CCAPP Seminars

# The Characterization of the Gamma-Ray Signal from the Central Milky Way: A Compelling Case for Annihilating Dark Matter

1/20/15
Tim Linden
(Chicago)

In scenarios where dark matter particles can annihilate to produce standard model particles, the galactic center of the Milky Way is expected to provide the highest flux from dark matter in the sky. This has allowed galactic center observations to set extremely stringent limits on the parameters of the dark matter particle. Recently, we have worked on gamma-ray observations from the Fermi-LAT telescope, and have detected a significant extended excess, which is spherically symmetric around the position of the galactic center, and does not trace any known astrophysical emission profile. In this talk, I will summarize the current status of these observations and discuss dark matter and astrophysical interpretations of the data. I will show forthcoming results which strongly constrain the properties and the possible interpretations of the observed excess. Finally, I will posit upcoming tests which will strongly suggest, or rule out, a dark matter interpretation.

# Voids and Scaling in the Halo Model

1/27/15
Jim Fry
(Florida)

The popular halo model has implications that go beyond its usual applications to the galaxy autocorrelation function or power spectrum. I will examine how the model holds up in its predictions for higher order statistics of both mass and galaxy number, and I will revisit the hierarchical scaling behavior of void probabilities.

# The Fermi Bubbles and Future Gamma-ray Telescopes

2/3/15
Meng Su
(MIT)

The Fermi Bubbles are a pair of giant lobes at the heart of the Milky Way, extending roughly 50 degrees north and south of the Galactic Center, and emitting photons with energies up to 100 GeV. This previously unknown structure could be evidence for past activity of the central supermassive black hole. I will outline the current state of the multiwavelength observations of this structure, and discuss theoretical proposals and numerical simulations for their nature and origin. The Fermi Bubbles was discovered using data from the successful Fermi Gamma-ray Space Telescope, which has proved the great potential of studying astrophysics, cosmology, and fundamental physics in high energy gamma rays. I will talk about three future gamma-ray missions: DAMPE, HERD, and PANGU, and comment on what science could be learned from these future missions.

# Scrutinizing the Relationship Between Galaxies and Supermassive Black Holes

2/10/15
Jillian Bellovary
(Vanderbilt)

Supermassive black holes (SMBHs) have a ubiquitous presence in massive galaxies, but their formation and evolutionary history remain mysteries. One of the strongest observed trends between SMBHs and their host galaxies is the tight relation between black hole mass and the velocity dispersion of the stellar spheroid (aka the M-Sigma relation). While this relation hints at a fundamental link between galaxy and SMBH growth, there are also some challenges regarding scatter and outliers. I will present evidence that the M-Sigma relation is not the clear-cut trend it seems to be; for example, the orientation of galaxies on the sky affects the value of velocity dispersion by up to 30%. Additionally, the existence of SMBHs in dwarf galaxies and galaxies without spheroids challenges the standard paradigm of SMBH-galaxy coevolution. I will offer some alternative methods of forming and growing SMBHs which can provide explanations for these puzzling outliers.

# Radio and X-ray Studies of Type II-P Supernovae: What They Reveal About the Parents

2/17/15
Alak Ray
(Tata Institute)

I will describe our ongoing work on studying X-ray and radio emission from a class of supernovae that forms almost half of all core collapse supernovae, namely type II Plateau SNe. We have carried out these with Chandra, EVLA and GMRT telescopes. Because these bands probe the interaction of the SN with the medium it exploded in, they provide valuable information about the star that exploded and the nature of the medium. Our Chandra observations of SN 2004dj for the first time measured the separate contributions of thermal emission from the SN shocks and the power-law nonthermal part arising out of accelerated particles. For this and a few other SNe the combination of radio and X-ray measurements indicate the extent of equipartition between magnetic fields amplified by the shock and the relativistic particles accelerated by it.

# Chasing our Cosmic Dawn: Opening the 21cm cosmological window on the universe

2/24/15
Danny Jacobs
(Arizona State)

The Epoch of Reionization (EoR) marks when the first stars ionized the primordial hydrogen half a billion years after the big bang. Direct observation of cosmological hydrogen is possible via the 21cm line and is now being hotly pursued as a new cosmological probe. Danny Jacobs is helping to lead observations with both the Precision Array for Probing the Epoch of Reionization (PAPER) and the Murchison Widefield Array (MWA). PAPER currently holds the deepest limits on the epoch of reionization power spectrum, while the MWA is leading the charge towards understanding foregrounds at a level necessary for imaging large scale structure. The two intertwined themes in these new instruments are the degree to which experimentation is an essential component of the science process and the ascendance of the software pipeline. Experiments currently under-way to refine array sensitivity include high-bandwidth forward-modeling of instrumental response, developing high level heuristics for data flagging, accelerating imaging pipelines, pushing more analysis steps into real-time systems, and in-situ calibration with external drone-mounted transmitters. Lessons learned and systems developed for PAPER and the MWA are being incorporated into the next generation Hydrogen Epoch of Reionization Array (HERA) that will yield 20 times the sensitivity of first generation arrays and also into the upcoming Square Kilometer Array.

# Shedding Light on Distant Galaxies with ~400,000 SDSS Quasars

3/3/15
Britt Lundgren

Distant quasars backlight the Universe over most of cosmic time, revealing the evolving gas distribution of the cosmic web. Intervening metal absorption lines in quasar spectra can provide sensitive tracers of the gaseous processes regulating star formation in foreground galaxies to high redshift, but observational difficulties have traditionally limited comparisons to the stellar properties of the absorbing galaxies. The extraction and statistical analysis of absorption line systems from 377,809 unique quasar spectra in the Sloan Digital Sky Survey (SDSS I-III) has recently catalyzed our understanding of the physical environments of the most prolific metal absorption species (e.g., Mg II, C IV) observed in optical quasar spectra. I will present recent stacking and clustering measurements made possible by the vast quasar absorption line samples from the SDSS, and present new, direct observations of Mg II absorber host galaxies at high redshift using the Hubble Space Telescope. Together these studies reveal compelling links between strong Mg II absorbers and large-scale star formation-driven outflows, providing insights into the role of feedback in the evolution of galaxies from z~2.

