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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)
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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)
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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)
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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)
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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)
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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)
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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)
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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)
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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)
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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)
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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.)
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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)
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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)
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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)
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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)
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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)
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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)
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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)
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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
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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)
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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)
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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)
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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)
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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)
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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)
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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
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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)
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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
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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
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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
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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
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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
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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
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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.
The Ohio State University |
Department of Physics |
Department of Astronomy |
Astrophysics and Cosmology
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