CCAPP Seminars

Hidden Sectors and Dark Forces

1/12/16
Jesse Thaler
(MIT)

With the overwhelming gravitational evidence for dark matter, the hunt is on for nongravitational interactions of dark matter with the standard model. In this talk, I consider the possibility that dark matter is part of a larger hidden sector, which interacts feebly with the standard model via dark forces. The hidden sector paradigm introduces a wealth of new search strategies, and I discuss the discovery prospects for dark forces at experiments ranging from Super-K to LHCb.

Studying the Galaxies inside Clusters with the Dark Energy Survey

1/19/16
Yuanyuan Zhang
(Michigan)

Being the largest virialized cosmic structures, galaxy clusters are important subjects of study for cosmology and also astrophysics research. The enormous amount of dark and baryonic matter inside clusters provides rich tracers of the astrophysical evolution of galaxies and hot intracluster plasma. Wide-field sky survey programs have been steadily pushing the precision limit of research into these rare objects in the universe. Ongoing optical surveys like the Dark Energy Survey (DES) are observing tens of thousands of clusters to redshift 1.0 and beyond, and cosmological studies demand a more refined understanding of cluster observable properties. In this talk, I will demonstrate the power and potential of DES to improve our understanding of cluster astrophysics. I will present new results characterizing the evolution history of cluster central galaxies and cluster red sequence galaxies with DES early data, and discuss prospects for future cluster studies.

Peculiar Transients as Probes of Stellar Evolution and Mass-Loss

1/26/16
Maria Drout
(Harvard)

Multi-wavelength observations of supernovae not only probe the explosion mechanism, but also carry information about the configuration of the star at the moment of collapse and the mass-loss history of the progenitor system in the years immediately preceding its death. The study of supernovae therefore offers us one of our only observational views of the final stages of stellar evolution. As a result, the discovery by wide-field dedicated surveys of new classes of astronomical transients at an ever-increasing rate has both expanded the types of stellar systems that we can directly probe and challenged some of our existing views of how these uncertain final stages proceed. In this talk I will discuss several types of new and peculiar astronomical transients and what their properties and intrinsic rates are teaching us about stellar evolution and stellar death.

Galaxy merger simulation

The Dynamic Lives of Supermassive Black Holes in Merging Galaxies

2/2/16
Laura Blecha
(U. of Maryland)

An evolutionary link between supermassive black holes (BHs) and their host galaxies is well established, but the physical processes driving this co-evolution are still uncertain. In particular, the importance of galaxy mergers for BH fueling and feedback is a matter of active debate. I will review the rapid recent progress in identifying active BH pairs (or "dual BHs") in merging galaxies, and I'll describe how combined observations and numerical modeling of these systems can constrain the connection between mergers and BH growth. In addition, the BH pairs themselves can eventually merge, producing powerful gravitational waves. Asymmetry in this gravitational wave emission can eject the merged BH from its host nucleus, leaving the galaxy without a central BH and producing an offset ("recoiling") quasar. I will describe the handful of candidate recoiling BHs discovered to date, as well as recent theoretical results that indicate promising avenues for identifying a population of recoils in wide-field surveys.

Supernova remnants interacting with molecular clouds

2/9/16
Katie Auchettl
(OSU CCAPP)

Supernova remnants (SNRs) are the long lived structures that result from the explosive end of a massive star and they play an important role in the dynamics of the interstellar medium. The shock-front produced by the supernova explosion heats and mixes metal-rich stellar ejecta and swept-up ISM to X-ray emitting temperatures, and are sites in which populations of relativistic particles can be efficiently accelerated to the knee of the Cosmic-ray spectrum. As massive stars tend not travel far from their original birth site, SNRs are usually born in the same dense environment in which their progenitor was born. The interaction between the SNR with this dense molecular material has a profound effect on the morphology and emission properties of these objects. In this talk, I will review the importance of studying these SNRs and their properties. In particular, I will highlight investigations into the high energy emission of these remnants using X-ray and gamma-ray satellites which give an insight into the original progenitor, the properties of the surrounding environment and their abilities to accelerate particles.

A review of uhe neutrino detection using the Askaryan effect

2/9/16
Jordan Hanson
(OSU CCAPP)

Interaction of the highest energy cosmic rays with the cosmic microwave background would produce neutrinos with energies of ~1 EeV. The spectrum of these cosmogenic neutrinos is now being constrained, and a generation of experiments based on the Askaryan effect are underway. We review the creation of high-energy cascades created in dielectric materials by electroweak interactions, and discuss how the Askaryan effect in this situation leads to a radio-frequency electromagnetic pulse. Further, we have studied two corrections to the basic approach: the Landau-Pomeranchuk-Migdal (LPM) effect, and the shower form factor. Both effects modify the electromagnetic pulse, and we present an open-source code that attempts to include these effects. A future direction for this work includes using the form factor technique to model the radio emission from extensive air-showers.

Results from the Search for eV-Sterile Neutrinos with IceCube-86

2/16/16
Carlos Arguelles
(MIT)

The IceCube neutrino telescope at the South Pole has measured the atmospheric muon neutrino spectrum as a function of zenith angle and energy. Using IceCube's full detector configuration we have performed asearch for eV-scale sterile neutrinos. Such a sterile neutrino, motivated by the anomalies in short-baseline experiments, is expected to have a significant effect on the muon-antineutrino survival probability due to matter induced resonant effects for energies of order 1 TeV. This effect makes this search unique and sensitive to small sterile mixings. In this talk, I will present the results of the IceCube sterile neutrino search.

Secluded Neutrinos: From the Early Universe to IceCube

2/23/16
Ian Shoemaker
(Penn State)

Sterile neutrinos are predicted in many theories beyond the Standard Model and may be hinted at in short-baseline data. However cosmological data seems to rule out these neutrinos. Intriguingly, this tension is ameliorated when these new neutrinos are self-interacting. I will explore the impact of this self-interaction on their evolution in the early universe and on the spectrum and flavor of IceCube's ultrahigh energy neutrinos.

Low Mass Galaxies and their Gas at the Peak Epoch of Star Formation

3/1/16
Dawn Erb
(Haverford College)

Because faint, low mass galaxies are numerous at high redshifts, their impact on the Universe is expected to be significant. They may host a substantial fraction of the Universe's star formation, provide many of the energetic photons needed to reionize the hydrogen gas surrounding galaxies, and affect their surroundings via powerful, starburst-driven galactic outflows. Because of their faintness, however, the properties of these galaxies are difficult to determine. I will discuss a variety of observations aimed at characterizing the physical conditions in low mass, low metallicity galaxies during the peak epoch of star formation, when the Universe was ~20% of its current age, with particular emphasis on the study of galactic outflows in faint galaxies.

Fundamental Physics with the Smallest Galaxies

3/8/16
Alex Drlica-Wagner
(Fermilab)

The population of Milky Way satellite galaxies includes the least luminous, least chemically evolved, and most dark matter dominated galaxies in the known universe. Due to their proximity, high dark matter content, and low astrophysical backgrounds, dwarf spheroidal galaxies are unique probes of cosmology and promising targets for indirect searches for dark matter. Prior to 2015, roughly two dozen dwarf spheroidal galaxies were known to surround the Milky Way. Since the beginning of last year, new optical imaging surveys have discovered over twenty new dwarf galaxy candidates, potentially doubling the population of Milky Way satellite galaxies in a single year. I will discuss recent optical searches for dwarf galaxies, focusing specifically on results from the Dark Energy Survey (DES) and the implications for gamma-ray searches for dark matter annihilation with the Fermi Large Area Telescope.

