization sensitive camera (SPTpol) to be deployed in 2011. SPTpol will measure the polarization of the CMB to provide constraints on neutrino mass and potentially, the energy scale of inflation. In this talk I provide an overview of the SPT instrument, and discuss the prospects for cosmology with these data.

The Early Reionization of Voids

1/15/09
Kristian Finlator
Univ. of Arizona

I introduce a new method for computing cosmological reionization by coupling cosmological hydrodynamic simulations with an accurate solution to the moments of the radiative transfer equation. Applying this method to precomputed density and emissivity fields reveals that reionization proceeds rapidly from the overdense regions that host sources into voids before "mopping up" self-shielded filamentary regions. Our finding that most filamentary regions reionize late owes to a low-mass cutoff in the ratio of halo mass to ionizing luminosity that is expected in situations where low-mass halos form stars inefficiently or have low ionizing escape fractions. Previous works have largely overlooked the possibility for such a reionization topology because they implicitly agreed on the bias of the emissivity field. I discuss implications of the topology of reionization for current and upcoming observations.

Co-evolving star formation and AGN activity within the zCOSMOS density field

1/20/09
John Silverman
ETH Zurich

An understanding of the influence of environment on both AGN activity and star formation is crucial to determine the physical mechanism(s) regulating the coeval growth of supermassive black holes (SMBHs) and the galaxies in which they reside. Deep multi-wavelength surveys (e.g. COSMOS) now offer the tools to explore in detail such studies up to z~1 by providing least biased samples of AGN, a characterization of the underlying parent sample of galaxies, and an assessment of the local environment. I will present new results based on 7543 zCOSMOS galaxies with quality optical spectra and XMM-Newton observations that identify those galaxies hosting X-ray selected AGNs including the obscured population. We specifically measure star formation rates of AGN host galaxies, growth rates of these SMBHs, and the influence of the environment on triggering AGN activity. Our findings shed light on the key ingredients for a galaxy to harbor an actively accreting SMBH and possible migration onto the local SMBH-bulge relations.

Cluster Detection in Sunyaev-Zel'dovich Surveys

1/21/09
Laurie Shaw
McGill Univ.

Measuring the redshift evolution of the cluster mass function provides us with a sensitive means of constraining cosmological parameters. Sunyaev-Zel'dovich Effect (SZE) surveys are currently searching for clusters via their imprint on the CMB. In the first part of my talk I will discuss work done to determine the effectiveness of SZ cluster-finding algorithms in detecting clusters and measuring their integrated flux -- a quantity predicted to be tightly correlated with cluster mass -- using synthetic sky-maps constructed from high-resolution cosmological 'lightcone' simulations. In the second part of my talk I will present recent results from the South Pole Telescope, including the first blind detections of galaxy clusters via the SZE.

Falsifying Paradigms for Cosmic Acceleration

1/22/09
Michael Mortonson
Univ. of Chicago
Download Seminar PPT

Future measurements of cosmic distances and growth can test and potentially falsify classes of dark energy models including the cosmological constant and quintessence. The distance-redshift relation measured by future supernova surveys will strongly constrain the expansion history of the universe. Under the assumption of a particular dark energy scenario, limits on the expansion rate place bounds on the evolution of the growth of large-scale structure. I will discuss the anticipated predictions for growth and expansion observables from a future SNAP-like supernova sample combined with CMB data from Planck. Although simple models like flat LCDM are easiest to falsify, strong consistency tests exist even for general dark energy models that include dynamical dark energy at low redshift, early dark energy, and nonzero spatial curvature.

N-Body Simulations and Photometric Redshifts

1/28/09
Hans Stabenau
Univ. of Pennsylvania

Recent measurements have shown that the expansion of the universe is accelerating. In order to extrapolate GR to fit the properties of the universe on horizon scales, a strange form of energy density (``dark energy'') is required. An alternative to dark energy is a modification of GR that accounts for accelerating expansion. In this talk, I will discuss my work on predicting large-scale structure formation in an alternative gravity model by N-body simulation. A related problem that I have worked on is efficiently obtaining accurate redshifts for galaxies using photometric redshifts with surface brightness priors. Finally I will talk briefly about my ongoing work on analyzing data from the Balloon-borne Large Aperture Submillimeter Telescope (BLAST) experiment.

Astrophysical Signatures of Dark Matter Annihilation

1/29/09
Greg Dobler
Harvard
Download Seminar PDF

One of the most significant outstanding unknowns in cosmology is the fundamental nature of dark matter. One hypothesis is that the dark matter consists of Weakly Interacting Massive Particles (WIMPs) that are a thermal relic of the Big Bang. Generic WIMP models predict self-annihilation cross sections and masses that not only give roughly the correct relic density, but also have astrophysical consequences which may be observable by current and near-term experiments. I will discuss the implications for WIMP annihilation in the context of three recent observations: the anomalously hard synchrotron radiation centered on the Galactic center that is observed by WMAP (the WMAP "haze"), the rise in the PAMELA local positron spectrum above 10 GeV, and the excess electrons centered on 650 GeV in the ATIC local electron spectrum. In addition, I will show that, not only is the spectrum of the haze from 23 to 33 GHz consistent with numerous annihilation channels, but the existence of the haze electrons implies an inverse Compton (scattering of starlight photons by the electrons which produce the haze)signal towards the Galactic center which may be observable by the Fermi Gamma-Ray Space Telescope.

Estimating cosmological parameters with cosmic shear

2/13/09
Tim Eifler
Univ. of Bonn

In recent years weak lensing by the large-scale structure of the Universe, called cosmic shear, has become a valuable probe in cosmology. Large upcoming surveys such as KIDS, Pan-STARRS, DES, SNAP/JDEM, and Euclid will improve the quality of cosmic shear data significantly, enabling us to measure its signal with less than 1% statistical error. In order to obtain cosmological parameters from these high precision data properly, there remain issues to be addressed. On the observational side, systematic errors, mainly from insufficient PSF-correction, must be reduced, and a possible contribution to the shear signal coming from intrinsic alignment or shape-shear correlation must be excluded. On the theoretical side, we need accurate predictions for P_delta(k) and precise statistical methods to infer cosmological parameters. In this seminar talk I review the basics of cosmic shear with the focus on how to constrain cosmological parameters. I illustrate the impact of cosmic shear covariances on the parameter estimation and present an improved likelihood analysis for cosmic shear data. As a second topic, I address the issue of E- and B-modes in the shear signal and outline a new method (the ring statistics) to separate E-modes from B-modes. I explain advantages and problems of the ring statistics and compare its information content to that of other second-order cosmic shear measures. Finally, I present results from the first shear measurement using the ring statistics on data of the CFHTLS survey.

Surveying the TeV sky with Milagro and HAWC

2/17/09
Tyce DeYoung
Penn. State U.

The past few years have seen a wealth of new results in TeV astronomy, produced by wide field-of-view air shower detectors such as Milagro and by air Cherenkov telescopes. These results are shedding light on the very energetic objects such as the accelerators of cosmic rays, but also contain puzzling surprises, such as fine structure in the arrival directions of TeV cosmic rays. In the near future, the High Altitude Water Cherenkov (HAWC) observatory will provide an order of magnitude better sensitivity than Milagro and, in conjunction with other new instruments such as IceCube and Fermi (GLAST), will provide an even richer understanding of the very high energy universe.

Signatures of Dissipation and Relaxation in the Stellar Orbit Structure of Merger Remnants (Galactic Archaeology)

2/24/09
Loren Hoffman
Northwestern U.

Many of the observed properties of elliptical galaxies indicate that they had a violent formation history. They are dynamically hot systems, with high velocity dispersions dominating over ordered stellar streaming. Gas-rich tidal tails, and rings and shells indicative of the recent disruption of a spiral galaxy, often surround systems otherwise resembling ordinary giant ellipticals. These observations led Toomre & Toomre (1972) to suggest that elliptical galaxies are the products of mergers between spirals, a hypothesis that today fits naturally into the context of hierarchical structure formation. The violent relaxation in galaxy mergers is incomplete - the stellar distribution is scrambled enough to produce hot, ellipsoidal systems resembling early-type galaxies, but substantial memory of the initial conditions is retained as features in the remnant distribution function. This "fine structure" serves as a fossil record of the galaxy's formation history. High-resolution integral field spectroscopy with instruments such as SAURON and OASIS has enabled us to reconstruct the full 3D stellar orbital distributions of nearby galaxies, taking "galactic archaeology" to a whole new level. In this talk I will present simulation work aimed at meeting the challenge of parsing this new profusion of information on the buildup of local galaxies. In particular I will focus on the signatures of gas dissipation in the remnant orbit structure, and on the unique characteristics of "dry" merger remnants, produced by the re-merger of two gas-poor ellipticals.

The Quest for Dark Matter: from hints to discovery

3/16/09
Gianfranco Bertone
IAP, France

The possibility of explaining the positron and electron excess recently found by the PAMELA and ATIC collaborations in terms of dark matter (DM) annihilation or decay has attracted considerable attention. However, DM has been invoked in the past to explain many other experimental results, such as the 511 KeV line detected by INTEGRAL, the EGRET 'bump', or the WMAP 'Haze'. It is therefore natural to ask what we can actually learn from these experiments, and whether it is possible to obtain conclusive evidence for DM from Particle Astrophysics experiments. To answer these questions, I will review the prospects to detect "smoking-gun" features that would unambiguously point to DM, and argue that in absence of them, conclusive evidence can probably be obtained only through by crossing the results from accelerator, direct and indirect searches.