# The Role of Dwarf-Dwarf Interactions in the Evolution of Low Mass Galaxies

3/10/15
Sabrina Stierwalt
(Virginia)

I will present the initial results from TiNy Titans, the first systematic study a sample of interacting dwarf galaxies and the mechanisms governing their star formation. Mergers of massive galaxies provide a significant mode of galaxy evolution and are observed to inspire intense starbursts and significant rearranging of the galaxies' gas and dust. However, despite the fact that mergers among low mass galaxies outnumber those between massive ones, whether these effects occur in the shallower gravitational potential wells of dwarf galaxies remains completely unconstrained. A few intriguing examples of dwarf-dwarf interactions exist in the literature, but the efficiency of gas removal and the enhancement of star formation in dwarfs via pre-processing (i.e. dwarf-dwarf interactions occurring before the accretion by a massive host) has never been studied for a uniformly selected sample of dwarfs. Our multiwavelength approach gathers high resolution optical, UV, and radio imaging to probe the effects of interactions on the star formation and ISM in a complete sample of dwarf pairs selected from the Sloan Digital Sky Survey. We find star formation is enhanced in paired dwarfs over isolated dwarfs to an even greater extent than is observed in massive galaxies, but the dwarfs involved in interactions still have large gas reservoirs (and thus capacity for future star formation). Our interacting dwarfs tend to be low metallicity and thus offer a unique window into modes of star formation that were important at earlier epochs.

# It's Always Darkest Before the Cosmic Dawn

3/17/15
Josh Dillon
(MIT)

Realizing the promise of 21 cm cosmology to provide an exquisite probe of astrophysics and cosmology during the cosmic dark ages and the epoch of reionization has proven extremely challenging. We're looking for a small signal buried under foregrounds orders of magnitude stronger. We know that we're going to need very sensitive, and thus very large, low frequency interferometers, which present their own set of difficulties. And, as I will explain, we're going to need a rigorous statistical analysis of the maps we make to extract interesting cosmological information. I will discuss the steps we've taken to overcome these obstacles with early data from the Murchison Widefield Array by isolating foregrounds to a region of Fourier space outside a clean "epoch of reionization window." Additionally, I will present some of most recent and exciting predictions for what 21 cm cosmology can tell us as we move to larger telescopes like the Hydrogen Epoch of Reionization Array and higher redshifts.

# On Neutrino Decay and Other Non-Unitary Effects on Neutrino Propagation

3/24/15
Andre de Gouvea
(Northwestern U)

After providing a brief overview of the neutrino physics landscape - what we know and what we know we don't know - I discuss in some detail the possibility that neutrino propagation is not unitary, e.g., the neutrinos could, generically, decay, and review what is currently known about the lifetime of the different neutrino mass-eigenstates.

# Primordial non-Gaussianity in the CMB and Large-Scale Structure

3/31/15
Kendrick Smith
(Perimeter Inst)

I'll give a pedagogical review of inflation and explain how its physics can be constrained by searching for "primordial non-Gaussianity", i.e. differences between the statistics of the initial curvature field in our universe and the statistics of an ideal Gaussian field. Then I'll talk about observational CMB constraints, including some new results from Planck. Finally I'll discuss future prospects for improving Planck constraints with large-scale surveys such as Euclid and LSST.

# The Zen of Core Collapse: How to See A Supernova That Doesn't Happen

4/7/15
Elizabeth Lovegrove
(UCSC)

It now seems likely that stars over 8 solar masses, rather than all exploding as some form of core-collapse supernova, have a range of explosion probabilities that depend on mass. Different masses of progenitor may explode with regular CCSNe energies, explode with much lower energies, or fail to explode entirely. Narrowing down these possible outcomes and associating them with stellar masses is important to galactic evolution models and a key step in achieving accurate end-to-end core-collapse simulations. But making progress on this question requires a large sample of observed supernova progenitors, and these are hard to come by. True observational limits, too, require being able to see not only supernovae that happen but also supernovae that don't. In this talk I will present and discuss results from the CASTRO simulation code on both transients from completely-failed supernovae as well as shock breakouts from faint supernovae, a promising channel for retrieving progenitor information.

# Exploring the Third Dimension: Spectroscopic Needs for Imaging Dark Energy Experiments?

4/14/15
Jefferey Newman
(Pittsburgh)

This talk will explore a number of applications of wide-area spectroscopic surveys, such as those which the Dark Energy Spectroscopic Instrument (DESI) will enable, to solving key problems for imaging dark energy experiments. First, I will discuss the problem of photometric redshifts (a.k.a. photo-z's): i.e., estimates of the redshifts of objects based only on flux information obtained through broad filters. Higher-quality, lower-scatter photo-z's will result in smaller random errors on cosmological parameters; while systematic errors in photometric redshift estimates, if not constrained, may dominate all other uncertainties from these experiments. In this talk, I will describe the key challenges for training and calibrating photometric redshift algorithms in the next two decades, and describe how DESI can help us address these challenges. Second, I will discuss the contributions the DESI instrument can make to LSST supernova studies by measuring spectroscopic redshifts for large samples of supernova hosts, which would require comparatively modest time allocations. Finally, I will describe the LSST Dark Energy Science Collaboration, highlighting how all of us can get involved now in preparing for this extraordinarily rich imaging dataset.

# The Cosmic Laboratory: Probing Fundamental Physics with Large-Scale Structure and the CMB

4/21/15
Roland de Putter
(JPL)

In the current era of precision cosmology, the Universe on large scales has become a unique probe of many fundamental physics questions that are hard or even impossible to address with conventional terrestrial experiments. In this seminar, I will discuss examples of what this "Cosmic Laboratory" teaches us about neutrino physics, inflation, dark matter and dark energy. I will discuss the following two topics in particular detail. First, I will discuss how currently, and for the foreseeable future, cosmology places the tightest bounds on the absolute mass scale of neutrinos. I will pay specific attention to the robustness of these bounds with respect to assumptions about the primordial power spectrum and thus inflation. Secondly, I will explain what it would take for a galaxy survey to use primordial non-Gaussianity to distinguish between single-field and multi-field inflation and I will introduce a specific proposal for such a survey, called SPHEREx.