Anisotropies as a probe the Diffuse Gamma-Ray Background

3/15/16
Mattia Fornasa
(GRAPPA/Amsterdam)

The Diffuse Gamma-Ray Background (DGRB) collects the radiation produced by all those sources that are not bright enough to be resolved individually. Therefore, it represents an essential tool to study faint gamma-ray emitters, like star-forming or radio galaxies and the exotic Dark Matter. The anisotropy pattern of the DGRB is extremely informative: I will review the recent measurement of the anisotropy angular power spectrum performed by the Fermi LAT Collaboration with almost 80 months of data. This brand-new result can be used to infer the composition of the DGRB. In particular, I will show how it constrains the emission expected from Dark Matter.

Growth of cosmic structure - the next frontier

3/22/16
Dragan Huterer
(Michigan)

The physical mechanism behind the acceleration of the universe remains one of the great mysteries of modern cosmology, with little progress to date in understanding dark energy. In the near future, we need new kinds of tests in addition to better data. One very effective way to test the consistency of the current LCDM paradigm is to isolate and separately constrain the growth of structure in cosmological measurements and compare to constraints from the purely geometrical measures. I will review such recent work applied to current data. I will also review efforts to use the galaxy maps in order to reconstruct the late-time Integrated Sachs-Wolfe contribution to the CMB anisotropy maps. Key to the success of these efforts and other tests with large-scale structure is exquisite control of the photometric calibration errors, and I will describe a general formalism to account for these pervasive systematics.

New Constraints on Cosmic Reionization from Planck and Hubble Space Telescope

3/29/16
Brant Robertson
(UCSC)

Understanding cosmic reionization requires the identification and characterization of early sources of hydrogen-ionizing photons. The 2012 Hubble Ultra Deep Field (UDF12) campaign acquired the deepest blank-field infrared images with the Wide Field Camera 3 aboard Hubble Space Telescope and, for the first time, systematically explored the galaxy population deep into the era when cosmic microwave background (CMB) data indicate reionization was underway. High-redshift observations with HST including UDF12, CANDELS, and the Frontier Fields provide the best constraints to date on the abundance, luminosity distribution, and spectral properties of early star-forming galaxies. We synthesize results from these HST campaigns and the most recent constraints from Planck CMB observations to infer redshift-dependent ultraviolet luminosity densities, reionization histories, and the electron scattering optical depth evolution consistent with the available data. We review these results, and discuss future avenues for progress in understanding the epoch of reionization.

Joining Forces Against the Dark Universe:
From the Cosmic Microwave Background to Large Scale Structure

4/5/16
Shirley Ho
(Carnegie Mellon)

Despite tremendous recent progress, gaps remain in our understanding of the Universe. We have not yet pinned down the properties of dark energy, nor have we confirmed Einstein's theory of Gravity at the largest scales. Current and upcoming large sky surveys of the Cosmic Microwave Background (CMB), Large Scale Structure (LSS) in galaxies, quasars and the Lyman-alpha forest present us with the best opportunity to understand properties of the Universe.
I will first review recent cosmology results from the CMB and LSS, concentrating on BOSS results using Baryon Acoustic Oscillations and Redshift Space Distortions. I will then introduce novel cosmological probes which combine CMB with LSS directly. These novel probes will open new windows into the momentum field of the Universe and Gravity at the largest scales. I will finally put these in context with the upcoming surveys such as Dark Energy Spectroscopic Instrument (DESI), Large Synoptic Survey Telescope (LSST), Wide Field Infrared Survey Telescope (WFIRST) and CMB S4.

Weighing the Giants: Cluster Masses and Cosmology

4/12/16
Anja von der Linden
(Stony Brook)

Surveys of galaxy clusters provide a sensitive probe of cosmology by measuring the evolution of the halo mass function. However, already current cluster surveys are systematically limited by uncertainties in the relation between cluster mass and observables (e.g. X-ray luminosity, cluster richness). Cluster weak lensing is the most promising observational method to calibrate the mass scaling to the required precision, but requires the control of systematic errors to a few percent each. In the "Weighing the Giants" project, we carefully investigated and quantified all sources of systematic uncertainty, resulting in accurate weak lensing masses for 51 clusters. We use these measurements to improve the precision of cosmological constraints from X-ray selected clusters by a factor of two. Already from a sample of ~200 clusters selected from the ROSAT All-Sky Survey, we place some of the tightest, most robust constraints on a number of cosmological parameters, including the dark energy equation of state, neutrino masses, and modified gravity. Furthermore, we show that when adopting the "Weighing the Giants" mass scale, the results from Planck CMB temperature anisotropies and Planck cluster counts are consistent without invoking the need for new physics. These results bode extremely well for future cluster surveys. In particular, I will show how the "Weighing the Giants" work lays out the path for LSST to become a key cornerstone for cluster experiments in the next decade.

The Universe According to Planck

4/19/16
Douglas Scott
(UBC)

The Planck satellite has completed its mission to map the entire microwave sky at nine separate frequencies. A new data release was made in February 2015, based on the full mission, and including some polarization data for the first time. The team is now working towards the final (2016) data release. More than 100 papers have already been produced, covering many different aspects of the sky at these wavelengths. We have learned in detail about the physics of the interstellar medium in our Galaxy, and to remove this foreground emission in order to extract the cosmological information from the cosmic microwave background (CMB). Planck's measurements lead to an improved determination of the basic model that describes the Universe on the very largest scales. In particular, a 6-parameter model fits the CMB data very well, with no strong evidence for extensions to that scenario. There are constraints on inflationary models, neutrino physics, dark energy and many other theoretical ideas. New cosmological probes include CMB lensing, CMB-extracted clusters of galaxies, the Cosmic Infrared Background and constraints on large-scale velocities. This talk will highlight some of the newest results, including the improvements coming from the addition of the polarization dimension.

Fermi's role in the era of multi-messenger astronomy

4/26/16
Valerie Connaughton
(USRA)

With the uncovering of a mysterious backdrop of astrophysical neutrinos by IceCube and the detection of gravitational wave (GW) radiation by LIGO, the era of multi-messenger astronomy is rich in discovery space. I will discuss the role of the Fermi Gamma-ray Space Telescope in this vibrant field, concentrating on the contributions of the Gamma-ray Burst Monitor (GBM) to the detection of electromagnetic counterparts to gravitational waves. The recent discovery of a weak signal in the GBM data, close in time to a GW produced during the merger of two stellar-mass black holes, and consistent in arrival direction with the GW event, was both exciting and unexpected. Future joint observations by LIGO/Virgo and high-energy astrophysical satellites will be needed to establish a firm connection between electromagnetic radiation and black-hole mergers. Over the next few years, LIGO/Virgo will become sensitive enough to detect signals from the mergers of binary systems involving a neutron star. The role of these mergers as the progenitors of short Gamma-Ray Bursts will be confirmed or refuted.