Evolution and modelling of dwarf spheroidal galaxies

3/24/09
Jaroslaw Klimentowski
Nicolaus Copernicus Astronomical Center

Dwarf spheroidal galaxies of the Local Group are key objects for understanding of many aspects of current cosmology. Their dark matter contents are much higher than in typical galaxies. Due to their vicinity studies can be made with more sophisticated methods, not available for classical cosmological objects like clusters. Unfortunately we still lack knowledge about how they were formed and evolved. The missing satellites problem has shown that modern simulations cannot correcty explain the numbers of dwarf galaxies observed on the sky. Modelling of their dark matter halos differs between authors using even the same observational data. This shows that we still rely on different, often doubtful assumptions. In this talk I will present results of our work on dwarf spheroidal galaxies. We have studied different scenarios of formation and evolution of satellites based on a cosmological simulation. We have also studied in detail tidal evolution of disk galaxy in Milky Way potential. We show how a stellar disk can be transfomed into a spheroid and how this scenario can be confirmed by observations of real objects. We show how tidal debries can affect mass modelling and we test methods to do it properly. Finally we apply them to real sky galaxies and draw conclusions.

Hot Spots and Cold Spots in Universe

The Missing Baryons and Quasars Missing in Action

3/31/09
Shirley Ho
Berkeley National Lab

I will present a new method in finding the missing baryons by generating a template for the kinematic Sunyaev-Zel'dovich effect. The template is computed from the product of a reconstructed velocity field with a galaxy field; we find that the combination of a galaxy redshift survey such as SDSS and a CMB survey such as ACT and PLANCK can detect the kSZ, and thus the ionized gas, at significant signal-to-noise. Unlike other techniques that look for hot gas or metals, this approach directly detects the electrons in the IGM through their signature on the CMB. The signal-to-noise ratio for various combination of experiments will be shown.
I will also discuss preliminary results on cross-correlation between Quasars (from SDSS) and WMAP, which puts upper limit on the average amount of energy that is imparted onto the gas surrounding the quasars.

William of Occam and Enrico Fermi

Fundamental Physics from the Sky

4/7/09
Stefano Profumo
UC Santa Cruz
Download Seminar PDF

Can we learn about New Physics with astronomical and astro-particle data? Understanding how this is possible is key to unraveling one of the most pressing mysteries at the interface of cosmology and particle physics: the fundamental nature of dark matter. Rapid progress may be within grasp in the context of an approach which combines information from high-energy particle physics with cosmic-ray and traditional astronomical data. I discuss recent puzzling data on cosmic-ray electrons and positrons and their interpretation. I show how the Fermi Space Telescope will soon shed light on those data as well as potentially on several dark matter particle properties. I then introduce a novel approach to particle dark matter searches based on the complementarity of astronomical observations across the electromagnetic spectrum, from radio to X-ray and to gamma-ray frequencies.

Dark Matter Halos

The phase-space structure of dark matter halos

4/14/09
Monica Valluri
Univ. of Michigan
Download Seminar PDF

LambdaCDM simulations of structure formation have reached unprecedented numerical resolution in recent year. Yet some aspects of the evolution of dark matter halos are still not well understood. I will present results of a study of the evolution of the coarse-grained phase space distribution function in LCDM halos with particular focus on the importance of mixing in collisionless evolution. I will also describe results of a recent study of the evolution of the orbital properties of dark matter particles in response to the growth of baryonic components. The goal of this talk is to provide some insights into the evolution of dark matter halos using tools of classical dynamics.

Galaxy

Clues about Disk Evolution from the Outermost Reaches of Galaxies

4/28/09
Rok Roskar
Univ. of Washington
Download Seminar PDF

Outer disks of galaxies defy our understanding of disk formation. Their profiles deviate from simple exponentials, they are at once the sites of current galaxy assembly and places where a galaxy's history can be effectively preserved owing to long dynamical times. We investigate the nature of outer disks with an N-body/SPH approach. We simulate a suite of idealized models representative of galaxy formation through dissipational collapse after the last major merger. We find that a disk break is seeded by a drop in star formation density, while the outer disk is populated almost exclusively by stars that migrated there from the interior on surprisingly circular orbits. The degree of such radial migrations is large and unexpected. I will discuss the theoretical basis for this phenomenon and present some observational evidence that lends support to the theory. I will also briefly chart out some far-reaching implications of such migrations for studies ranging from the solar neighborhood to extragalactic stellar populations.

Black and White Plot

the stellar population synthesis technique

5/5/09
Charlie Conroy
Princeton
Download Seminar PDF
Price Prize Lecture

The SPS technique is deceptively simple. Relying on stellar evolution calculations, stellar spectral libraries, and dust models, practitioners of SPS aim to convert the observed spectral energy distributions of galaxies into physical properties. Knowledge of these physical properties, which range from total stellar masses to star formation rates and metallicities, are essential for understanding the formation and evolution of galaxies. The SPS framework thus provides a fundamental link between theory and observations. Despite its importance, a systematic investigation of the uncertainties in SPS is lacking. In this talk I will describe ongoing work exploring the panoply of uncertainties in SPS, including uncertainties in stellar evolution, dust models, and initial mass functions, amonst others, and their propagation into the derived physical properties of galaxies. I will also discuss attempts to constrain these uncertain aspects with existing and future observations.

Artic Instrument in Sun

An Update from the South Pole Telescope

5/19/09
Jeff McMahon
University of Chicago

The South Pole Telescope (SPT) is a 10-meter telescope optimized for arcminute scale observations of the cosmic microwave background (CMB). Construction and commissioning were completed in January 2007, and since then we have acquired more than two years of data. Using these observations we recently published the first detection of galaxy clusters selected with the Sunyaev-Zel'dovich (SZ) effect. Through the SZ effect, we will create a mass limited catalog of galaxy clusters out to t on. In addition, the survey data will provide an improved measurement of the temperature power spectrum of the CMB out to arcminute scales. In this talk I will describe the telescope and instrumentation, provide an update on current and forthcoming results, and discuss plans for future science with SPT.

Indirect Dark Matter Detection -- Robust Bounds on Annihilation to Electrons, Neutrinos and Gamma Rays

5/26/09
Nicole Bell
Melbourne University

We examine dark matter annihilation in galaxy halos to neutrinos, gamma rays, and e+e-. We show that annihilation to neutrinos, the least detectable final state, defines a robust upper bound on the total cross section and implies that annihilation cannot significantly modify dark matter halo density profiles. Dark matter annihilation into charged particles is necessarily accompanied by gamma rays produced via radiative corrections. Internal bremsstrahlung from final state charged particles produce hard gamma rays up to the dark matter mass, with an approximately model-independent spectrum. In addition, significant electromagnetic radiation is produced via energy loss processes of e+e- annihilation products. We discuss dark matter interpretations of the PAMELA anomaly in light of these results.

JDEM MINOS

Dark Energy Calibrations for JDEM and Neutrino Oscillations with MINOS

6/09/09
Bob Armstrong
Indiana University

Essential to future measurements of dark energy will be an understanding of systematic errors. One important component to the error budget will be from photometric calibration. Upcoming surveys will need to reduce the calibration uncertainty to less than 1% to distinguish between different models for dark energy. I will describe ongoing work to achieve this goal for JDEM. In addition, I will discuss neutrino oscillation results from the MINOS experiment. MINOS is a long baseline neutrino oscillation experiment that sends neutrinos from Fermilab to northern Minnesota. By comparing the neutrino energy spectrum at both locations, a precision measurement of the atmospheric mixing parameters can be done. I will report on these results as well our recent measurement of electron neutrino appearance.

black and white galaxy merger

Inspiralling Supermassive Black Holes as Tracers of Galaxy Mergers

9/08/09
Julie Comerford
UC Berkeley
Download Seminar PDF

When two galaxies with central supermassive black holes (SMBHs) merge, the SMBHs inspiral in the resultant merger-remnant galaxy and eventually coalesce. However, very few inspiralling SMBH pairs have been identified observationally. In this talk, I will describe a new technique I use to build a significantly larger sample of inspiralling SMBHs, where I spectroscopically identify inspiralling SMBHs that power AGN. I search the DEEP2 Galaxy Redshift Survey for galaxy spectra that exhibit AGN emission lines that are offset in velocity relative to the mean velocity of the host galaxy's stars, suggesting bulk motion of the AGN within the host galaxies. Within the set of DEEP2 red galaxies at 0.3 < z < 0.8, I find 32 AGN with statistically significant (greater than 3 sigma) velocity offsets, ranging from ~50 km/s to ~300 km/s. After exploring physical effects such as AGN outflows that could cause such velocity offsets, I find that these offsets are most likely the result of SMBHs inspiralling within merger-remnant galaxies. With this new technique of identifying galaxy mergers, I find that roughly half of red galaxies hosting AGN are merger-remnant galaxies. This result implies that galaxy mergers may trigger AGN activity in red galaxies and sets a merger rate of ~3 mergers/Gyr for red galaxies at 0.3 < z < 0.8. Finally, I will discuss the utility of HST imaging and optical slit spectroscopy in increasing the number of known inspiralling SMBHs.