# Observational Tests of Cosmic-ray Diffusion in the Magellanic Clouds

4/24/15
Laura Lopez
(OSU Astronomy)

Cosmic rays (CRs) play an important role in the interstellar medium: they ionize dense molecular gas, they are responsible for the light elements in the periodic table, and they account for 20% of the ISM energy budget. However, the means by which CRs are first accelerated and then transported through external galaxies are not well understood. I will present results from a recent study of the Magellanic Clouds to constrain CR transport using Fermi gamma-ray observations. I will show how we have characterized the spatial distribution of gamma rays in the LMC and SMC and used the findings, in conjunction with available multiwavelength data, to constrain CR transport based on how the emission depends on physical parameters, such as gas density, massive star formation, magnetic field structure, and turbulence properties.

# Searching for Dark Matter with Gamma Rays

4/28/15
Andrea Albert
(SLAC)

There is overwhelming evidence that non-baryonic dark matter constitutes ~27% of the energy density of the Universe. Weakly Interacting Massive Particles (WIMPs) are promising dark matter candidates that may produce gamma rays via annihilation or decay in the Universe. A detection of gamma rays from WIMPs would not only confirm the existence of dark matter through a non-gravitational force, but also indicate the existence of physics beyond the Standard Model. I will present recent Fermi-LAT Collaboration results from WIMP searches including looking for gamma-ray spectral lines and gamma-ray excesses in areas of large dark matter concentration like the Galactic center and dwarf spheroidal galaxies.

# Interpretation of IceCube results in the multi-messenger context

5/1/15
Walter Winter
(DESY Zeuthen, Germany)

The discovery of high-energetic cosmic neutrinos is one of the recent major breakthroughs in science. We discuss the concept of the neutrino production, and interpret recent results taking into account the information from other messengers (gamma-rays, cosmic rays). For example, one question is if these neutrinos come from the most powerful accelerators in the universe, i.e., the ones which can accelerate cosmic rays to the highest observed energies. We also discuss future perspectives for neutrino astronomy.

# The PAU Survey at the William Herschel Telescope

5/4/15
Ignacio Sevilla
(Illinois)

We present PAUCam: a multi-narrow band camera designed to conduct a photometric survey from the 4m William Herschel Telescope, starting this year. The scientific goals (which include intrinsic galaxy alignments, magnification and redshift space distortions), camera design and current status will be described.

# The future of weak gravitational lensing in cosmology

5/5/15
Ludo van Waerbeke
(UBC)

Gravitational lensing by large scale structures is poised to play a central role in the future of precision cosmology, to help constraining modified theories of gravity, the neutrino mass, quantifying the dark energy equation of state and a lot more. It is quite possible however that gravitational lensing will first shed light on some of the complex astrophysical processes taking place in galaxy and galaxy cluster formation, before it can reach the high precision needed for dark energy and modified gravity tests. After a short introduction of the basic concepts and ideas, I will discuss some of the recent and most promising developments and what challenges lie ahead for future surveys such as LSST, Euclid or WFIRST.

# New Physics at IceCube

5/7/15
Bhupal Dev
(U. of Manchester)

The recent observation of ultra-high energy neutrino events at IceCube has spurred a lot of interest in the scientific community. From a particle physics viewpoint, several exotic phenomena have been proposed to explain the data. For a reliable interpretation, it is however important to first scrutinize the Standard Model (SM) expectations carefully, including all known theoretical and experimental uncertainties. In this talk, we discuss the compatibility of the 3-year IceCube dataset with some well-motivated flavor and spectral compositions of the astrophysical flux within the SM framework, and show that the SM interactions are sufficient to explain the current data within the known uncertainties. Using this result, we can derive constraints on some New Physics scenarios, which are otherwise difficult to probe at the LHC or other existing low-energy experiments.

# Cosmological Constraints on Newton's Gravitational Constant for Matter and Dark Matter

5/12/15

In this talk I will present a study using cosmological data of the gravitational interaction in ordinary and dark matter sectors. In the first part I will present a new constraint of Newton's gravitational constant by making use of the latest CMB data from the Planck experiment and independent constraints coming from Big Bang Nucleosynthesis and low redshift experiments. In the second part of the talk I will focus on the dark matter sector, giving a constraint to the dark matter equivalence principle and long-range fifth forces.

# The Fermi Galactic Center excess

5/15/15
Ilias Cholis
(Fermilab)

The possible gamma-ray excess in the inner Galaxy and the Galactic center suggested by Fermi-LAT observations has triggered great interest in the astro-particle physics community. Among its various interpretations have been WIMP dark matter annihilations, gamma-ray emission from a population of millisecond pulsars, or emission from cosmic rays injected in a sequence of burst-like events or continuously at the GC. Given that the galactic diffuse emission is the dominant (by an order of magnitude or more) at any direction greater than 2 degrees from the GC understanding the background systematics has been a vital missing piece in the discussion. I will present the first comprehensive study of model systematics coming from the Galactic diffuse emission in the inner part of our Galaxy and their impact on the inferred properties of the excess emission at Galactic latitudes between 2 and 20 degrees and energies 300 MeV to 500 GeV. I will show both theoretical and empirical model systematics, which are deduced from a large range of Galactic diffuse emission models and a principal component analysis of residuals in numerous test regions along the Galactic plane. The hypothesis of an extended spherical excess emission with a uniform energy spectrum is compatible with the Fermi-LAT data in the region of interest at 95% CL. Assuming that this excess is the extended counterpart of the one seen in the inner few degrees of the Galaxy, a lower limit of 10 degrees (95% CL) can be derived on its extension away from the GC. In light of the large correlated uncertainties that affect the subtraction of the Galactic diffuse emission in the relevant regions, the energy spectrum of the excess is equally compatible with both a simple broken power-law of break energy 2.1 $\pm$ 0.2 and with spectra predicted by the self-annihilation of dark matter, implying in the case of $\bar{b}b$ final states a dark matter mass of 49$^{+6.4}_{-5.4}$ GeV. I will also briefly discuss interpretations of this excess, based on annihilating DM, leptonic CR outburst and a population of millisecond pulsars.

# Simulating Massive Neutrinos

5/19/15
Simeon Bird
(Carnegie)

Massive neutrinos make up a fraction of the dark matter, but due to their large thermal velocities, cluster significantly less than cold dark matter (CDM) on small scales. An accurate theoretical modelling of their effect on structure formation is required to fully exploit large-scale structure data. I will discuss different methods to add neutrinos to cosmological simulations, finishing with one which is accurate for current cosmological mass bounds and requires minimal extra computational cost. I will then talk about the impact of neutrino masses on cluster counts and perhaps the Lyman-alpha forest.