A Case for the 3.55 keV Line:
Claims, Counterclaims, Reasons, and Evidence

5/3/16
Esra Bulbul
(MIT)

X-ray observations of dark matter dominated objects have the potential to reveal a signal from decaying or annihilating dark matter. We previously reported the detection of an unidentified emission line at 3.55 keV in the stacked XMM-Newton observations of galaxy clusters. The origin of this unidentified line could be attributed to decay of dark matter particles. I will provide a comprehensive review on the detections and non-detections of the 3.55 keV line in dark matter dominated objects in the literature.

Light from Dark Matter

5/6/16
Juri Smirnov
(Max-Planck)

In this talk I will discuss photon spectra from annihilating Dark Matter. In particular the search for monochromatic lines is of great interest, as from the particle physics perspective it allows to determine the DM mass. And from the astrophysical perspective it is unlikely to be mimicked by a compact source. I will address the general question, under which circumstances in a given model a line is in principle observable given a finite instrument resolution. Two mechanisms which allow to see such gamma line features will be presented and the corresponding model realisations discussed. I will discuss how line searches open the window of possibility to scrutinise possible observed continuous gamma ray excesses. As concrete examples the Galactic Center excess and the Reticulum II excess will be considered.

Revealing the Progenitors of Explosive Transients with Spectroscopic Surveys

5/10/16
Or Graur
(NYU/Harvard)

We still do not know what types of stellar systems end up exploding as most types of supernovae (SNe). In my talk, I will show how we can use observed correlations between the SN explosion rates and various host-galaxy properties to constrain the progenitor scenarios of different types of SNe. Most of the results I will present were achieved via a spectroscopic SN survey conducted among galaxy spectra from the Sloan Digital Sky Survey. I will also show how this survey paves the way to transform any massive spectroscopic galaxy survey into a transient survey at no extra cost. This has particular applications to upcoming projects such as WFIRST and the Dark Energy Spectroscopic Instrument (DESI) survey.

Forging the heaviest elements

5/17/16
Rebecca Surman
(Notre Dame)

While the origins of the light (hydrogen, helium) and intermediate mass (carbon through iron) elements found in our solar system are well understood, we still don't know where roughly half of the elements heavier than iron were made. From the solar system abundance pattern of these nuclei, we can tell they were synthesized via rapid neutron captures in the r-process of nucleosynthesis. Exactly where the appropriate astrophysical conditions for the r-process exist, however, is still uncertain. Here we will discuss two attractive potential sites---core-collapse supernovae and neutron star mergers---and describe how progress in open issues in neutrino and nuclear physics may be the key to unlocking this longstanding mystery.

Hubble Space Telescope

Rare Elements from the First Stars to Today

5/24/16
Ian Roederer
(Michigan)

Understanding the origin of the elements is one of the major challenges of modern astrophysics. Elements listed along the bottom two-thirds of the periodic table---including arsenic, selenium, barium, europium, lead, thorium, uranium, and others---are mainly produced by neutron-capture reactions. Some had not been detected previously in late-type stars, and the origins of all are not fully understood at present. My work focuses on abundances derived from ultraviolet and optical high-resolution spectroscopic data of dwarf galaxies, globular clusters, and field stars in the stellar halo. I will present recent observations of these elements that change our understanding of when and how they were first produced in the early Universe.
Image Credit: NASA

A sub-kiloparsec scale view of star formation in M31

5/31/16
Alexia Lewis
(U. of Washington)

In the nearby Universe, observations of resolved stellar populations enable the measurement of star formation rates as a function of position and time - spatially-resolved star formation histories (SFHs) - within a single galaxy. Combined with multi-wavelength observations of dust and gas, these resolved SFHs represent the most direct way to holistically probe galaxy evolution. I will discuss my work in M31, where we have leveraged observations from the Panchromatic Hubble Andromeda Treasury program to measure the spatially-resolved recent SFH of M31's disk on 100 pc spatial scales over the past 500 Myr. My work has shown that the M31's 10 kpc ring is long-lived, posing a challenge to galactic dynamics. Additionally, I find that most (90%) of the star formation in M31 is obscured by dust. This obscuration is not well-captured by conventional integrated tracers of embedded star formation (e.g., 24 micron). I will also disucss my ongoing work to examine attenuation curve variations in M31 using a combination of HST + GALEX observations. As a whole, these studies reveal the most finely spatially-resolved view of star formation in an L_star galaxy to date.

Accurate Cosmology with Observations of Galaxies

6/7/16
Jonathan Blazek (Physics)

A number of large international collaborations are planning massive surveys - including DESI, LSST, Euclid, and WFIRST - which will allow us to probe the cosmological model using observations of hundreds of millions of galaxies. However, the complex relationships between the underlying large-scale structure and the positions and shapes of galaxies remain poorly understood. I will discuss how these interesting astrophysical questions are critical for the success of next-generation cosmological surveys. I will conclude with a discussion of the potential impact on galaxy clustering of supersonic streaming baryonic velocities, including a significant effect recently identified with other CCAPP researchers.

Results from the Completed SDSS-III Baryon Oscillation Spectroscopic Survey

6/7/16
Ashley Ross (Physics)

I will talk about results from the completed SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS), which mapped the structure of the Universe via spectroscopic redshift measurements of 1.2 million galaxies within a volume of 19 Gpc3. I will describe how BOSS data is used to measure distances via the localization of the baryon acoustic oscillation feature and the rate of structure growth via the modeling of redshift-space distortion effects. I will then describe how these measurements can be combined with those of cosmic microwave background experiments in order to test models of dark energy, measure the sum of the mass of neutrinos, and test general relativity.

FAST-PT: a novel algorithm to calculate convolution integrals in cosmological perturbation theory

6/21/16
Joe McEwen (Physics)

In this talk, I present a newly developed numerical algorithm to perform convolution or mode-coupling integrals that appear in nonlinear cosmological perturbation theory. The algorithm makes use of special function identities to reduce the convolution integral to a one dimensional integral calculable by Fourier transforms. This yields extremely fast performance, enabling mode-coupling integral computations fast enough to embed in Monte Carlo Markov Chain parameter estimation. As a first example of FAST-PT, I presents results for one-loop calculations.

Reverberation Mapping of AGN Accretion Disks

6/21/16
Michael Fausnaugh (Astronomy)

I will discuss new reverberation mapping results that allow us to investigate the temperature structure of AGN accretion disks. By measuring time-delays between broad-band continuum light curves, we can determine the size of the disk as a function of wavelength, which allows us to map the disk's temperature profile. I will discuss the recent detection of continuum lags in NGC 5548 reported by the AGN STORM project and the implications of these lags for the accretion disk. I will also present results from a 6-month reverberation mapping campaign that has found evidence for continuum lags in several other AGN. Most of these targets do not have previously published black hole masses, and our measurements of these masses allow us to directly compare the inter-band continuum lags with predictions from standard thin-disk theory.