Cosmological hydrogen recombination: the effect of very high-n states and quadrupole transitions

9/22/09
Dan Grin
Caltech
Download Seminar PDF

Thanks to the ongoing Planck mission, a new window will be opened on the properties of the primordial density field, the cosmological parameters, and the physics of reionization. Much of Planck's new leverage on these quantities will come from temperature measurements at small angular scales and from polarization measurements. These both depend on the details of cosmological hydrogen recombination; use of the CMB as a probe of energies greater than 1016 GeV compels us to get the ~eV scale atomic physics right.

One question that remains is how high in hydrogen principle quantum number we have to go to make sufficiently accurate predictions for Planck. Using sparse matrix methods to beat computational difficulties, I have modeled the influence of very high (up to and including n=200) excitation states of atomic hydrogen on the recombination history of the primordial plasma, resolving all angular momentum sub-states separately and including, for the first time, the effect of hydrogen quadrupole transitions. I will review the basic physics, explain the resulting plasma properties, discuss recombination histories, and close by discussing the effects on CMB observables.

galaxy merger

Pairing of Supermassive Black Holes in galaxy mergers

10/6/09
Simone Callegari
University of Zurich

Theoretical and observational efforts have been devoted in recent years to trace the coevolution of the populations of galaxies and of the Supermassive Black Holes (SMBHs) inhabiting their centers. In particular, the hierarchical assembly of a galaxy through mergers could lead to the formation of SMBH pairs in its nucleus. Such pairs can give rise to many interesting processes, among which the emission of gravitational waves, detectable by forthcoming experiments such as LISA. In this talk I will present results from a campaign of N-body/SPH simulations aimed at studying the conditions that drive or inhibit SMBH pairing in galaxy mergers. Gasdynamics affects the formation and the properties of SMBH pairs in a complex way, especially in the cosmologically relevant unequal-mass regime. Exploring the evolution of SMBHs during mergers is therefore a key ingredient in the study of the cosmic history of the SMBH population.

Graph

Cosmology with the shear-peak statistics

10/19/09
Joerg Dietrich
University of Michigan

Weak-lensing searches for galaxy clusters are plagued by low completeness and purity, severely limiting their usefulness for constraining cosmological parameters with the cluster mass function. A significant fraction of `false positives' are due to projection of large-scale structure and as such carry information about the matter distribution. We demonstrate that by constructing a ``peak function'', in analogy to the cluster mass function, cosmological parameters can be constrained. To this end we carried out a large number of cosmological N-body simulations in the \Omega_m-\sigma_8 plane to study the variation of this peak function. We demonstrate that the peak statistics is able to provide constraints competitive with those obtained from cosmic-shear tomography from the same data set. By taking the full cross-covariance between the peak statistics and cosmic shear into account, we show that the combination of both methods leads to tighter constraints than either method alone can provide.

Toward Unveiling the Sources of the Highest Energy Cosmic Rays

10/20/09
Hajime Takami
(IPMU, Univ. of Tokyo)

The origin of the highest energy cosmic rays (HECRs) is one of the biggest mysteries in modern astrophysics. A main reason why we have not able to identify their sources is magnetic fields in the Universe, i.e., the trajectories of HECRs are deflected by them. Recent progress of very large detectors for HECRs like Pierre Auger Observatory has unveiled their anisotropic arrival distribution and also spatial correlation between the arrival directions of HECRs and matter distribution of local Universe. These facts point out that magnetic fields are not so large that the arrival directions of HECRs lose information on their sources, but, the magnetic fields are not negligible, since we do not find any plausible source candidates to the arrival directions of detected HECRs. Thus, in order to explore the origin of HECRs by HECRs themselves, it is essential to take their propagation in magnetized Universe into account. In this seminar, I will briefly review our current understanding of HECR sources and mainly discuss the possibility to find the sources by cosmic ray astronomy, especially focusing on the propagation of the highest energy protons in intergalactic and Galactic magnetic fields.

Probing extragalactic high-energy cosmic-ray sources with high-energy neutrinos and gamma rays

10/26/09
Kohta Murase
(Kyoto University)
Download Seminar PDF

The origin of high-energy cosmic rays is one of the big mysteries in the Universe. Observations of high-energy neutrinos and gamma rays are important to know properties of the cosmic-ray sources, especially for transients. Now, not only high-energy gamma-ray but also neutrino observations have provided information. Identification of the sources may be possible in the near future. In my talk, we will discuss possibilities and consequences of high-energy cosmic-ray acceleration in extragalactic astrophysical objects. We will especially focus on transient sources such as gamma-ray bursts, active galactic nuclei and newly born magnetars. We might also discuss persistent sources such as clusters of galaxies, if we have time.

Tassos Fragos Image

Models for X-Ray Binaries: Galactic and extragalactic populations

10/27/09
Tassos Fragos
Northwestern University

X-ray binaries are unique astrophysical laboratories as they carry information about many complex physical processes such as star formation, compact object formation, and evolution of interacting binary systems. I will initially present an analysis that allows us to reconstruct the full evolutionary history of known Galactic X-ray binaries back to the time of compact-object formation; the results provide us with the most robust constraints on black-hole kicks due to asymmetries in the collapse. Motivated by deep Chandra observations of extra-galactic populations of X-ray binaries, I will also present population studies of low-mass X-ray binaries in elliptical galaxies. These simulations are targeted at understanding the origin of the shape and normalization of the observed X-ray luminosity functions as well as the transient behavior of X-ray binaries. Finally, I will briefly talk about an ongoing project towards developing a new advanced computational tool for the study of X-ray binary populations formed in both galactic fields and dense stellar clusters.

Getting the most out of dark matter observations and experiments

11/3/09
Annika Peter
Caltech
Download Seminar PDF

Dark matter, constituting a fifth of the mass-energy in the Universe today, is one of the major "known unknowns" in physics. There are currently four approaches to determining the nature of dark matter, assuming it is composed of at least one new species of particle: 1) creation in collider experiments; 2) indirect detection via its annihilation products; 3) and direct detection; and 4) observations sensitive to the gravity of dark matter. For the latter three approaches, event rates are not only sensitive to the "physics" of dark matter (mass, cross sections, and the theory in which the dark matter particles live) but to the "astrophysics" of dark matter as well, namely the phase space density of dark matter throughout the Milky Way and other galaxies and its evolution through cosmic time. There is much theoretical uncertainty in phase space density, a fact which tends to either be ignored or simply acknowledged as a problem. I will highlight a few recent developments in understanding the local dark matter phase space density. Then, I will propose a shift in the way we think about uncertainties in the dark matter phase space density. Namely, we should be treating the astrophysics and physics properties of dark matter on equal footing, as things we want to derive from myriad data sets. I will show how this shift in thinking may yield more robust determinations of both the physics and astrophysics of dark matter, and show how this works in practice.

The Quest For the Nature of Dark Matter

11/10/09
Mike Kuhlen
(IAS/UC Berkeley)

Despite having observational evidence of its existence for more than seventy years now, we still don't know the nature of Dark Matter. Recent progress in astronomical observations, laboratory experiments, theory, and numerical simulations has led to an explosion in research on this topic. Here I review some of the recent developments, with an emphasis on how ultra-high resolution cosmological numerical simulations can contribute to the quest to unravel the mystery of Dark Matter.

CDM Accelerating Cosmology as an Alternative to ΛCDM

11/17/09
J.A.S. Lima
(IAG, University of São Paulo, Brazil)

A new accelerating cosmology driven only by baryons plus cold dark matter (CDM) is proposed in the framework of general relativity.

In this scenario, the present accelerating stage of the Universe is powered by the negative pressure describing the gravitationally-induced particle production of cold dark matter particles. The new cosmology is presented in 3 steps: 1) The Mechanism, 2) The Classical Formalism (Correction to the Energy Momentum Tensor), and, 3) The Cosmological Scenario.

The resulting cosmology has only one free parameter and the differential equation governing the evolution of the scale factor is exactly the same of the ΛCDM model. For a spatially flat Universe, as predicted by inflation (Ωdm + Ωbar =1), it is found that the effectively observed matter density parameter is Ωeff = 1- α, where α is the constant parameter specifying the CDM particle creation rate. The supernovae test based on the Union data (2008) requires α ≅ 0.71 so that Ωeff ≅ 0.29, as independently derived from weak gravitational lensing, large scale structure and other complementary observations. Some caveats of the model (mainly the ones related to the quantum formulation) are discussed in the conclusion.

What the most metal-poor stars tell us about the early Universe

11/20/09
Anna Frebel
(CfA)

The chemical evolution of the Galaxy and the early Universe is a key topic in modern astrophysics. Since the most metal-poor Galactic stars are the local equivalent of the high-redshift Universe, they can be employed to reconstruct the onset of the chemical and dynamical formation processes of the Galaxy, the origin and evolution of the elements, and associated nucleosynthesis processes. They also provide constraints on the nature of the first stars and SNe, the initial mass function, and early star formation processes. The discovery of two astrophysically very important metal-poor objects recently lead to a significant advance regarding these topics. One object is the most iron-poor star yet found (with [Fe/H]=-5.4). The other star displays the strongest known overabundances of heavy neutron-capture elements, such as uranium, and nucleo-chronometry yields a stellar age of ~13 Gyr. Metal-poor stars, once also identified in dwarf galaxies, are vital probes also for near-field cosmology. Their chemical signatures now suggest that systems like these were building blocks of the Milky Way's low-metallicity halo. This opens a new window to study galaxy formation through stellar chemistry.