# Gravitational Signals from Noise in the Hubble Diagram

5/21/15
Edward Macaulay
(Univ of Queensland)

Understanding the nature of the dark universe requires precise measurements of the background expansion history, and also the growth rate of density fluctuations. In this talk, I'll consider both regimes with supernova lensing for the OzDES spectroscopic survey - which is measuring the redshifts of hundreds of supernova and thousands of galaxies identified by the Dark Energy Survey. I'll start by reviewing the more established method of growth rate measurements with Redshift Space Distortions, and discuss possible tension between RSDs and expectations from Planck CMB measurements. I'll then consider how OzDES can place novel constraints on the growth rate and amplitude of density fluctuations by correlating noise in the supernova Hubble diagram with the gravitational effects of lensing and peculiar velocities expected from the observed density field.

# Not with a Bang, but a Whimper: Evidence for Low-energy Supernovae

5/22/15
(OSU/CCAPP)

I will present new HST and Spitzer late-time imaging of SN 2008S and NGC 300-OT, the prototypes of a class of stellar transients whose true nature is debated. Both objects have faded below the luminosity of their progenitors and are now undetected in both the near and mid-IR, providing strong evidence that these events were terminal. This, combined with the mass constraints on the progenitors, indicates that this class of transients likely arise from electron-capture supernovae.

# Indirect Detection of Dark Matter Using MeV-Range Gamma-Ray Telescopes

5/22/15
Kimberly Boddy
(Hawaii)

The astrophysics community is considering plans for a variety of gamma-ray telescopes in the energy range 1--100 MeV, which can fill in the so-called "MeV gap" in current sensitivity. We investigate the utility of such detectors for the study of low-mass dark matter annihilation or decay. For annihilating (decaying) dark matter with a mass below about 140 MeV (280 MeV) and couplings to first generation quarks, the final states will be dominated by photons or neutral pions, producing striking signals in gamma-ray telescopes. We determine the sensitivity of future detectors to the kinematically allowed final states. In particular, we find that planned detectors can improve on current sensitivity to this class of models by up to a few orders of magnitude.

# Weighing the Giants: Anchoring Galaxy Cluster Cosmology

5/26/15
(KICP Chicago)

The gas mass fractions and the distribution in mass and redshift of the galaxy cluster population provide powerful probes of cosmology, constraining the cosmic matter density, the amplitude of the matter power spectrum, properties of dark energy, and the mass of neutrinos, among other parameters. Historically, these tests have been limited by the absolute accuracy of cluster mass determinations. Here, mass measurements from weak lensing have an advantage over estimates based on observations of the intracluster medium (ICM), because the former are nearly unbiased and can be straightforwardly tested against simulations. I will describe recent cosmological constraints obtained from an analysis of X-ray selected cluster samples, incorporating extensive gravitational lensing data from the Weighing the Giants project -- the first cluster cosmology study to consistently integrate a lensing mass calibration, including a rigorous quantification of all systematic uncertainties. The results highlight the power and potential of galaxy clusters, which constrain both the expansion of the Universe and the growth of cosmic structure, and their complementarity with other probes such as type Ia supernovae, large-scale galaxy surveys, and the cosmic microwave background.

# Probing the nature of AGN coronae through future X-ray and sub-mm observations

5/29/15
Yoshiyuki Inoue
(ISAS / JAXA)

While the cosmic X-ray background is likely to originate from individual AGNs, the origin of the cosmic MeV gamma-ray background is not fully understood. We proposed that AGNs having non-thermal electrons in coronae may explain the MeV background. Such non-thermal electrons are expected to exist if a corona is heated by magnetic reconnections. However, the sensitivity of current MeV gamma-ray instrument is not sufficiently good to detect the expected power-law tail in the MeV band from individual AGNs. Furthermore, the heating mechanism of coronae in AGNs is still unknown, although magnetic reconnection heating is one possibility. In this talk, I would like to introduce how we can probe the origin of the MeV background and the nature of AGN coronae such as magnetic field and non-thermal content through future observations by ASTRO-H and ALMA.

# High energy neutrinos from extra-galactic astrophysical sources

5/29/15
Irene Tamborra
(Amsterdam)

The IceCube neutrino telescope recently discovered a flux of astrophysical neutrinos with energies up to few PeV. In light of the new born high-energy neutrino astronomy era, I will discuss the expected high-energy neutrino emission from extra-galactic astrophysical sources as well as our chances to unveil the physics of the cosmic accelerators by employing neutrinos and their photon counterparts.

# Neutron Astronomy with IceTop

6/2/15
Michael Sutherland (OSU Physics)

IceTop can detect an astrophysical flux of neutrons from Galactic sources as an excess of cosmic ray air showers arriving from the source direction. Neutrons are undeflected by the Galactic magnetic field and can typically travel 10 ($E$ / PeV) pc before decay. Two searches through the IceTop dataset are performed to look for a statistically significant excess of events with energies above 10 PeV ($10^{16}$ eV) arriving within a small solid angle. The blind search method covers from -90$^{\circ}$ to approximately -50$^{\circ}$ in declination. A targeted search is also performed, looking for significant correlation with candidate sources in different target sets.

# Honing in on the flavor composition of high-energy astrophysical neutrinos: the view from theory

6/2/15
Mauricio Bustamante (OSU Physics)

After years of searching, IceCube has recently detected the first high-energy astrophysical neutrinos, in the form of a diffuse flux in the range between 100 TeV and 2 PeV. While still in its infancy, neutrino astronomy opens an exciting new window to the Universe. One of the issues that IceCube has recently started to look into is the flavor composition of the flux: what is the proportion of electron-, muon-, and tau-flavor in the signal. This will reveal information about the physical conditions at the cosmological neutrino sources. In this talk I will present some broad considerations, coming from simple theoretical arguments, that show that the number of possible flavor compositions of the incoming flux is surprisingly small. This is true even in the presence of many types of potential new physics effects in the neutrino sector. These arguments provide theoretical guidance that can be used to constrain the experimental flavor searches.