Dark Forces in the Sky: Signals from Z' and the Dark Higgs

6/28/16
Rebecca Leane (Melbourne/Physics)

In this talk I will discuss the indirect detection signals for a self-consistent hidden U(1) model containing a fermionic dark matter candidate, dark Z' gauge boson and a dark Higgs. Compared with a model containing only a dark matter candidate and Z' mediator, the presence of an additional scalar provides a mass generation mechanism for the dark sector particles and can be required in order to avoid unitarity violation at high energies. I will show that the inclusion of the additional scalar opens up a new two-body s-wave annihilation channel, providing rich phenomenology for indirect detection searches. This phenomenology is missed in the usual simplified model approaches. This new process allows indirect searches to explore regions of parameter space not accessible with other commonly considered s-wave annihilation processes, and enables both the Z' and scalar couplings to be probed. I will discuss the phenomenology of the sector with a focus on this new process, and determine the limits on the model parameter space from Fermi data on Dwarf Spheriodal Galaxies and other relevant experiments.

Ice giant exoplanets

6/28/16
Radek Poleski (Astronomy)

Among possible analogues of planets observed in the Solar System, objects similar to Uranus and Neptune are hardest to detect. Their long orbital periods make transits and radial velocities signals very hard to detect, even the planets are common. It turns out that microlensing is the only technique that can detect analogues of Uranus or Neptune. I will present a few examples of ice giant exoplanets detected using existing microlensing experiments. I will show that planet properties can be derived using Nested Sampling algorithm (useful also for other optimization problems), even if a standard MCMC run on the same data fails badly. Finally, I will show that deriving properties of microlensing ice giants is harder than it is for microlensing planets lying closer to their host stars.

Probing neutrino lifetime using high-energy astrophysical neutrinos

7/5/16
Mauricio Bustamante (Physics)

In theories beyond the Standard Model, neutrinos may be unstable and decay with rates that have detectable effects. The cumulative effect of decay on a neutrino flux will be larger the longer the neutrino travel time, or baseline. Therefore, the high-energy (10 TeV -- 2 PeV) astrophysical neutrinos recently discovered by IceCube --- with estimated baselines from several megaparsecs to a few gigaparsecs --- are fertile ground to test decay. I will show how decay distorts the flavor composition of these neutrinos and the rate of neutrino-induced showers. Using these observables, existing and near-future IceCube data improve the lifetime bounds by several orders of magnitude, in the normal and inverted neutrino mass hierarchy.

Introducing Agnostic Spectral Stacking as a Powerful Method to detect and characterise Weak QSO Absorber Populations

7/5/16
Stephan Frank (Astronomy)

We have developed a new spectral stacking method that allows for the detection and characterisation of absorber populations that are individually too weak to be detected by traditional line search methods. I will demonstrate how this technique works for the specific case of searching for signatures of NeVIII absorbers, thought to arise in warm to hot circumgalactic gas, at intermediate redshifts (z=0.7-1.2) in an ensemble of high-quality COS-spectra.

Using Simulations of the X-ray Emission from Black Holes to Dissect the Inner Regions of Accretion Flow

7/12/16
Janie Hoormann
(Washington U, St. Louis)

X-ray observations of black holes provide an opportunity to probe the strong gravity regime of General Relativity (GR) and the properties of inner accretion flow. To this end, a ray tracing code was developed to simulate the X-ray spectral, timing, and polarization signatures surrounding stellar and supermassive black holes for both the thermal disk and power-law emission. These simulations can be used to study the recently observed reverberation between the direct coronal emission and the reflected emission forming the Iron K alpha line and the Compton hump with particular attention taken to examine the effect the ionization of the disk will have on the reverberation observations. In addition, these calculations can also be performed for various spacetime backgrounds (both GR and non-GR) to constrain potential deviations from the No-Hair theorem which states black holes are only described by their mass, spin, and charge.

Asteroseismic Tests of Stellar Isochrones

7/12/16
Jamie Tayar (Astronomy)

With the combination of APOGEE spectroscopic data and asteroseismology from Kepler, there are now thousands of evolved stars with known masses, metallicities, temperatures, gravities, compositions, and evolutionary states. We find strong evidence for metallicity dependent offsets between the actual HR diagram position of evolved stars and that predicted by isochrones. Such offsets have been suggested previously, but they are particularly obvious in our uniquely well characterized data set. We compare this to results from 3D atmosphere calculations and discuss consistency with convection theory. We also show that these temperature offsets can cause large errors in the ages derived from HR diagram position.

Background Rejection in the ARA Experiment

7/19/16
Carl Pfendner (Physics)

The Askaryan Radio Array (ARA) is a radio frequency observatory under construction at the South Pole that is searching for ultrahigh energy neutrinos via the Askaryan effect. Thermal fluctuations currently dominate the trigger-level background for the observatory and anthropogenic sources also introduce a significant source of noise. By taking advantage of the observatory's regular geometry and the expected coincident nature of the RF signals arriving from neutrino-induced events, this background can be filtered efficiently. This contribution will discuss techniques developed for the ARA analyses to reject these thermal signals, to reject anthropogenic backgrounds, and to search for neutrino-induced particle showers in the Antarctic ice. The results of a search for neutrinos from GRBs using the prototype station using some of these techniques will be presented.

Observing Neutron Stars

7/26/16
Shirley Li (Physics)

Neutron stars are interesting objects both for astronomers and for nuclear physicists. They are abundant in our galaxy, yet we have only observed a tiny fraction of their population. In my talk, I will discuss a new approach to surveying neutron stars. Our preliminary results show that the current generation telescope may have sensitivity to detect neutron stars.

On the Reliability of CIV-based Black Hole Masses: We're Making Progress

7/26/16
Kelly Denney (Astronomy)

Being able to reliably determine quasar black hole masses based on the rest-UV CIV emission line has benefits for understanding black hole growth and galaxy evolution in the early universe because this line redshifts into the more easily-accessible visible wavelength regime for z >~ 1.5. However, there has been an unresolved and continuing controversy attached to using CIV as a virial mass indicator due to apparent inconsistencies between masses based on this line and the more robustly-tested Hbeta emission line. I will discuss how these inconsistencies largely appear to be due to the lack of understanding of the origin and object diversity in the CIV emission components, how this is connected to the geometry and kinematics of the variable broad line region, and how this in turn affects our ability to use simple line width characterizations as a proxy for the velocity dispersion of the variable CIV-emitting BLR gas. I will then present selected results of several recent projects that have been aimed to improve our understanding of CIV emission as a means to improve the reliability of this emission line as a virial black hole mass indicator.

Searching for Dark Matter with NuSTAR

8/2/16
Kenny Ng (Physics)

I will talk about a novel usage of NuSTAR observation in searching for sterile neutrino dark matter. With NuSTAR, it may be possible to close off the high energy part of the parameter space for a class of sterile neutrino dark matter models, where sterile neutrinos are produced through active-sterile neutrino mixing.

ANITA 4: A TUFF new mission to discover ultra-high energy neutrinos

8/2/16
Oindree Banerjee (Physics)

The Antarctic Impulsive Transient Antenna (ANITA) is a NASA Long Duration Balloon project primarily looking for ultra-high energy neutrinos. We are launching ANITA 4 at the end of this year with hardware updates to filter narrow band noise and to trigger more efficiently. In this talk, I will give an overview of the Tunable Universal Filter Frontend (TUFF) boards. We built these boards here at OSU for the purposes of amplification and filtering, and recently integrated them with the ANITA instrument at NASA Columbia Scientific Balloon Facility in Palestine, Texas for a hang test.