Galaxy Cluster

What *don't* we know about galaxy formation?

11/24/09
Darren Croton
(Swinburne)

Much progress has been made in recent years in our understanding of the co-evolution of galaxies and AGN, and their connection to the underlying large-scale structure. In this talk I will discuss simulation and modeling techniques that bridge theories of galaxy and quasar formation with the properties of observed galaxy populations. In addition, I will discuss a number of open questions important for extra-galactic astronomy and cosmology, and explain how future large-scale surveys and galaxy formation models may jointly address them.

sky distribution of magnetars

Magnetar Observations in the Fermi Era

12/1/09
Chryssa Kouveliotou
(NASA's MSFC)

Magnetars are magnetically powered rotating neutron stars with extreme magnetic fields (over 1014 Gauss). They are discovered and emit predominantly in the X- and gamma-rays. Very few sources (roughly 15) have been found since their discovery in 1987. NASA's Fermi Observatory was launched June 11, 2009; the Fermi Gamma Ray Burst Monitor (GBM) began normal operations on July 14, about a month after launch, when the trigger algorithms were enabled. In the first year of operations we recorded emission from four magnetar sources; of these, only one was an 'old' magnetar: SGR 1806+20. The other three detections were two brand new sources, SGR J0501+4516, discovered with Swift and extensively monitored with both Swift and GBM, SGR J0418+5729, discovered with GBM and the Interplanetary Network (IPN), and SGR J1550-5418, a source originally classified as an Anomalous X-ray Pulsar (AXP 1E1547.0-5408). In my talk I will give a short review on magnetars and describe the current status of the analyses efforts of the GBM data with our magnetar team.

Satellite

Naturally hidden dark matter

12/8/09
Francesc Ferrer
(Washington Univ, St. Louis)

Models addressing the naturalness problems of the Standard Model of Particle Physics often contain particles that could constitute the dark matter (DM) in the universe. Such DM particles could give rise to detectable fluxes of cosmic and gamma-rays. The simplest scenarios, like the neutralino in the Minimal Supersymmetric extension of the Standard Model (MSSM), cannot account for the anomalous cosmic ray fluxes observed by the PAMELA experiment. We study extensions of the MSSM that alleviate some of its remnant fine-tuning problems, and whose dark sector can fit the reported positron fraction excess.

Pop goes the neutrino: acoustic detection of astrophysical neutrinos

12/14/09
Justin Vandenbroucke
(Stanford/KIPAC)

High energy particle showers developing in a dense medium heat the medium locally, causing it to expand and emit a shock wave detectable as an acoustic pulse. The idea of detecting particle tracks and showers with this method was proposed in the 1950's, and was confirmed in the laboratory in the 1970's. Interest has grown recently in discovering and then characterizing extremely high energy neutrinos, particularly cosmogenic ("GZK") neutrinos of energy ~10^18 eV. The acoustic technique has been proposed as a possible method to detect GZK neutrinos. Large volumes (10-100 km^3) of naturally occurring target media such as water, ice, and salt could be instrumented relatively inexpensively with this technique. I will describe the status of several acoustic projects, focusing in particular on the South Pole Acoustic Test Setup (SPATS), a small array deployed by the IceCube collaboration to determine the acoustic properties of South Pole ice. SPATS has completed many of its goals, including measuring the sound speed, noise level, transient background, and attenuation length in South Pole ice. Although these measurements were originally made for neutrino astronomy R&D, they can also help address open questions in glaciology.

Fermi and Ultra-High Energy Cosmic Rays

12/15/09
Charles Dermer
(Naval Research Laboratory)

The Fermi Gamma ray Space Telescope, now midway through the second year of its mission, has given us a temporally evolving panorama of the GeV sky. Fermi's legacy includes a catalog of extragalactic sources containing star-forming galaxies, active galaxies, and gamma-ray bursts which display unexpected high-energy behavior. We consider Fermi and correlated data as potential evidence for hadronic acceleration leading to ultra-high energy cosmic ray (UHECR) production in blazars and GRBs. Centaurus A is considered as a nearby UHECR source. The features considered in theoretical analyses of relativistic jets are the gamma-gamma opacity and power constraints. Minimum outflow Lorentz factors have important implications for UHECR and UHE neutrino production, e.g., with IceCube, which are described in this talk.

part of Coadd images with a cluster highlighted

Analysis of Galaxy clusters in the SDSS Coadd data

2/8/10
Marcelle Soares-Santos
(U. of São Paulo)

Galaxy cluster counts in spatial pixels and mass bins constitute a sensitive probe for Cosmology. Analyses based on this fact are part of the scientific program of experiments such as the upcoming Dark Energy Survey and have been pursued using the state of the art data. We perform a measurement of cosmological parameters using cluster counts in the SDSS Coadd. A measurement using clusters requires galaxy photometric redshifts, cluster finding algorithms, cluster mass calibration, cosmological parameter estimation and a data set of sufficient scope. For the SDSS Coadd, photometric redshifts are obtained with a neural network algorithm. A cluster catalog from this sample of 13M galaxies covering 250 sq-degrees up to redshift ~1 is constructed using a Voronoi Tessellation cluster finder. The selection function is computed using DES mock galaxy catalogs. A weak lensing analysis provides the mass calibration of the cluster sample binned into observables. A joint likelihood method using the mean abundance and spatial distribution is used to obtain cosmological constraints.

Cosmic ray anisotropy measurement with IceCube

2/9/10
Rasha Abbasi
(U. of Wisconsin, Madison)
Download Seminar PDF

IceCube is cubic kilometer scale neutrino observatory located at the geographical South Pole. The kilometer cubed detector construction is on schedule to be completed in 2011. At the moment it is taking data with 59 deployed strings, when completed it will comprise 80-strings plus 6 additional strings for the low energy array Deep Core. The strings are deployed in the deep ice between 1,450 and 2,450 meters depth, each string containing 60 optical sensors. In this talk I will present selected results of ongoing analysis of IceCube detector data including the search reporting the measurement of 0.06% of large scale anisotropy. The data used in the large scale anisotropy analysis contains billions of downward going muon events with a median energy per nucleon of ~14 TeV and a median angular resolution of 3 degrees. The energy dependence of this anisotropy is also presented. The observed anisotropy has an unknown origin and we will discuss various possible explanations. Studies of the anisotropy could further enhance the understanding of the structure of the galactic magnetic field and possible cosmic ray sources.

Two galaxies

Unraveling the Formation History of Elliptical Galaxies

3/2/10
TJ Cox
(Carnegie Observatories)
Download Seminar PDF

The idea that galaxies in general, and elliptical galaxies in particular, are shaped by their merger history has gained widespread acceptance. However, a detailed mapping between specific merger histories, and the wide variety of galaxies observed is still uncertain. By using a comprehensive set of state-of-the-art numerical simulations, we show that a single disk-disk merger, as originally proposed by the "merger hypothesis," is a plausible mechanism to form many elliptical galaxies provided that dissipation is involved. We also show that additional (merger?) processes are likely needed to form the largest ellipticals and we outline several properties commonly observed in elliptical galaxies that may provide insight into their formation history.

Understanding Core-Collapse Supernovae in the Transient Era

3/16/10
Chris Fryer
(Los Alamos National Laboratory)

Supernova surveys have taught us much about supernovae. But the surveys of the past focused on "normal" supernovae. Today's transient surveys are discovering a wide variety of stellar explosions. These new explosions potentially will teach us as much about supernovae as focused supernovae. I will discuss a variety of specific examples where we can use the "new" explosions discovered in transient surveys to help us understand supernovae.

Observations of Prompt Gamma-ray Burst Emission

3/30/10
Takanori Sakamoto
(NASA-GSFC)

I will review prompt emission observations from HETE-2 and Swift, which are both satellite missions dedicated to the detection of Gamma-ray Bursts (GRBs). HETE-2 and Swift have on-board computers to process the data and localize GRBs in real-time without a "human-in-the-loop" delay. Thanks to the fast and accurate position localization of GRBs, our understanding of their afterglow emission and host galaxies (birthplace of GRBs) has been dramatically improved. However, the origin of GRB prompt emission is still far from being resolved. I will talk about the observational properties of the prompt GRB emission phase in the context of HETE-2 and the Swift data. I will also discuss the nature of future observations needed to understand GRB prompt emission.

Unraveling gamma-ray Blazars in the Era of Fermi and VERITAS

4/6/10
Luis Reyes
(U of Chicago)

The field of high-energy astrophysics is experiencing a revolution due to recent observations that have revealed a universe that is surprisingly rich, variable and complex at gamma-ray energies. This revolution has now switched into high gear with the launch of the Fermi Gamma-ray Space Telescope and the full-fledged operation of a new generation of ground-based instruments such as VERITAS, H.E.S.S. and MAGIC. Among the different classes of gamma-ray sources observed by these instruments, a particular subset of active galactic nuclei (AGN) known as blazars stand out as some of the most energetic and variable objects observed at any wavelength. In my talk I will describe how the complementary capabilities of space and ground-based instruments are leading us to a better understanding of gamma-ray blazars as high-energy sources, as a population, and as a cosmological tool to probe the background radiation known as extragalactic background light (EBL). Finally, I will discuss the important scientific return that a next-generation instrument such as AGIS would bring to the field of AGN astrophysics.