# The Era of Accurate Cosmology?

6/9/15
Jonathan Blazek (OSU Physics)

Several great successes of the past decade have placed us firmly in the era of precision cosmology. An upcoming generation of cosmological surveys - including LSST, DESI, Euclid, and WFIRST - promise tremendous statistical power. But will we be able to harness this power? Without significant progress on several astrophysical and observational issues, we may find ourselves with extremely precise (and expensive) measurements that are systematically biased. In this talk, I will briefly discuss a few of the primary goals of these upcoming surveys along with the some of the questions we must understand to achieve these goals. In particular, I will focus on correlations between intrinsic galaxy shapes, which must be included when interpreting weak lensing measurements. I will describe recent work in modeling these correlations and discuss future steps.

# Preliminary Results on the Ages of Ultracool Dwarfs from SDSS

6/9/15
Sarah Schmidt (OSU Astronomy)

The ages of ultracool dwarfs (the smallest stars and the warmest brown dwarfs) are particularly interesting due to the degeneracy in mass, age, and luminosity at spectral types M7-L3. We examine the ages of these low mass from the BOSS Ultracool Dwarf sample of 13000 late-M and early-L dwarfs using a combination of colors, kinematics, and spectral features. I will discuss some preliminary results from the BUD sample, including an examination of kinematic differences between late-M and early-L dwarfs and a spectroscopic search for young, low gravity objects and low-metallicity subdwarfs.

# From data to theory at the cosmic dawn

6/16/15
Paul Sutter (OSU Physics)

Understanding the epoch of reionization is extremely difficult due to significant observational challenges and only a vague theoretical picture of the underlying physical processes. Fortunately, in the past year there have been many developments that address these difficulties. As examples I will discuss HERA, a proposed radio interferometer designed specifically to observe this epoch, and a new semi-blind Bayesian method for foreground removal and signal recovery. Finally, I will present some ideas for extracting physically meaningful quantities from the upcoming measurements.

# Crowdfunding Physics with Fiat Physica

6/24/15
Mark Jackson (Fiat Physica)

Theoretical physics suffered budget cuts of nearly 25% in 2014. Researchers report spending as much as 75% of their time writing grant proposals. Labs want to hire new talent but don't have the funds. With steep declines in funding, physics and its many derivative sciences are unable to rely on the support of grants alone. Yet a vast community of science enthusiasts exists who want to get involved at the ground floor of discovery.
In this talk I will be introducing Fiat Physica, the new crowdfunding platform connecting scientists, educators, and communicators with these avid science enthusiasts. Fiat Physica provides the unique opportunity to build a supportive, engaged community who cares about your research, invention, or other scientific endeavor. The success and adoption of new findings, technologies, and initiatives often lie in the non-financial support they receive. Crowdfunding allows you to raises funds, support and awareness in one combined effort. Unlike generic crowdfunding sites, Fiat Physica drives targeted traffic directly to your campaign to amplify your campaign's reach. Our niche blog and social media channels attract visitors to your campaign who are already interested in what you have to offer.

# The ExaVolt Antenna: Concept and Development

6/30/15
Carl Pfendner (OSU Physics)

In the past decade, searches for the cosmogenic neutrino flux produced by the interactions of ultra-high energy cosmic rays with the cosmic microwave background have not yet resulted in detection. Radio detection of ultra-high energy neutrinos provides a cost-effective means of probing a large effective volume. The Antarctic Impulsive Transient Antenna (ANITA) balloon-borne experiment, with sensitivity to neutrinos with energies >1019 eV, has provided some of the most stringent limits on cosmogenic neutrino production models by searching for coherent radio Cherenkov signals produced by the Askaryan effect in Antarctic ice. The ExaVolt Antenna (EVA) is a mission concept to extend the sensitivity of balloon-borne radio neutrino detection to energies 1017 eV. EVA uses a novel antenna design that exploits the surface area of the balloon itself to provide a reflector antenna with 30 dBi gain (compared to 10 dBi on ANITA). I will present recent developments in this detector concept including results of a scale model hang test.

# The luminosity function of satellite galaxies to z = 1.5

6/30/15
Anna Nierenberg (OSU Physics)

The standard cold dark matter model predicts that halos should contain large numbers of subhalos, characterized by a steep mass function. In contrast, the visible satellites of the Milky Way have a much flatter mass function. The solution to this so-called "missing satellite" problem is most likely tied to the fact that galaxy formation becomes more inefficient in low mass halos, due to a complex combination of cooling, star formation, and feedback processes. It may also involve a modification to the standard CDM model. Distinguishing between these alternatives, and even quantifying the nature of the problem, has been hampered by the fact that the lowest mass satellites are only well measured in our own Galaxy and at low redshift. I discuss my current work to use the near-IR HST CANDELS survey in order to study the properties of luminous satellites down to unprecedented depths and look-back times. I will be presenting some *all new* very preliminary measurements of the luminosity function of faint (sub SMC luminosity) satellite galaxies to redshift 1.5, and comparing with theoretical predictions from Millennium and a warm dark matter simulation.

# Is the Galactic Bulge Devoid of Planets?

7/7/15
Matthew Penny (OSU Astronomy)

Gravitational microlensing surveys have comparable sensitivity to planets orbiting stars in both the Galactic bulge and disk. I will show that the distribution of measured and inferred distance estimates to microlensing planet hosts differs substantially from what one would naively expect. I will outline a number of possible solutions to this puzzle, including that planet abundance may depend on a star's birth environment or perhaps more simply that the distance estimates may be unreliable. I will end by discussing how upcoming microlensing campaigns by Spitzer and K2 will help to resolve this issue.

# Zel'dovich decomposition: Probabilistic segmentation of the cosmic web into clusters, filaments, walls, and voids

7/7/15
Peter Melchior (OSU Physics)

I will present a novel method to analyze the cosmic web, based on the well-known Gaussian Mixture Model (GMM). By distinguishing GMM components with oblate, prolate or spherical shape, I can assign probabilities for any test sample to reside in walls, filaments, or clusters. I will demonstrate the method by decomposing the entire BOSS survey and discuss the potential of the method for photometric surveys, in particular DES. I will also give an outlook to a number of cosmological and astrophysical studies that are enabled with such a decomposition.