Strong narrow-line lensing and the subhalo mass function

8/9/16
Anna Nierenberg (Physics)

The quasar narrow-line region provides an attractive background source for gravitational lensing studies of dark matter substructure, because it is smooth, large enough to be free from microlensing contamination, and significant narrow-line flux is detectable in virtually all quasars, making it possible to more than double the previous sample of strong lenses which could be used for this purpose. I will present an initial analysis of data from a dedicated HST grism survey in which we measure narrow-line fluxes from multiply imaged background quasars and place constraints on the presence of substructure in these systems.

Experimental Particle Astrophysics in Antarctica

8/9/16
Jordan Hanson (Physics)

In conjunction with the New Vistas in Astronomy Outreach Program with the Perkins Observatory and the Columbus Astronomical Society, I will present a review of the field of experimental particle astrophysics in Antarctica. The photo and radio Cherenkov-based experimental results will be discussed, as well as the future of the field. The way forward for detection of neutrinos with world-record setting energies lies with the radio Cherenkov based projects.

The Galactic Center Environment and the Galactic Center GeV Excess

8/16/16
Tim Linden (Physics)

The Milky Way Galactic Center is the most extreme astrophysical environment that is currently resolvable at gamma-ray energies -- and cosmic-rays accelerated in the inner degrees of our galaxy power numerous excesses observable across the electromagnetic spectrum. Recently, Fermi-LAT observations have discovered a significant gamma-ray excess centered coincident with the position of Sgr A*. While this excess may be explained by populations of gamma-ray pulsars or by dark matter annihilation, it is worth noting that the intensity of this excess is comparable to the systematic uncertainties in the diffuse astrophysical gamma-ray emission near the Galactic plane. Thus, a detailed understanding of the intensity, spectrum, and morphology of gamma-rays from hadronic and leptonic processes in the Galactic center is necessary to determine both the existence and characteristics of the gamma-ray excess. In this talk, I will discuss significant improvements in gamma-ray diffuse emission modeling that enhance our understanding of high energy astrophysics near the Galactic center, and will describe the impact of these models on our understanding of the gamma-ray excess.

Unraveling the History of the Milky Way

9/13/16
Yuan-Sen Ting

Understanding physical processes responsible for the formation and evolution of galaxies like the Milky Way is a fundamental problem in astrophysics. However, a key challenge is that the properties and orbits of the stars can only be observed at present: in order to understand what happened in the Milky Way at earlier epochs, one must explore “archeological” techniques. One idea, "chemical tagging”, aims to probe the history of the Milky Way via the unique imprint in chemical abundance space of long-disrupted star clusters. I will discuss the opportunities and challenges associated with chemical tagging, including a first constraint on the disrupted cluster mass function in the Milky Way. I will also describe a new set of tools for efficient fitting large quantities of stellar spectra and opportunities for extracting many stellar parameters from low-resolution data.

Using Neutral Hydrogen to Measure Cosmic Magnetism and CMB Foregrounds

9/20/16
Susan Clark

Sensitive, high resolution observations of Galactic neutral hydrogen (HI) reveal an intricate network of slender linear features. Across the high Galactic latitude sky, this HI is aligned with the magnetic field as traced by both starlight polarization (Clark et al. 2014) and Planck 353 GHz polarized dust emission (Clark et al. 2015). The structure of the neutral interstellar medium is more tightly coupled to the magnetic field than previously known. At high Galactic latitudes, where the Planck data are noise-dominated, the HI data provide an independent constraint on the Galactic magnetic field orientation, and hence the local dust polarization angle. The HI data thus provide a new tool in the search for inflationary gravitational wave B-mode polarization in the cosmic microwave background, which is currently limited by dust foreground contamination. By using HI orientation as a Bayesian prior on the dust polarization angle, we can better constrain the properties of the polarized CMB foreground. This gives us a new mechanism for testing models of ISM-magnetic field interactions.

Unveiling the Low Surface Brightness Universe: The Dragonfly
Nearby Galaxies Survey

9/27/16
Allison Merritt

The Dragonfly Telephoto Array, comprised of 48 individual Canon telephoto lenses operating together as a single telescope, is an innovative approach to low surface brightness imaging. Sub-nanometer coatings on each optical element reduce scattered light from nearby bright stars and compact galaxy centers -- typically a key obstacle for integrated light observations -- by an order of magnitude, and Dragonfly's large field of view (2 x 2.6 degrees for a single frame) provides a large-scale view of galactic stellar halos and satellite systems. Using extremely deep (>30 mag/arcsec^2) optical imaging in g and r bands from the Dragonfly Nearby Galaxies Survey (DNGS), we have characterized the stellar halos of a sample of nearby luminous galaxies. I will present measurements of the stellar halo mass fractions of an initial sample of spiral galaxies from the survey, and discuss these in the context of the assembly histories of individual galaxies. Finally, I will present recent results on the presence of ultra diffuse galaxies in a nearby group.

Dwarf Galaxy Archaeology with the r-process Galaxy Reticulum II

10/4/16
Alex Ji

Detailed chemical abundances of metal-poor stars offer an observational window to the era of first stars and galaxies. Ultra-faint dwarf galaxies contain a coherent population of metal-poor stars, providing important environmental context to this record of early chemical enrichment. I will present an example of dwarf galaxy archaeology with the ultra-faint dwarf galaxy Reticulum II. Seven of nine stars in this galaxy display extremely enhanced r-process abundances 2-3 orders of magnitude higher than in the other ultra-faint dwarfs. Stars with such extreme r-process enhancements are only rarely found in the Milky Way halo. The r-process abundances imply that the neutron-capture material in Reticulum II was synthesized in a single prolific event, possibly a neutron star binary merger or a magnetically driven supernova. The single r-process enrichment event also provides a unique probe of the star formation and metal mixing history of this galaxy. Reticulum II illustrates how continued observations of faint dwarf galaxies can constrain the very high-redshift universe.

Supernovae as Drivers of Dust Evolution in Galaxies

10/11/16
Tea Temim (Space Telescope Science Institute, STScI)

The presence of dust in galaxies has a profound effect on the physical, chemical, and thermal state of their interstellar media (ISM). Despite its significant role in the astrophysical processes governing galaxy evolution, the nature, origin, and evolution of dust are still not well-understood. Dust grains are primarily formed in the ejecta of core collapse supernovae (SNe) and mass outflows from evolved stars, and then subsequently processed and destroyed by SN shocks expanding into the surrounding ISM. The amount of dust destruction in the ISM determines whether a galaxy's dust budget can be balanced by dust formation in stellar sources, or if an additional supply of dust is required. I will summarize the recent progress on the study of dust formation and processing in supernova remnants (SNRs), including observations of dust heated by pulsar winds that reveal important information about the properties of pristine SN-condensed grains. I will also discuss the balance between dust formation and destruction by SNe and its implications for dust evolution models and our understanding of the origin of dust in the Universe.

First year of data from the High Altitude Water Cherenkov observatory

10/18/16
Miguel Mostafa

High-energy gamma-ray observations are an essential probe of cosmic-ray accelera-tion mechanisms because they are created by cosmic rays interacting near their origin. The characteristics of the gamma-ray flux variability and spectra constrain the accel-eration mechanisms and the environment of the accelerators. The detection of the highest energy gamma rays and the shortest timescales of variability are the key sci-entific motivations for building a continuously operating gamma-ray experiment with a large effective area.