NIght Sky

Clues about Dark Matter: Studying the Milky Way in 6-D

4/27/10
Nitya Kallivayalil
(MIT)

Tidal Streams provide a powerful probe of the potential of the Milky Way halo over large Galactocentric distances and their detailed phase-space structure gives us clues as to the nature of dark matter. Powerful theoretical techniques are now available to re-construct the underlying potential from the six-dimensional phase-space parameters that describe stellar tracers. Notably absent from the presently available data-sets are full 3-D velocities. I will describe ongoing efforts to remedy this aimed at tracers that sample the Milky Way halo at a large range of distances: the inner stellar halo, the Sagittarius Stream and Globular Clusters, and the Magellanic Clouds. I will also describe efforts to expand the number of reference QSOs suitable for space-based astrometry, and what we ultimately hope to learn about halo shape and distribution.

galaxy cluster simulation

Measuring gravitational lenses

5/18/10
Peter Melchior
(Heidelberg University)

With current and upcoming lensing surveys, massive datasets are or will become available, which enable us to constrain the cosmological parameters governing the formation of gravitationally bound structures in the universe. I will discuss the principles employed for inferring the mass distribution of individual galaxy clusters and of the large-scale structure as a whole. I will also go through the problems we encounter, especially in estimating the lensing-induced distortions from background galaxies, and how we seek to overcome them with novel methods and dedicated simulations.

Bullet Cluster

Cosmological Constraints from the Growth of X-ray Luminous Galaxy Clusters

5/25/10
Adam Mantz
(NASA/GSFC)

Over the past few years, constraints on the growth of cosmic structure have become available from observations of the galaxy cluster population and its evolution. This advance is largely due to the painstaking identification of clusters at redshifts z>0.3 in the X-ray flux-limited ROSAT All-Sky Survey (with ongoing Sunyaev-Zel'dovich and optical surveys not far behind). I will present cosmological constraints obtained from a sample of 238 X-ray flux-selected clusters, which, including the recently released MACS sample, extend to redshift 0.5. The cluster data provide robust constraints on the amplitude of the matter power spectrum as well as the dark energy equation of state (+-0.2 for a constant w model). The ability to trace the growth of structure as a function of time also allows us to test the observed growth rate against that predicted by General Relativity, independent of the background expansion history. Ultimately, this provides a tool for testing alternative theories of gravity and potentially distinguishing them from dark energy models. Finally, I will present constraints on cluster mass-observable scaling relations, a necessary and parallel aspect of the cosmological tests, which has some interesting implications for future work.

Exploring the Ends of the Rainbow: Cosmic Rays in Star-Forming Galaxies

9/21/10
Brian Lacki
OSU/CCAPP
Download Seminar PDF

The cosmic rays (CRs) in star-forming galaxies dominate their emission at gamma-ray and radio wavelengths. The observed linear correlation between the nonthermal radio emission and the thermal infrared emission of galaxies, the far infrared (FIR)-radio correlation (FRC), links together the CR electron population, star-formation rate, and magnetic field strength of galaxies. Furthermore, gamma-ray data links the CR proton population, the star-formation rate, and gas density. We construct one-zone steady-state models of cosmic ray (CR) spectra in star-forming galaxies ranging from normal galaxies to the densest starbursts, calculating both the radio and gamma-ray emission. We then calculate the broadband emission of primary and secondary CR protons, electrons, and positrons. We find the FRC is caused by conspiracies of several factors for galaxies across the range of the correlation, including CR escape from galaxies, UV opacity, non-synchrotron cooling, and secondary electrons and positrons generated by CR protons. The conspiracies have great implications for the evolution of the FRC at high z, actually preserving it to higher redshift than previously thought but allowing variations in the FIR-radio ratio with different galaxy properties. I describe how the recent gamma-ray observations of M82 and NGC 253 compare with our models. These starbursts are somewhat less gamma-ray bright than we expect, but still indicate substantial pionic losses for CR protons and non-synchrotron cooling for CR electrons and positrons, supporting the conspiracy. Finally, I will describe our more recent work on the highest energy CR electrons in starbursts and the gamma-rays they produce. Starburst galaxies ought to be opaque to 30 TeV gamma-rays through pair production; in the strong magnetic fields of starbursts, these created electrons and positrons radiate synchrotron X-rays. We find that these synchrotron X-rays could make up ~10% of the diffuse hard X-ray emission from M82-like starbursts and even more in the brightest starbursts like Arp 220.

Electromagnetic Flares from the Tidal Disruption of Stars by Massive Black Holes

9/28/10
Linda Strubbe
(Berkeley)
Download Seminar PDF

A star that wanders too close to a massive black hole (BH) gets shredded by the BH's tidal gravity. Stellar gas soon falls back to the BH at a rate initially exceeding the Eddington rate, releasing a flare of energy as gas accretes. How often this process occurs is uncertain at present, as is the physics of super-Eddington accretion (which is relevant for BH growth and feedback at high redshift as well). Excitingly, transient surveys like the Palomar Transient Factory (PTF), Pan-STARRS and LSST should shed light on these questions soon -- in anticipation, we predict observational properties of tidal flares. Early on, much of the falling-back gas should blow away in a wind, producing luminous optical emission imprinted with blueshifted UV absorption lines, and the observational signatures can be qualitatively different for M_BH ~ 105 - 106 Msun relative to more massive BHs. Possible X-ray emission can complicate the spectroscopic predictions. I will describe predicted detection rates for PTF, Pan-STARRS and LSST, and discuss the substantial challenge of disentangling these events from supernovae. These surveys should significantly improve our knowledge of stellar dynamics in galactic nuclei, the physics of super-Eddington accretion, the demography of IMBHs, and the role of tidal disruption in the growth of massive BHs.

Multi-wavelength studies of Galactic satellites and implications for dark matter detection

10/5/10
Louie Strigari
(Stanford/KIPAC)
Download Seminar PDF

The census of local group dwarf galaxies has changed dramatically in recent years. By studying both their number counts and internal kinematics, faint Galactic satellites uniquely test the standard cosmological model and the properties of dark matter in a regime that is not probed by large scale observations such as the distribution of galaxy clusters and the cosmic microwave background. In this talk, I will discuss the confrontation of new data with theoretical predictions, highlighting a developling new twist on the lingering issue of the overproduction of Galactic satellites in the theory of cold dark matter. I will further discuss the importance of multi-wavelength probes of satellites, following a path of discovery in optical surveys, to targeted follow up spectroscopy of individual objects, and then to searches for particle dark matter annihilation using high energy gamma-rays and neutrinos. Following this trail I argue that Galactic satellites present the most robust constraints on the dark matter annihilation cross section. Given the current constraints, I will review the status of a search for optically dark satellites with the Fermi gamma-ray telescope.

The Coyote Universe and Beyond

10/12/10
Katrin Heitmann
(Los Alamos National Lab)
(Download Seminar Talk)

Cosmological evidence for dark energy and dark matter poses an exciting challenge to fundamental physics. Next-generation surveys will investigate new physics beyond the Standard Model by targeting the nonlinear regime of structure formation, observed using powerful probes such as weak gravitational lensing. In order to fully exploit the information available from these probes, accurate theoretical predictions are required. Currently such predictions can only be obtained from costly, precision numerical simulations. In this talk, I will introduce the "Coyote Universe" project, a combined computational and statistical program to obtain precision predictions for the nonlinear power spectrum of density fluctuations. Such a program is essential for the interpretation of ongoing and future weak-lensing measurements to investigate and understand the nature of dark energy. I will discuss planned extensions of the Coyote Universe to include more cosmological parameters and physics. This work will be carried out with a new simulation capability recently developed at Los Alamos and targeted at future hybrid computing architectures. I will give a brief overview of these new developments.

IceCube

Neutrino Oscillations and (dis)appearance prospects for IceCube-DeepCore

10/19/10
Jason Koskinen
(Penn State)
(Download Seminar PDF)

The recent commissioning of the full DeepCore sub-array, a low-energy extension of the IceCube neutrino observatory, offers exciting opportunities for neutrino oscillation physics in the multi-GeV energy region. The improved energy reach, use of the surrounding IceCube detector as an active veto and immense size of DeepCore will produce one of the largest neutrino datasets ever acquired, annually containing tens of thousands of atmospheric neutrinos after oscillating over a baseline of up to one earth diameter. I will cover some current non-DeepCore oscillation results as well as the prospects for a DeepCore muon neutrino disappearance and possibly a tau neutrino appearance measurement. Proposed future extensions to DeepCore designed to drive the energy reach down to ~1 GeV will conclude the talk.