# Cosmology with Baryon Acoustic Oscillation Scale Measurements: Current Status and Future Prospects

7/14/15
Ashely Ross (OSU Physics)

I will describe how measurements of the baryon acoustic oscillation (BAO) scale embedded in the distribution of galaxies can be used to constrain cosmological parameters, especially the nature of dark energy. I will then describe the current state-of-the BAO measurements, which have been made using data from the Sloan Digital Sky Survey (SDSS) I-III. I will conclude by describing results to look forward to in the future, including those using the completed data set of the SDSS-III baryon oscillation spectroscopic survey (BOSS), data from the SDSS-IV extended BOSS, and data from the Dark Energy Survey.

# Cosmic Rays and Neutrinos as Complementary Probes of Ultra-High Energy Astrophysics

7/14/15
Nathan Griffith (OSU Astronomy)

Ultra-high energy cosmic ray and neutrino spectra provide us with a complementary set of information that can be used to uncover a variety of information about ultra-high energy astrophysics. Via a model that employs probability tables calculated by CRPropa, this work uses fits to the Pierre Auger Observatory's 2013 data to generate neutrino spectra. Then, looking through the lens of current and next generation neutrino detectors (ANITA, ARA EVA), the parameters of source evolution and source spectrum cutoff at ultra-high energies are inspected.

# The Sensitivity of Astrophysical Anisotropy Signals to Spectral Lines

7/21/15
Sheldon Campbell (OSU Physics)

There has recently been new progress in the theoretical understanding of the statistical uncertainty for measurements of the angular power spectra of sources in high energy astrophysics experiments. The implications of these results for anisotropy experiments are explored; in particular, we'll discuss using the detected radiation's angular distribution to detect a weak spectral line in a diffuse background. Applied specifically to the problem of observation of a line from annihilating dark matter in the diffuse gamma-ray background, anisotropy analyses will be shown to be important for unravelling dark matter information from their annihilation signatures.

# The AGN Space Telescope and Optical Reverberation Mapping Project

7/28/15
Kelly Denney (OSU Astronomy)

Several members of the department are involved in a monumental reverberation mapping project (PI - B.M. Peterson) that consisted of ~170 HST/COS orbits and >1000 supporting SWIFT and ground-based observations across the UV/optical wavelength range. This project represents the most detailed (in terms of combined wavelength and temporal coverage) study of reverberation mapping of a single target, namely, NGC 5548, a nearby Seyfert 1 galaxy. While I am not the primary person completing any part of the analysis of this project, I thought this would nonetheless be a good opportunity to present locally the program and initial results that have been completed by many members of the OSU astronomy community and other collaborators from the STORM Project.

# Far-IR temperature-insensitive metallicity diagnostics

7/28/15
Kevin Croxall (OSU Astronomy)

With the advent of the latest generation of NIR spectrographs, new diagnostics of the physical properties of high-redshift galaxies have now become attainable. These instruments have facilitated high-quality measurement of multiple nebular emission lines for 100s of high-z galaxies. The first results from large surveys have shown there to be important discrepancies between the physical conditions and abundance patterns within high-z and local star-forming galaxies. In light of these new observations, a more sophisticated understanding of massive stellar evolution and photoionization modeling is required to interpret the data in a physically meaningful manner. I will discuss the use and usefulness of the FIR fine-structure lines in understanding the abundance patterns and physical conditions of ionized gas in local galaxies. Additionally, I will discuss their use in high-z star forming galaxies.

# Come, and learn all about agnostic stacking!

8/11/15
Stephan Frank (OSU Astronomy)

Utilising the largest available high-quality COS dataset towards quasars at redshifts z>0.7, we probe weak NeVIII absorbers to an unprecedented low column density limit (log N > 12.3) via a new method ('agnostic stacking'). The total pathlength for NeVIII detection (Δz = 9.3), and high S/N of the data in conjunction with this new statistical method allow us to place tight constraints for the slope and normalisation of the column density distribution function over a range of column densities not accessible to direct searches. We find that this method (with modelling of the absorber population and noise characteristics) is a powerful tool for measuring doublet absorption at the noise limit.

# Data Science From the Ground Up: Introducing a New FITS Viewer: A New Hope

8/11/15
Dmitri Muna (OSU Astronomy)

The astronomical community is ahead of most other scientific disciplines in that we've standardized on a single data format. This is not a trivial accomplishment. However, the software we use to work with FITS files is incomplete, out of date, and is a user interface nightmare. The most basic step of working with data is visualization, regardless of format. I will demonstrate a new, modern FITS viewer I am developing to address these issues.

# Astrophysical Models for the Galactic Center GeV Excess

8/18/15
Tim Linden (OSU Physics)

Observations taken over the last 5 years have indicated an excess in gamma-rays emanating from the center of the Milky Way galaxy. While one potential explanation for this emission is dark matter annihilation, several astrophysical explanations have also been proposed. I will briefly describe the current state of both observations and models of the galactic center excess, concentrating specifically on several recent studies evaluating the possibility that millisecond pulsars in the galactic center region contribute significantly to the gamma-ray excess.

# No galaxy left behind: Precision cosmology at the limits of imaging surveys with Balrog

8/18/15
Eric Huff (OSU Physics)

Accurate statistical measurement with large imaging surveys has traditionally required throwing away a sizable fraction of the data. This is because most measurements have have relied on selecting nearly complete samples, where variations in the composition of the galaxy population with seeing, depth, or other survey characteristics are small. I will describe Balrog, our implementation of a new method for accurately characterizing the likelihood function of astronomical imaging survey measurements using simulated galaxies injected into the real images. I'll show an example where we are able to make accurate (statistics-limited) measurements of the angular clustering of the faintest galaxies in the Dark Energy Survey imaging.