The Milagro experiment was the first-generation of gamma-ray detectors based on the water-Cherenkov technique, and demonstrated that it is possible to monitor a large fraction of the TeV sky on a 24/7 basis. The second-generation water-Cherenkov experiment, the High Altitude Water Cherenkov (HAWC) observatory, consists of an array of 300 water-Cherenkov detectors covering an area of 22,000 m2 at 4,100 m above sea level. The larger effective area, the higher altitude, and the optical isolation of the detectors led to a 15-fold increase in sensitivity relative to Milagro. The improved performance allows us to survey the TeV sky, to map the diffuse emission, to detect emission from extended regions, and to observe transient events such as gamma-ray bursts. In addition, we also have the potential for discovering electromagnetic coun-terparts to gravitational waves and astrophysical neutrinos. The full HAWC array has been taking data since March, 2015. I will present the preliminary results using data from the first year of operation of the HAWC observatory.

Compact and diffuse dark matter

10/25/16
Juilan Munoz

A significant part of dark matter could be compact, in particular in the form of primordial black holes. I will review the signatures of primordial black holes, both in the form of gravitational-wave events and as gravitational lenses of fast radio bursts. Alternatively, a diffuse dark-matter component could interact with baryons. I will explain how these interactions cause heating of the baryons, becoming observable prior to the epoch of reionization.

Measuring BAO in galaxy surveys

11/1/16
Angela Burden
(Yale)

I will give a brief introduction on how baryon acoustic oscillations measured in galaxy surveys are used to make cosmological measurements. I will then talk about the reconstruction technique used in the BOSS survey and present some of the cosmological measurements made from the final data set. Finally I will talk about the upcoming DESI survey and describe preliminary work on techniques we hope to use to ensure our BAO measurements are not biased by the instrument.

The Destructive Birth of Massive Stars and Massive Star Clusters

11/8/16
Anna Rosen
(UCSC)

Massive stars play an essential role in the Universe. They are rare, yet the energy and momentum they inject with their intense radiation fields and stellar winds into the interstellar medium (ISM) dwarfs the contribution by their vastly more numerous low-mass cousins. These mechanisms can halt accretion onto massive stars and limit star formation in massive star clusters (MSCs), which can host thousands of massive stars. For stellar winds, I discuss how we can use observations to constrain a range of kinetic energy loss channels for the shock-heated gas from stellar winds in MSCs. I demonstrate that the kinetic energy injected by stellar winds in MSCs is not a significant contributor to stellar feedback for young MSCs. I argue instead that radiation pressure is likely the dominant feedback mechanism in massive star and MSC formation. Therefore detailed simulation of their formation requires an accurate treatment of radiation. For this purpose, I have developed a new, highly accurate hybrid radiation algorithm that properly treats the absorption of the direct radiation field from stars and the re-emission and processing by interstellar dust. With this new method, I performed a suite of three-dimensional radiation-hydrodynamic simulations of the formation of massive stars and MSCs. For individual massive stellar systems, I find that mass is channeled to the massive stellar system via gravitational and Rayleigh-Taylor instabilities. I will also present a simulation of the formation of a MSC from the collapse of a dense, turbulent, magnetized million solar mass molecular cloud. I find that the influence of the magnetic pressure and radiative feedback slows down star formation. These early results suggest that the combined effect of turbulence, magnetic pressure, and radiative feedback from massive stars is responsible for the observed low star formation efficiencies in molecular clouds.

Tidal disruptions of stars by supermassive black holes: dynamics, light, and relics

11/15/16
James Guillochon
(CfA)

Most supermassive black holes in the local universe lie dormant, with only one in a hundred accreting at their Eddington limits. Aside from this active minority, and the black holes in nearby galaxies that we can observe to influence the dynamics of stars and gas, most remain difficult to study. Tidal disruptions of stars by supermassive black holes give these dormant black holes a chance to be seen once every ~10,000 years, and each tidal disruption brings along with it a host of observable signatures that can be studied from gigaparsecs away, from the moment of the disruption to millennia after a disruption has occurred. In my talk I will present work I have done on tidal disruptions of stars, and describe their dynamics, observational signatures from real-time monitoring, and relics of disruption that may exist in plain sight.

Hidden sectors and dark matter

11/29/16
Jessie Shelton
(UIUC)

The lack of experimental signals of WIMP dark matter to date has brought hidden sector dark matter models into sharp focus: a broad and general class of theories where the relic abundance of dark matter is determined through its interactions with other dark fields, rather than directly with the Standard Model. I'll point out that the existence of an internally thermalized hidden sector places new requirements on early universe cosmology, and talk about physical consequences from two possible scenarios for the origin of a dark sector.

Probing the Dark Universe with Galaxy Surveys

11/30/16
Risa Wechsler
(Stanford)

The expansion of the universe and the growth of structure in it are dominated by two constituents that make up the 95 percent of the universe that is unexplained by the standard model, known as dark matter and dark energy. An understanding of these unseen components is critical to answering the most fundamental questions about our universe: how the it began, why it is accelerating, and what is the nature of most of its mass. A new generation of sky surveys are beginning to map the universe’s expansion history and evolution of structure over the last ~ 12 billion years, using statistical constraints from hundreds of millions of galaxies. I will outline the landscape of current and near future cosmological sky surveys, including early results from the Dark Energy Survey, and expected measurements from the upcoming Dark Energy Spectroscopic Instrument and the Large Synoptic Survey Telescope. Making use of these data to understand the nature of dark energy and dark matter also requires large numerical simulations of the evolution of the matter distribution and a modeling approach for connecting these simulations to observations of the galaxy distribution. I will present recent developments in how we are using simulations, modeling of the galaxy-halo connection, and large galaxy surveys together to probe the physics of the dark universe as well as the physics of galaxy formation.

Cosmic Axion Detection with an Amplifying B-field Ring Apparatus

12/6/16
Ben Safdi
(MIT)

Dark matter may be composed of ultralight axions that interact very weakly with ordinary matter. When axion dark matter encounters a static magnetic field, it sources a small effective electric current that follows the magnetic field lines and oscillates at the axion Compton frequency. I will describe a new idea for a laboratory experiment to detect this axion effective current. In the presence of axion dark matter, a toroidal magnet will act like an oscillating current ring, whose induced magnetic flux can be measured by an external pickup loop inductively coupled to a SQUID magnetometer. I will demonstrate that a meter-scale toroid could potentially probe the QCD axion with a GUT-scale decay constant, and I will describe ongoing efforts at MIT to build a small-scale prototype, with a toroid of O(10 cm).

Lensing Cross Correlations: Measurements and Applications

1/17/17
Sukhdeep Singh
(Carnegie Mellon U)

Weak gravitational lensing has emerged as an important cosmological probe for our understanding of dark matter and dark energy. Cross correlations between lensing and galaxy over-density map the stacked matter distribution around galaxies and provide important information about matter correlations and the growth of large scale structure. I will present our measurements of galaxy-galaxy lensing and galaxy-CMB lensing cross correlations using SDSS III-BOSS spectroscopic samples with SDSS galaxy lensing and Planck CMB lensing. I will also address some of the issues in covariance estimation for these measurements. When combined with galaxy clustering and RSD measurements, these measurements also provide strong constraints on the standard cosmological model. As applications of these measurements, I will discuss the recent measurements of the E_G parameter that quantifies deviations from general relativity, cosmological parameter constraints and the estimation of relative calibration uncertainties between galaxy shear and CMB lensing.