Observational Signatures of Neutron Star Mergers

10/26/10
Brian Metzger
(Princeton U.)
(Download Seminar PDF)

A fraction of neutron star (NS) and black hole binaries are formed sufficiently compact that they in-spiral and merge due to the emission of gravitational waves within the lifetime of the Universe. Such compact object mergers are among the most promising sources for the direct detection of gravitational waves with ground-based interferometers such as LIGO and Virgo. Maximizing the science of such a detection will, however, require identifying a coincident electromagnetic (EM) counterpart. One possible source of EM emission is a gamma-ray burst (GRB), powered by the accretion of material that remains in a rotationally-supported torus around the central black hole. I will overview the observational and theoretical status of the connection between NS mergers and the "short duration" subclass of GRBs. Although new observations from NASA's Swift observatory have provided some evidence in favor of the merger model, the puzzling discovery has also been made that many short GRBs are followed by late-time X-ray flaring activity, which does not fit current theory and may require modifying or considering alternative progenitor models. Another source of EM emission from NS mergers is a supernova-like optical transient, powered by the radioactive decay of heavy elements synthesized in neutron-rich ejecta from the merger. I will present the first calculations of the radioactively-powered transients from mergers that include both realistic nuclear physics and radiative transport, and I will discuss the prospects for detecting and identifying such events with present and future telescopes.

balloon above earth

The Cosmic Radio Background: Recent Measurement and Implications

11/2/10
Jack Singal
(SLAC/Stanford)

Results from the ARCADE 2 experiment reveal for the first time an extragalactic radio background that is brighter than some had assumed, in excess of the integrated contribution of discrete radio sources that have been characterized by interferometric surveys to their current flux limits. The origin of the radio background has thus provoked some debate. Given what is known from an analysis of radio source count surveys and constraints from the other cosmological backgrounds, we have developed a model to explain the radio background which has interesting astrophysical implications. This talk will combine experiment and theory. In the first part of the talk I will present the measurements performed by the balloon-borne ARCADE 2 experiment. In the second part, I will discuss how existing constraints disfavor some explanations of the radio background that have been proposed, and present the case that the background is produced largely by the radio emission of ordinary star forming galaxies above redshift 1 characterized by an evolving radio to far-infrared correlation.

QUIET experiment for CMB polarization measurement

11/8/10
Akito Kusaka
(U. of Chicago)

Cosmic microwave background (CMB) polarization is the ultimate probe of primordial gravity waves in the early universe, via the B-mode (or parity odd) signal on degree angular scales. A detection of such a signal would rule out most non-inflationary models and represent indirect observation of a fundamentally new phenomenon near the grand unification energy scale. With its unique radiometer technology, QUIET is among the most competitive experiments aiming to detect such a signature in the CMB. QUIET started its observation with its 44GHz receiver in October 2008. After nine months of successful observation, we deployed the 95GHz receiver replacing the 44GHz one and the observation resumed in August 2009. In this talk, I will review its instrumentation, site, and observation strategy, as well as the current status of the analysis.

The Cosmic Diffuse Gamma-ray Background: a puzzle to unveil

11/16/10
Marco Ajello
(SLAC/Stanford)
(Download Seminar PDF)

The Extragalactic Gamma-ray background might encrypt in itself the signature of some of the most powerful and exotic phenomena in the Universe. Recently, Fermi-LAT measured its intensity with unprecedented accuracy. At the same time Fermi, with its unprecedented sensitivity, detected over a thousand point-like sources. Most of the extragalactic sources are blazars, but a growing fraction of the detected sources comprises also starburst/starforming galaxies as well as radio galaxies. In this talk I will review and address the current efforts to sort out the different components of the extragalactic gamma-ray background, focusing in particular on the blazar class and the starforming galaxies. I will also discuss future developments and the possibility to study the fluctuations of the gamma-ray sky to gain knowledge about the 'truly' diffuse component of the gamma-ray background. Finally I will also address the variability of the gamma-ray sky and what can be learned from its systematic study.

Resonant Stripping as the origin of dwarf spheroidal galaxies

11/23/10
Elena D'Onghia
(Havard-Smithsonian CfA)

The most dark matter dominated galaxies known are the dwarf spheroidals, but their origin is still uncertain. The recent discovery of ultra-faint dwarf spheroidals around the Milky Way further challenges our understanding of how low-luminosity galaxies originate and evolve because of their even more extreme paucity of gas and stars relative to their dark matter content. By employing numerical simulations I will show that interactions between dwarf disc galaxies can excite a gravitational resonance that immediately drives their evolution into spheroidals. This effect, which is purely gravitational in nature, applies to gas and stars and is distinct from other mechanisms which have been proposed up to now to explain the origin of dwarf spheroidals, such as merging, galaxy-galaxy harassment and more general heating processes, or tidal and ram pressure stripping. Using a new analytic formalism we developed based on the linear perturbation theory I will show the nature and the efficiency of the resonant process and its applicability to the formation of tails of stars and streams of gas.

The Fermi LAT as a cosmic-ray electron observer

11/30/10
Francesco Loparco
(U. of Bari, Italy)
(Download Seminar PDF)

Even though it was designed to be a high sensitivity gamma-ray observatory, the Large Area Telescope (LAT) onboard the Fermi satellite has proved to be also an excellent electron/positron detector. The data collected by the LAT during its first year of operation have been used to measure the cosmic-ray electron and positron (CRE) energy spectrum in the energy range from 7 GeV to 1 TeV and to search for possible anisotropies in their arrival directions. An overview on the data analysis will be given and the main results will be illustrated.

Ultra High Energy Cosmic Rays from Mildly Relativistic Supernovae

12/7/10
Sayan Chakraborti
(Tata Institute)

Understanding the origin of the highest energy cosmic rays, is a crucial step in using them as probes of new physics, at energies unattainable by terrestrial accelerators. However their sources remain an enigma nearly half a century after their discovery. They must be accelerated in the local universe, as otherwise background radiations would severely suppress the flux of protons and nuclei, at energies above the Greisen-Zatsepin-Kuzmin (GZK) limit. Nearby GRBs, Hypernovae, AGNs and their flares, have all been suggested and debated in the literature as possible sources. A local sub-population of type Ibc supernovae with mildly relativistic ejecta have been detected for some time as sub-energetic GRBs or X-Ray Flashes and more recently as radio afterglows without detected GRB counterparts, such as SN 2009bb. In this talk we shall discuss the measurement of the size-magnetic field evolution, baryon loading and energetics, of SN 2009bb using its radio spectra obtained with the VLA and GMRT. This will allow us to see where the engine-driven SNe lie in the Hillas diagram and whether they can explain the post-GZK UHECRs?

A Bayesian Analysis of a Milky Way Ultra-Faint Satellite

1/18/11
Greg Martinez
(UC Irvine)

With the advent of SDSS the number of known Milky Way satellites has more than doubled. There new members, such as Segue 1, are extremely optically faint. Accurate mass measurements require careful analysis of velocity data. Here I describe the analysis of the multi-epoch velocity measurements of Segue 1 to determine its intrinsic velocity dispersion. Our method includes a simultaneous Bayesian analysis of both membership probabilities and the contribution of binary orbital motion to the observed velocity dispersion. Our analysis strongly disfavors the possibility that segue 1 is a bound star cluster. The inferred dark matter density is one of the highest measured, making Segue 1 a prime source for indirect dark matter detection. I will discuss the possibility of indirect detection in the context of SUSY models.

A Search for Point Sources with the IceCube Neutrino Observatory

2/1/11
Jon Dumm
(UW Madison)
(Download Seminar PDF)

Construction of the IceCube Neutrino Observatory was only recently completed on Dec 18, 2010. IceCube is the first 1km3 detector of it's kind, monitoring 1 billion tons of ice. Deep under the South Pole, IceCube looks for rare high energy neutrino interactions (> ˜100 GeV). While the observatory was under construction for 5 years, data was being collected and analyzed continuously. Some of the science highlights so far include searches for astrophysical neutrinos, a measurement of the atmospheric neutrino spectrum above 1 TeV, observation of a cosmic ray anisotropy in the southern hemisphere, and indirect searches for dark matter. This talk will describe IceCube, the motivations for building such a detector, and highlight the effort to find point-like sources of astrophysical neutrinos.

detector schematic

Channeling and daily modulation in direct dark matter detectors

2/8/2011
Nassim Bozorgnia
UCLA

The channeling of the ion recoiling after a collision with a WIMP in direct dark matter detectors produces a larger signal than otherwise expected. Channeling is a directional effect which depends on the velocity distribution of WIMPs in the dark halo of our galaxy, and could lead to a daily modulation of the signal. I will discuss channeling and blocking effects using analytic models produced in the 1960's and 70's, and present estimates of the expected amplitude of daily modulation in the data already collected by the DAMA experiment.

Dark matter annihilation and spherical harmonics of Fermi gamma-rays

2/15/11
Dmitry Malyshev
(NYU)

Gamma-ray production by dark matter annihilation is one of the most universal indirect dark matter signals. In order to avoid intensive astrophysical background, one can study the gamma-rays away from the Galactic plane. The problems is that the dark matter annihilation signal at high latitudes is smooth and most probably subdominant to Galactic and extragalactic fluxes. I will discuss the use of spherical harmonics decomposition as a tool to distinguish a large scale small amplitude dark matter signal from astrophysical fluxes. The sensitivity of this method for currently available Fermi data is similar to the signal from thermal WIMP dark matter annihilation into, e.g., W+W-

Vagins and Beacom

GADZOOKS! How to See Extragalactic Neutrinos By 2016

3/8/11
Mark Vagins
(IPMU/UCI)
(Download Seminar PDF)

Water Cherenkov detectors have been used for many years to study neutrino interactions and search for nucleon decays. Super-Kamiokande, at 50 kilotons the largest such underground detector in the world, has enjoyed over ten years of interesting and important physics results. Looking to the future, for the last eight years R&D on a potential upgrade to the detector has been underway. Enriching Super-K with 100,000 kilograms of a water-soluble gadolinium compound - thereby enabling it to detect thermal neutrons and dramatically improving its performance as a detector for supernova neutrinos, reactor neutrinos, atmospheric neutrinos, and also as a target for the new T2K long-baseline neutrino experiment - will be discussed.