# Physics at the End of the Electromagnetic Spectrum: First Results from HAWC

8/25/15
Segev BenZvi
(U. of Rochester)

During the past decade, the study of TeV gamma rays -- the highest energy electromagnetic radiation yet observed -- has opened up a new window on the Universe. A new generation of powerful detectors has led to the discovery of more than 150 Galactic and extragalactic sources of TeV gamma rays. These measurements provide evidence for cosmic-ray acceleration, enable studies of cosmological radiation fields, and allow searches for dark matter annihilation. Unfortunately, nearly all measurements in this energy band have been pointed observations and are affected by survey bias. The High-Altitude Water Cherenkov (HAWC) Observatory, an air shower array recently completed in Sierra Negra, Mexico, is designed to provide an unbiased survey of TeV gamma rays from the northern sky. The HAWC detector has an instantaneous field of view of 2 sr, operates with >95% uptime, and observes two-thirds of the sky each day. HAWC is built to fill in gaps in TeV measurements that are difficult to accomplish with pointed observations, including measurements of extended Galactic sources, diffuse gamma-ray emission, and monitoring of transients such as gamma-ray bursts. I will review the current status of HAWC, present first results from the observatory, and discuss prospects for measurements with HAWC during the next 5 years.

# Hunting the First Galaxies with Gravitational Lensing

9/1/15
Dan Coe
Space Telescope Science Institute (STScI)

The first galaxies are the next frontier of extragalactic research. How and when did they form, and how did they contribute to reionization? Answers to these questions require both census taking and detailed studies of individual galaxies. The highest redshift searches (z ~ 9 - 12) have yielded fewer candidates than expected, leaving luminosity functions highly uncertain while hinting at accelerated evolution in the first 600 Myr. I will present preliminary new results from the Frontier Fields program that support this finding. Gravitational lensing improves the discovery efficiency of high-redshift candidates bright enough for follow-up study. The Cluster Lensing And Supernova survey with Hubble (CLASH) Multi-Cycle Treasury program revealed candidates as distant as MACS0647-JD at z ~ 10.8 (420 Myr) lensed to 26th magnitude AB. I will present preliminary results from our Hubble grism and Spitzer imaging programs following up the z ~ 11 candidate. To discover even more brightly lensed distant galaxies, I am leading a new 190-orbit Hubble Treasury program called RELICS, the Reionization Lensing Cluster Survey. By observing 41 massive clusters, RELICS will deliver the best and brightest high-redshift candidates in time for the November 2017 JWST GO Cycle 1 call for proposals.

# Sweating the small stuff: Simulating dwarf galaxies, ultra-faint dwarf galaxies, and their own tiny satellites

9/8/15
Coral Wheeler
(UC Irvine)

If LCDM is correct, then all dark matter halos hosting galaxies, from those hosting dwarfs to those hosting giant clusters, are filled with abundant substructure down to very low mass scales (<< 10^9 Msun). Specifically, even the dark matter halos of Local Group field dwarfs should be filled with subhalos, many of which should be fairly massive (~ 10^8 Msun), and thus are potential targets for hosting small (ultrafaint) galaxies. Here we make predictions for the existence of ultrafaint satellites of dwarf galaxies using the highest resolution cosmological dwarf simulations yet run (mgas~ 250 Msun). We simulate four halos -- two each at the mass of classical dwarf galaxies (Mvir ~10^10 Msun) and ultrafaint galaxies (Mvir ~ 10^9 Msun) -- down to z=0 using the GIZMO (Hopkins 2014) code. This code relies on state-of-the-art MFM hydrodynamics and implements the FIRE (Feedback in Realistic Environments) recipes (Hopkins et al. 2014) for converting gas into stars and capturing the energy fed back from those stars into the surrounding medium. We predict that ultrafaint galaxies (M* ~ 3,000 Msun) should exist as satellites around more massive dwarf galaxies (M* ~ 10^6 Msun) in the Local Group. These tiny satellites, as well as the two isolated ultrafaints, have uniformly ancient stellar population (> 10 Gyr) owing to reionization-related quenching. The more massive systems, in contrast, all have late-time star formation. Our results suggest that Mhalo ~ 5 x10^9 Msun is a probable dividing line between halos hosting reionization "fossils" and those hosting dwarfs that can continue to form stars in isolation after reionization. Importantly, we show that the extended ~50 kpc regions around Local Group "field" dwarfs may provide efficient search locations for discovering new ultrafaint dwarf galaxies, and discuss the prospects for their discovery in light of the new generation of large surveys and giant telescopes. If these tiny satellites are observed, this would provide evidence that dark matter substructure is truly hierarchical, as predicted in the standard paradigm.

# Confronting the Fermi Galactic Center Excess with 3D Models of the Gamma-Ray Sky

9/16/15
Eric Carlson
(UCSC)

Physical models of diffuse Galactic gamma-ray emission have so-far assumed that the injection and propagation of cosmic rays in the Galaxy may be treated as axisymmetric, with cosmic-ray sources tracing the observed distributions of OB stars, pulsars, or supernova remnants; all of which underestimate the cosmic-ray population at the Galactic center. In this seminar I will move beyond these assumptions and introduce three novel model elements: fully 3D cosmic-ray propagation, 3D gas maps, and 3D cosmic-ray injection tracing the distribution of supernovae-harboring H2 regions in the Galaxy. I will show that a physically well-motivated fraction f_H2~15%-20% of cosmic-ray sources tracing H2 (i) globally improves the quality of the fit to the observed diffuse gamma-ray emission, and (ii) highly suppresses residual gamma-ray emission from the Galactic center region. I will then discuss in detail, the impact of these changes on the Fermi Galactic Center GeV excess as well as briefly outlining future avenues for 3D diffuse gamma-ray emission modelling.

# Impact of Gravitational Slingshot of Dark Matter on Galactic Halo Profiles

9/18/15
Yao-Yu (Joshua) Lin
(National Taiwan U)

We study the impact of gravitational slingshot effect from massive astrophysical objects (e.g. stars, black holes) on the distribution of cold dark matter in Milky Way sized galaxies and dwarf galaxies. Multiple gravitational encounters of a lower mass dark matter particle with massive astrophysical bodies would lead to an average energy gain for the dark matter, similar to second order Fermi acceleration. We calculate the average energy gain and model the integrated effect on the dark matter profile. We find that such slingshot effect due to the intermediate mass black holes in dwarf galaxies were significant in certain cases, which changes the dark matter distribution at the galactic center and alleviate small scale problems of cold dark matter.