Simulating nonlinear cosmological structure formation with massive neutrinos

1/24/17
Arka Banerjee
(U of Illinois Urbana-Champaign)

Accurate measurements of neutrino mass eigenstates could offer a window into physics beyond the Standard Model of particle physics. Apart from oscillation experiments, cosmological probes offer a promising avenue for neutrino mass measurements. One such probe is the effect of massive neutrinos on cosmological structure formation. In this talk, I will first discuss a technique which can simulate cosmologies with massive neutrinos accurately down to scales where the clustering of neutrinos is fully nonlinear, while not suffering from shot noise effects that are seen in standard N-body simulations. I will then talk about a specific observable signature that massive neutrinos produce on Large Scale Structure - a strong scale dependent bias of voids even on large scales.

Outskirts of Dark Matter halos

1/25/17
Susmita Adhikari
(U of Illinois Urbana-Champaign)

Recent work has shown that density profiles in the outskirts of dark matter halos can become extremely steep over a narrow range of radius, deviating from well-known fitting functions like the Navarro-Frenk-White(NFW) profile. This behavior is produced by splashback material on its first apocentric passage after accretion. The location of this splashback feature may be understood quite simply, from first principles. I will discuss how this feature may be used as a probe of fundamental physical phenomena like dynamical friction and also of exotic physics. I will also review the progress in the detection of this feature in observations of galaxy clusters.

How to Turn Off Star Formation: Assembling Early Passive Galaxies

1/31/17
Justin Spilker
(Arizona)

One of the most important realizations of the past fifteen years is the vital role that feedback processes must play in the evolution of galaxies, particularly at the massive end (M_star >~ 10^11 Msun). Beginning already within the first 2Gyr of the history of the universe, star formation in massive galaxies appears to be efficiently and rapidly shut off ("quenched") as they transition to the red sequence. I will discuss my ongoing efforts to understand the mechanisms by which galaxies quench, focusing on three populations across redshifts. First, I will show highlights from an ongoing ALMA program to spatially and spectrally resolve outflowing molecular winds from z=4-5 dusty star-forming galaxies. Next, I will show results from a VLA program studying a population of compact galaxies at z~2.5 thought to be rapidly transitioning to quiescence. Finally, I will describe upcoming ALMA observations of z~0.7 galaxies which have already quenched, in an effort to link these objects with local quiescent galaxies. The ultimate goal of these studies is to understand the mechanisms, direction, and consequences of feedback processes in galaxies, and I will outline areas where I expect progress to be made over the next several years.

Fundamental Physics with the Next Galaxy Surveys

2/7/17
Chang Hoon Hahn
(NYU)

Galaxies' connection to the cosmic web allows us to use them as tracers of the matter distribution in the Universe and make precise measurements of large scale structure. The next galaxy surveys with eBOSS and DESI will expand the cosmic volumes probed with galaxies and provide unprecedented statistical power. The main challenges for realizing their full potential are methodological.
I will present how major methodological challenges can be solved with robust treatment of systematics, higher order statistics, innovative approaches to probabilistic inference, and improved understanding of the galax-halo connection. By overcoming these challenges and unlocking the full potential the next galaxy surveys, I will present how we can measure the growth of structure and total neutrino mass with unprecedented precision.

Search For Type Ia Supernova NUV-Optical Subclasses

2/8/17
David Cinagro
(Wayne State U)

In response to a recently reported observation of evidence for two classes of Type Ia Supernovae (SNe Ia) distinguished by their brightness in the rest-frame near ultraviolet (NUV), we search for the phenomenon in publicly available light-curve data. We use the SNANA supernova analysis package to simulateSN Ia-light curves in the Sloan Digital Sky Survey Supernova Search (SDSS) and the Supernova Legacy Survey (SNLS) with a model of two distinct ultraviolet classes of SNe Ia and a conventional model with a single broad distribution of SN-Ia ultraviolet brightnesses. We compare simulated distributions of rest-frame colors with these two models to those observed in 158 SNe Ia in the SDSS and SNLS data. The SNLS sample of 99 SNe~Ia is in clearly better agreement with a model with one class of SN Ia light curves and shows no evidence for distinct NUV sub-classes. The SDSS sample of 59 SNe Ia with poorer color resolution does not distinguish between the two models.

APOSTLE Local Group simulations: why bother with faint galaxies?

2/14/17
Azadeh Fattahi
(U of Victoria)

Despite its success in explaining the large scale structure of the Universe, the standard model of cosmology (LCDM) has faced a number of problems in explaining the properties of dwarf galaxies. Detailed studies of these intrinsically faint objects are only possible in the Local Universe. The Local Group of galaxies therefore serves as a laboratory for studying galaxy formation scenarios and for testing predictions of cosmological models. I will introduce the APOSTLE project, a suite of high resolution hydrodynamical simulations of Local Group-like regions within the LCDM framework, and explain how the small scale problems of LCDM are resolved or persist in this model. I will particularly focus on the too-big-to-fail problem for MW satellites and its solution within LCDM and without requiring dwarf galaxies to have "cores" in their dark matter density profiles. I will extend the discussion to Andromeda satellites. Is there any too-big-to-fail problem in this system?

Re-Examining Astrophysical Constraints on the Dark Matter Model

2/21/17
Alyson Brooks
(Rutgers)

The cosmological model based on cold dark matter (CDM) and dark energy has been hugely successful in describing the observed evolution and large scale structure of our Universe. However, at small scales (in the smallest galaxies and at the centers of larger galaxies), a number of observations seem to conflict with the predictions CDM cosmology, leading to recent interest in alternative dark matter models. I will demonstrate a number of ways that baryonic physics can resolve the conflict between theory and observations, by significantly altering the structure and evolution of galaxies. Baryonic physics can create dark matter cores in galaxies, can form bulgeless disk galaxies, and alters the distribution and mass of satellite galaxies. I will also show that a proper consideration can reconcile theory with the observed velocity function of galaxies. Despite all of the successes of baryonic physics in reconciling CDM with observations, I will explain why alternative dark matter models are still viable and interesting.

Multi-messenger results at ultra-high energies from the Pierre Auger Observatory

2/28/17
Foteini Oikonomou
(Penn State)

The question of the origin of ultra high energy, > 10^19 eV, cosmic rays (UHECRs) remains unanswered, although experimental searches in the last decade have yielded important results, and insights about the universe at ultra-high energies. I will discuss the interpretation of the most recent measurements of the extensive air-showers produced by UHECRs at the Pierre Auger Observatory, and outline current strategies aiming to answer the question of UHECR origin. Emphasis will be given to studies of UHECR arrival directions, dedicated searches for neutral particles (neutrons, photons and neutrinos), and the nascent program of real-time searches for transient UHE emission, as part of multi-messenger monitoring networks.

Combining Cosmological Probes from the Dark Energy Survey, and Beyond

3/7/17
Elisabeth Krause
(Stanford)

Weak lensing, galaxy clustering, and the abundance of galaxy clusters probe different aspects of cosmic structure formation, and combining these measurements improves constraints on cosmology significantly. However, these observables probe the same underlying density field, and the information content is correlated. Additionally, they share correlated (astrophysical and observational) systematic effects. I will describe the ongoing joint analysis of probes of large-scale structure in the Dark Energy Survey. The unprecedented data quality and data volume of future surveys will require a new generation of analysis frameworks, and I will conclude by outlining some of the statistical and computational challenges for the interpretation of these data sets.