Generative modeling for the Milky Way and the Universe

3/14/11
Jo Bovy
(New York University)

At the interface between observational and theoretical astrophysics lies data analysis and inference. The most accurate and precise inferences require using a model that generates the data and that takes the noise into account. I give two examples where generative modeling performs better than other methods for parameter inference and classification. To put the Milky Way in a cosmological context we want to know its mass and dark matter distribution in detail. I will discuss in general how we can infer the gravitational potential—dynamics—from kinematics alone. As an application of this, I show how we can determine the Milky Way's circular velocity at the Sun from maser kinematics. As a second example, I discuss density-estimation-based classification for target selection. SDSS-III's BOSS aims to observe 150,000 quasars down to the faint limit of the SDSS in a redshift range (2.2 <= z <= 3.5) where the quasar and stellar color loci overlap significantly. I will show how we can determine models of the underlying distribution of quasars and stars in flux space. We can use these models to evaluate quasar probabilities for all potential targets and build an efficient survey.

Energy-Dependent Composition of UHECRs and the Future of Charged Particle Astronomy

3/29/11
Antoine Calvez
(UCLA)
(Download Seminar PDF)

Recent results from the Pierre Auger Observatory show an energy dependent chemical composition of ultrahigh-energy cosmic rays (UHECRs), with a growing fraction of heavy elements at high energies. These results suggest a possible non-negligible contribution from galactic sources. We show that in the case of UHECRs produced by gamma-ray bursts (GRBs), or by rare types of supernova explosions that took place in the Milky Way in the past; the change in the composition of the UHECRs can be the result of the difference in diffusion times between different species. The anisotropy in the direction of the Galactic Center is expected to be a few per cent on average, and the locations of the most recent/closest bursts can be associated with observed clustering of UHECRs.

Light WIMPs!

4/12/11
Dan Hooper
(U. of Chicago)
(Download Seminar PDF)

Observations from the direct detection experiments DAMA/LIBRA and CoGeNT, along with those from the Gamma Ray Space Telescope, have been interpreted as possible evidence of dark matter in the form of relatively light (5-10 GeV) WIMPs. I will discuss the implications of these observations for dark matter phenomenology and discuss how it will be possible with future measurements to either confirm or refute this interpretation. I will also discuss how recent results from the Tevatron could impact efforts to build models including a light WIMP.

Optimal Linear Image Combination

4/19/11
Barnaby Rowe
(Jet Propulsion Laboratory/Caltech)

I will describe a simple, yet general, formalism for the optimized linear combination of astrophysical images, developed here at JPL/Caltech with Christopher Hirata and Jason Rhodes. The formalism allows the user to combine multiple undersampled images to provide oversampled output at high precision. The proposed method is general and may be used for any configuration of input pixels and point spread function; it also provides the noise covariance in the output image along with a powerful metric for describing undesired distortion to the image convolution kernel. The method explicitly provides knowledge and control of the inevitable compromise between noise and fidelity in the output image.

We also present a first prototype implementation of the method then put it to practical use in reconstructing fully-sampled output images using simulated, undersampled input exposures that are designed to mimic the proposed dark energy mission WFIRST. Comparing results for different dither strategies we illustrate the use of the method as a survey design tool. Finally, we use the method to test the robustness of linear image combination when subject to practical realities such as bad pixels and focal plane plate scale variations, an important consideration for a mission such as WFIRST.

A Quest for Sources of Ultrahigh Energy Cosmic Rays

4/26/11
Kumiko Kotera
(U. of Chicago)
(Download Seminar PDF)

The origin of ultrahigh energy cosmic rays (UHECRs) has not been unveiled in spite of decades of experimental and theoretical research. In this talk, I discuss the observable signatures that would constrain the possible sources to one single suspect.
In particular, I will present the anisotropy signatures expected for various types of sources, and describe how the intergalactic magnetic field plays a prominent role in this picture. For this purpose, I will introduce an analytical formalism to study the propagation of UHECRs in the magnetized Universe.
Another constraint on the sources might come from multi-messenger signatures (in gamma-rays, neutrinos and gravitational waves) that can be produced together with UHECRs. I will present the expected fluxes for various astrophysical scenarios and discuss to which extent these signals could pin-point the actual sources of UHECRs.
In light of this discussion, I will briefly present the latest results of the Pierre Auger Observatory and give requirements for future detectors in UHECRs, neutrinos, gamma rays and gravitational waves, to solve this long-standing enigma.

21cm Slices

Constraining the Dawn of Cosmic Structure and the Epoch of Reionization with the 21cm Line

5/3/11
Jonathan Pritchard
Harvard/CfA

The first billion years of the Universe contains the formation of the first galaxies and reionization. This period lies beyond the current observational frontier presenting challenges to theory and observation. Low frequency observations of the redshifted 21 cm line of neutral hydrogen will be key in developing our understanding of this period. In this talk, I will describe two aspects of the 21 cm signal from the period of "cosmic dawn": the global 21 cm signal and 21 cm fluctuations. I will discuss what can be learnt about the first galaxies and reionization from this technique and explore some of the challenges and opportunities ahead for the first observations.

Indirect Detection of Dark Matter -
Electroweak Bremsstralung and Other Stories

5/13/11
Nicole Bell
(University of Melbourne)
(Download Seminar PDF)

Annihilation of dark matter to fermionic final states is often either helicity or velocity suppressed. We outline the circumstances under which bremsstrahlung processes can remove such suppressions, thereby dramatically improving prospects for indirect detection. In these cases, the three body final states such as e+e-gamma, e+e-Z and e\nuW dominate over the 2-body annihilation modes. Since the W and Z gauge boson have large hadronic decay modes, purely leptonic annihilation is impossible if the 3-body bremsstrahlung processes dominate. We also discuss dark matter annihilation via metastable mediators, and show that this can lead to greatly enhanced high energy neutrino signals from the Sun.

miguel

The High Altitude Water Cherenkov Gamma-ray Observatory

5/24/11
Miguel Mostafa
(Colorado State U.)

The High Altitude Water Cherenkov (HAWC) experiment is a large field of view, continuously operated TeV gamma-ray observatory to be constructed using a dense array of water Cherenkov detectors covering an area greater than 25,000 m2. HAWC will be located at an elevation of 4,100 m near the Sierra Negra mountain in Mexico. The instrument will use 900 photomultiplier tubes to observe the relativistic particles and secondary gamma rays in extensive air showers. This technique has been used successfully by the Milagro observatory to detect known (as well as new!) TeV sources. HAWC is a natural extension of Milagro, which has demonstrated the ability to detect {at TeV energies{ many of the galactic sources which have been observed by the Fermi LAT in the GeV energy range. The design of HAWC was optimized using the lessons learned from Milagro, and will be 15 times more sensitive than Milagro when completed. Improvements in sensitivity, angular resolution, and background rejection will allow HAWC to measure or constrain the TeV spectra of most of the Fermi discovered GeV sources. In addition, above 100 GeV, HAWC will be more sensitive than the Fermi satellite and be the only ground-based instrument capable of detecting prompt emission from gamma-ray bursts in this energy regime. In this seminar I will present the physics motivation, the HAWC observatory, and the activities of my group.

Supernova Feedback Keeps Galaxies Simple

9/26/11
Sayan Chakraborti
(TIFR, India)

Galaxies are complicated and history dependent. Yet, recent studies have uncovered surprising correlations among the properties of galaxies. Such simplicity seems, naively, to be at odds with the paradigm of hierarchical galaxy mergers. One of the puzzling results, is the simple linear correlation between the neutral hydrogen mass and the surface area, implying that widely different galaxies share very similar neutral hydrogen surface densities. We shall see in this presentation that self-regulated star formation, driven by the competition between gravitational instabilities and mechanical feedback from supernovae, can explain the nearly constant neutral hydrogen surface density across galaxies.

Shape Image

Weak Lensing Simulations and Precision Cosmology with Large-area Sky Surveys

9/27/11
Matt Becker
(KICP/U. of Chicago)

Weak lensing measurements are an essential part of near- and long-term large-area sky surveys aimed at an array of scientific goals, like understanding Dark Energy, elucidating further the connection between galaxies and dark matter halos, constraining modifications to General Relativity, etc. The weak lensing community has undertaken extensive simulation efforts, both CCD image simulations and computations of the cosmological weak lensing signals from large-scale structure simulations, in order to address the variety of systematic errors which can adversely effect these measurements and their interpretation. The next logical step in this effort is the construction of mock galaxy catalogs with weak lensing shear signals self-consistently from large-scale structure simulations. While these weak lensing mock galaxy catalogs have easily been made for small patches of sky (~10 square degrees), upcoming large-area sky surveys will image thousands of square degrees or more. I will describe a new multiple-plane ray tracing code which is able to produce full-sky weak lensing deflection, convergence, and shear fields suitable for the construction of weak lensing mock galaxy catalogs for large-area sky surveys. I will also highlight the application of this code to the Dark Energy Survey simulation effort. Finally, I will present a prototypical example of these simulation efforts, my recent work on interpreting weak lensing galaxy cluster mass measurements, emphasizing understanding their scatter and more importantly their potential biases. This work, and ongoing work by others in the Dark Energy Survey collaboration based on these new weak lensing mock galaxy catalogs, illustrates the utility these simulations in understanding systematic errors in current and future weak lensing measurements from large-area sky surveys.

radio halos and relics explained as arising from one homogeneous population of cosmic-ray protons

The Intracluster Medium of Galaxy Clusters

10/11/11
Uri Keshet
(CfA, Harvard)

Recent observations of galaxy clusters reveal new insights into the dynamical and nonthermal processes in the intracluster medium (ICM). Tangential discontinuities are directly seen in high resolution X-ray maps of cool cluster cores, in the form of cold fronts. They reveal bulk shear flows which magnetize the plasma, give rise to radio minihalos, and may play a key role in solving the cooling problem. The ICM shows a rich phenomenology of non-thermal radio emission, arguably arising from hadronic cascades involving cosmic-ray protons. While such a secondary signal is too weak to be observed by Fermi, the primary gamma-ray signal from strong virial shocks may be identifiable.