Energetic Windbags Young and Old: The Evolving Story of Pulsar Wind Nebulae

3/21/17
Pat Slane
(CfA, Harvard-Smithsonian)

In the death throes of a massive star's demise, the stellar core collapses to an ultra-dense state, generally a neutron star for all but the most massive progenitors. Born spinning rapidly, with immense magnetic fields, many such neutron stars act as pulsars, with cosmic generators producing teravolt potentials that result in winds of relativistic particles. The extended nebulae formed as pulsar winds expand into their surroundings provide information about the composition of the winds, the injection history from the host pulsar, and the supernova ejecta into which the nebulae are expanding. Recent modeling coupled with important observations from nearly all parts of the electromagnetic spectrum has placed important constraints on a significant number of individual systems and their host remnants, and on the population as a whole.
Here I provide a broad overview of the structure and evolution of pulsar wind nebulae, with specific examples of observations extending from the radio band to very high energy gamma-rays, and of hydrodynamical studies of these nebulae evolving within their host supernova remnants.

Dark Matter in the Smallest Galaxies

3/28/17
Matthew Walker
(CMU)

The Milky Way's dwarf-galactic satellites include the nearest, smallest, darkest and most chemically primitive galaxies known. I will summarize recent results -- and present new ones -- regarding the amount and spatial distribution of dark matter within these systems. I will discuss implications for two lines of inquiry regarding the nature of dark matter: 1) tests of the standard 'cold dark matter' model of cosmic structure formation, and 2) searches for dark matter annihilation/decay signals.

Using Type Ia Supernovae To Measure The Hubble Constant and Dark Energy

4/4/17
Dan Scolnic
(KICP, Chicago)

Type Ia Supernovae continue to prove to be an incredibly useful tool to measure cosmic distances. They are a critical pillar in the measurement of the local value of the Hubble constant, which has now been shown to be in over 3-sigma tension with the inferred value of the Hubble constant from measurements of the Cosmic Microwave Background. They are also a critical pillar in measurements of dark energy: numerous surveys, including Pan-STARRS, The Dark Energy Survey, LSST and WFIRST are positioning themselves to make the most accurate measurements of dark energy and any potential evolution of the equation-of-state parameter. I will discuss recent progress on both of these fronts and show how new data and new methods of standardization and calibration are yielding insights into the physics of Type Ia Supernovae and improved understanding of systematic uncertainties. I will talk about what to expect in upcoming years about the exciting tension in cosmology, and what new physics could be on the horizon.

Gamma-ray Constraints on Decaying Dark Matter and Implications for IceCube

4/7/17
Nick Rodd
(MIT)

Utilizing the Fermi measurement of the gamma-ray spectrum toward the inner Galaxy, I will explain how to derive some of the strongest constraints on dark matter lifetimes in the mass range from hundreds of MeV to above an EeV. The limits derived disfavour a decaying DM interpretation of the astrophysical neutrino flux observed by IceCube, and I will review why that possibility has received some attention in the literature recently.

High-energy X-rays from the Galactic Center: "zombie stars" and particle physics

4/11/17
Kerstin Perez
(MIT)

The inner parsecs of the Galaxy contain one of the highest concentrations of both high-energy sources and dark matter in the Milky Way. The supermassive black hole, pulsar wind nebulae, supernova remnants, X-ray binaries, and hot interstellar gas are copious emitters of X-rays and gamma-rays. In addition, this region contains a large density of dark matter, making it an important source of both dark matter interaction signatures and backgrounds to dark matter searches. NuSTAR provides a view of the hard X-ray (3-79 keV) band, a critical bridge between the soft X-ray and gamma-ray emission, with unprecedented angular resolution. I will present the first sub-parsec scale images of the Galactic Center in hard X-rays, obtained with NuSTAR, which offer leading constraints on the radiative decay of dark matter, in particular from sterile neutrinos, as well as new insight into the "zombie star" population that underlies this emission.

The Search for Extragalactic Dark Matter

4/18/17
Mariangela Lisanti
(Princeton)

The presence of all-sky, diffuse gamma-ray emission has been known for several decades, but its origin remains an open question. While astrophysical sources almost certainly contribute, dark matter annihilation in galaxies outside our own may also leave an imprint. I will present results from a series of studies aimed at extracting such dark matter signals with publicly available data from the Fermi Large Area Telescope. My primary focus will be on a new method that takes advantage of galaxy surveys, such as 2MASS, to provide thousands of targets in which to search for signals of dark matter.

Ex Luna, Scientia: The Lunar Occultation Explorer (LOX)

4/26/17
Richard Miller
(Alabama)

The Lunar Occultation Explorer (LOX) is a next-generation mission concept currently under consideration within NASA's MIDEX program. LOX will provide new capabilities for time-domain astronomy and establish the Moon as a platform for nuclear astrophysics. Performance requirements are driven by our focused science goal: resolving the enigma of Type-Ia supernova (SNeIa). Specifically LOX will reveal new details of these profoundly radioactive objects, including their intrinsic diversity, probing their fundamental thermonuclear parameter space, performing a census of progenitors and their explosion mechanisms, and evaluating the environmental conditions and intrinsic systematics of these enigmatic objects. LOX provides new capabilities for all-sky, continuous monitoring in the MeV regime (0.1-10 MeV) by leveraging the Lunar Occultation Technique (LOT). Key benefits of the LOX/LOT approach include maximizing the ratio of sensitive-to-total deployed mass, low implementation risk, and demonstrated operational simplicity that leverages extensive experience with planetary orbital geochemistry investigations; LOX also enables long-term monitoring of MeV gamma-ray sources, a critical capability for SNeIa science. Proof-of-principle efforts validated all aspects of the mission using previously deployed lunar science assets, and led to the first high-energy gamma-ray source detected at the Moon. LOX mission performance, development progress, and expectations for science investigations will be presented.

Discovery of a Thorne-Zytkow Object Candidate in the Small Magellanic Cloud

5/2/17
Emily Levesque
(Wisconsin)

Thorne-Zytkow objects (TZOs) are a theoretical class of star in which a compact neutron star is surrounded by a large, diffuse envelope. Supergiant TZOs are predicted to be almost identical in appearance to red supergiants (RSGs), with their very red colors and cool temperatures placing them at the Hayashi limit on the H-R diagram. The best features that can be used at present to distinguish TZOs from the general RSG population are the unusually strong heavy-element lines present in their spectra. These elements are the unique products of the star's fully convective envelope linking the photosphere with the extraordinarily hot burning region in the vicinity of the neutron star core. As part of a recent high-resolution spectroscopic search, my collaborators and I have discovered a TZO candidate in the Small Magellanic Cloud. It is the first star to display the distinctive chemical profile of anomalous element enhancements thought to be unique to TZOs. The positive detection of a TZO would provide the first direct evidence for a completely new model of stellar interiors, a theoretically predicted fate for massive binary systems, and never-before-seen nucleosynthesis processes that would offer a new channel for heavy-element production in our universe.

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