Dark Matter Parameters from Neutrino Telescopes

10/18/11
Katie Richardson
(U. of New Mexico)
(Download Seminar PDF)

In this talk, I will discuss how neutrino telescopes may help us extract dark matter parameters and can in fact place the most stringent bounds on the spin-dependent dark matter-nucleon scattering cross-section. In particular the dark matter annihilation final state provides a distinctive signature that allows us to discriminate among classes of dark matter models. Models with gauge boson or tau final states alongside neutrino final states are distinguishable, and the theoretically well-motivated U(1)_B-L extension of the MSSM produces just such a mixture of final states. It is feasible that the energy reconstruction capability of the IceCube neutrino telescope will preserve the important features. Finally, I will address the prospect for differentiating neutrino flavor final states from one another.

First Cosmic Shear Measurement in SDSS

10/25/11
Eric Huff
(Berkeley)
(Download Seminar PDF)

I discuss preliminary results from a first cosmic shear measurement in SDSS. We have coadded 250 square degrees of multi-epoch SDSS imaging along the celestial equator, optimizing for weak lensing measurement. We employ standard techniques for shape measurement, shear calibration, and inference of the redshift distribution, and perform a wide array of tests that show that the systematic errors for this measurement are probably negligible compared to the statistical errors. We analyze the shear autocorrelation with and without WMAP7 priors, and produce competitive constraints on the matter density and the amplitude of the matter power spectrum at redshift z=0.6.

I will also discuss some new results on lensing magnification. Motivated by the need for greater signal-to-noise in weak lensing measurements, we have used tight photometric galaxy scaling relations to measure a galaxy-galaxy magnification signal with many times the signal-to-noise of previous magnification results. I describe how minor improvements on this work may permit magnification measurements with signal comparable to shear.

Baryon Acoustic Oscillations:
Galaxy Bias Effect and Cosmological Measurements

11/1/11
Kushal Mehta
(Arizona)

I will talk about the work presented in Mehta et al (2011) regarding measuring the effects of galaxy bias on baryon acoustic oscillations (BAO) measurements in cosmological N-body simulations, and the technique of reconstruction used to refine the BAO signal. I will also talk about new SDSS-II LRG (Luminous Red Galaxies) BAO data and the measurements of cosmological parameters. These results will be presented in 3 papers (Padmanabhan et al, Xu et al, and Mehta et al, all in prep).

Propagation of Ultrahigh Energy Nuclei in the Galactic magnetic field

11/2/11
Gwenael Giacinti
(APC/Paris)

The composition of ultra-high energy cosmic rays (UHECR) at the highest energies is a matter of debate. The measurements from the Auger Observatory would suggest a shift towards heavier nuclei, whereas Telescope Array results can still be compatible with a proton composition. We present simulations for the propagation of ultra-high energy heavy nuclei, with E > 6x10^(19) eV, within recent Galactic Magnetic Field (GMF) models. Differences between the propagation of protons and heavy nuclei in the GMF may provide additional information about the charge composition of UHECRs.
For UHE heavy nuclei primaries, there is no one-to-one correspondence between their arrival directions at Earth and the directions of their extragalactic sources. We show the challenges, and possibilities, of "UHECR astronomy" with heavy nuclei. Finally, we present a quantitative study of the impact of the GMF on the (de-)magnification of source fluxes, due to magnetic lensing effects. For 60 EeV iron nuclei, sources located in up to about one fifth of the sky would have their fluxes so strongly demagnified that they would not be detectable at Earth, even by the next generation of UHECR experiments.

Exploring the Dark Universe with Gravitational Lensing

11/15/11
Sherry Suyu
(U. or California, Santa Barbara)
(Download Seminar PDF)

Understanding the nature of dark energy and dark matter is one of the biggest challenges in modern cosmology. Strong gravitational lens systems provide a powerful tool for measuring cosmological parameters and for probing dark matter in galaxies. In the first part of my talk, I will show how strong lens systems with measured time delays between the multiple images can be used to determine the "time-delay distance" to the lens. I will present the cosmological constraints, particularly on the Hubble constant and the dark energy equation of state, from a detailed analysis of the gravitational lens B1608+656, and discuss future prospects of time-delay lens cosmography. In the second part of my talk, I will present a joint lensing and kinematics analysis of the spiral gravitational lens B1933+503 at z=0.76 to disentangle the baryons and dark matter in the spiral galaxy and probe the stellar initial mass function.

Understanding Star-forming Galaxies across Cosmic Time

11/21/11
Matt Bothwell
(U. of Cambridge, UK)

The formation of stars from the interstellar medium is one of the primary drivers of galaxy evolution, and obtaining a full characterization of the processes involved is essential if we are to understand the physics behind the formation of galaxies. Viewing galaxies at high redshift gives us a direct window into the various formation processes, but the importance of a comprehensive understanding of the z~0 Universe cannot be overemphasized, as the early stages of galaxy evolution leave telltale footprints in the properties of local galaxies. I present work examining the star formation laws in galaxies at both low and high redshift. Firstly, I discuss the distribution function of star formation in the local Universe, calculated in a manner analogous to the luminosity function, and its implications for galaxy formation scenarios.

Looking to high redshift, I present molecular gas observations of a sample of z~2 ultra-luminous infrared galaxies (ULIRGs). These observations provide the best view of the star formation and kinematic properties of these enigmatic systems, allowing us to place them into the context of galaxy formation models.

Core Collapse Supernovae: Black Holes and Neutrinos

11/29/11
Evan O'Connor
(Caltech)

Core-collapse supernovae are some of the most explosive high-energy astrophysical events in our universe. They are the result of the collapse of the iron core in an evolved massive star (M > 8-10 solar masses). The collapse is halted when the collapsing core reaches nuclear densities, at which point the core-collapse supernova central engine takes over. We know that the central engine must eventually drive an explosion in some fraction of massive stars, however, after over 40 years of theoretical research we still do not completely understand this core-collapse supernova mechanism. In this talk, I will review the state of core-collapse supernova theory. I will also discuss our work at Caltech on both the success and failure of the core-collapse supernova mechanism. For looking at the success, we considered the possibility that collective neutrino oscillations may enhance the neutrino mechanism. If a core-collapse supernova fails, a black hole is the result. I will discuss our predictions for black hole populations from failed supernova.

Beyond the Standard Model of Cosmology:
Dark Energy, Neutrinos, and Primordial Non-Gaussianity

12/6/11
Shahab Joudaki
(UCI)
(Download Seminar PDF)

Some of the most outstanding problems of physics lie in the understanding of the dark sector of the universe, in particular dark energy, neutrinos, and inflation.
The dark energy and neutrinos are correlated through their effects on distances and the clustering of matter. I will review the present state of surveys sensitive to the effects of dark energy and neutrino mass. I will then forecast how well the present dark energy density and its equation of state along with the sum of neutrino masses may be constrained using multiple probes that are sensitive to the growth of structure and expansion history, in the form of weak lensing tomography, galaxy tomography, supernovae, and the cosmic microwave background. I will include all cross-correlations between these different probes and allow for non-negligible dark energy at early times (motivated by the coincidence problem) in spatially flat and non-flat cosmological models. In the latter portion of the talk, I will discuss a novel method to constrain non-Gaussianity of the primordial density perturbations by its impact on the ionization power spectrum from 21 cm emission during the epoch of reionization. I will show that 21 cm experiments in the near future may constrain inflationary models via primordial non-Gaussianity to the same precision as expected from Planck.

The LHCf experiment: Verification of high energy cosmic ray interactions

1/10/12
Yoshitaka Itow
(Nagoya University)

Recent progress in air shower observations of the highest energy cosmic rays with $\sim 10^{20}$ eV gives us an enigmatic problem about their origins and propagations. One difficulty is implication of air shower observations due to uncertainty of hadron interactions in such high energy. The particle production at the very forward region plays an important role in air shower development, since it carries most of collision energy. The LHCf experiment is dedicated to measure spectra of neutral particles at very forward region of the LHC collision point in order to verify interaction of cosmic rays of 10^{17} eV. The data taking had been carried out for $\sqrt{s}$=0.9TeV and 7TeV. The results of "inclusive" gamma-ray energy spectra at 0 degree has been obtained. Future plan for very forward measurement at p-A or A-A collisions is also discussed.

Department of Physics | Department of Astronomy | Postdoctoral Fellowships | Contact Us
191 West Woodruff Avenue, Columbus, OH 43210 Fax: (614) 292-7741