# Tidal streams in triaxial systems

9/23/15
(Columbia U)

Stellar tidal streams form from the steady disruption of stellar systems orbiting within the gravitational field of a larger host galaxy. Many streams and debris structures have been discovered in the halo of the Milky Way and it is hoped that these streams will provide strong constraints on the large-scale distribution of dark matter around the Galaxy. I will first describe a new dynamical model for using kinematic data for stream stars to infer the Milky Way potential. I'll then talk about theoretical expectations for the morphological evolution of tidal debris in complex potentials (e.g., triaxial, time-varying) where we expect an appreciable number of chaotic orbits. Here I'll focus on our recent work studying how chaos can alter the density evolution of streams and may be important for the survivability and observability of tidal streams. I'll discuss how the mere existence of thin tidal streams around the Milky Way may provide interesting constraints on the allowed shape and lumpiness of the Galactic dark matter halo.

# Some thoughts on Higgs portal to dark matter

9/25/15
Jure Zupan
(U. of Cincinnati)

In the talk I will cover two aspects of Higgs portal dark matter: the effect of non-standard Higgs Yukawa couplings, and the searches for the mediators that need to be present in the case of fermionic dark matter.

# CMB cosmology with ACT, Planck and ACTPol

9/29/15
Renee Hlozek
(Princeton)

CMB cosmology is currently undergoing a data-rich epoch, with measurements on small scales from experiments like the Atacama Cosmology Telescope (ACT) and it polarisation instrument, ACTPol, adding to measurements on larger scales by Planck, WMAP and most recently BICEP. I will contextualise the measurements and present constraints on parameters from the observations at 148 GHz and 217 GHz respectively by ACT from three years of observations. I'll discuss my recent re-analysis of data from the 2013 data release by the Planck satellite, where we found that the 217GHz x 217GHz detector set spectrum used in the Planck analysis is responsible for some of the tension between the Planck parameters and other astronomical measurements. I'll show evidence suggesting residual systematics in the detector set spectra used in the Planck likelihood code, and discuss how the picture has changed with updated Planck data, and put things in context with the BICEP results. I'll highlight the recent ACTPol results, and outline how upcoming information from various cosmological probes will open up the window on the epoch of reionisation; our least explored epoch to date.

# Star formation and high energy neutrinos at IceCube: a correlation?

10/6/15
Cecilia Lunardini
(Arizona State U)

The IceCube Neutrino Observatory has provided the first map of the high energy (~ 0.01 -- 1 PeV) sky in neutrinos. Since neutrinos propagate undeflected, their arrival direction is an important identifier for sources of high energy particle acceleration. Reconstructed arrival directions are consistent with an extragalactic origin, with possibly a galactic component, of the neutrino flux. We present a statistical analysis of positional coincidences of the IceCube neutrinos with known astrophysical objects from several catalogs. For the brightest gamma-ray emitting blazars and for Seyfert galaxies, the number of coincidences is consistent with the random, or "null", distribution. Instead, when considering starburst galaxies with the highest flux in gamma-rays and infrared radiation, up to n = 8 coincidences are found, representing an excess over the ~4 predicted for the null distribution. The probability that this excess is realized in the null case, the p-value, is p = 0.042. This value falls to p = 0.003 for a set of gamma-ray-detected starburst galaxies and star-forming regions in the galactic neighborhood. Therefore, it is possible that these might account for a subset of IceCube neutrinos. The physical plausibility of such correlation is discussed briefly.

# They need you (more than you need them)

10/13/15
Fraser Cain
(Universe Today)

The internet gives science communicators direct access to the audiences that want to learn more about the latest developments, bypassing traditional media. But there are so many social networks, apps and platforms that it's almost impossible to stay on top of everything. In this talk, Fraser Cain, publisher of Universe Today - a website read by 38 million people in 2014 - describes the systems and methods he used to build an enormous science communications platform. What worked, what didn't, and what role do journalists and the media play in this modern age?

# An Early-Universe Boost to the Dark Matter Annihilation Rate

10/20/15
(North Carolina)

Our ignorance of the Universe's evolution prior to the onset of Big Bang Nucleosynthesis profoundly limits our understanding of dark matter: we cannot calculate its relic abundance without knowing when the Universe became radiation dominated. Fortunately, there is another probe of the early Universe that could break this degeneracy. I will show how an effectively matter-dominated era prior to the onset of nucleosynthesis can radically enhance the population of microhalos for both thermal and nonthermal dark matter. I will then discuss how the resulting abundance of substructure affects the dark matter annihilation rate, which opens up the possibility of using gamma-ray observations to learn about the reheating of the Universe and the origins of dark matter.

# Galaxies in the balance

10/27/15
John Forbes
(UCSC)

Over the past decade a quiet revolution has taken place in our picture of galaxy evolution. Rather than evolving through violent mergers, galaxies live quiet lives on narrow scaling relations. This is explained elegantly by the so-called "equilibrium model" of galaxy formation. I will present a simple extension to this model that can explain the finite scatter observed in galaxy scaling relations as the consequence of variability in cosmological accretion. I will also show how the same variability that gives rise to scatter in these relations, when coupled with the physics of gas transport within disks, can explain the structure of gas disks in local and high redshift galaxies. Finally I will present the results of detailed simulations of isolated dwarf galaxies. With these simulations I will show how ordinary photoelectric heating can explain the long depletion times observed in dwarfs, and I will argue that the equilibrium model can still be applied to these galaxies despite their long depletion times.

# Common Envelope and the Formation of Close Neutron Star Binaries

11/3/15
Morgan MacLeod
(UCSC)

How do close neutron star binaries form? Many aspects of our understanding of the complex channels through which compact binaries form remain uncertain. This talk focuses on a crucial phase in the formation of close binary pairs of neutron stars: a common envelope episode. During the common envelope phase, one star evolves to engulf its companion in a shared gaseous envelope. Before this envelope is ejected, drag forces pull the two stellar cores into a tighter orbit. But the neutron star can also accrete from the surrounding gas. This talk explores the hydrodynamics of the common envelope phase in order to model the metamorphosis of the neutron star and the binary orbit.

# Limits on cascade annihilation models and decaying dark matter lifetime from dwarf galaxies using Fermi-LAT

11/6/15
Tathagata Ghosh
(Texas A&M)

Dwarf spheroidal galaxies are promising targets for the indirect detection of dark matter through gamma-ray emission due to their proximity, lack of astrophysical backgrounds and high dark matter density. They are often used to place restrictive bounds on the dark matter annihilation cross section. Many particle dark matter models predict that the dark matter undergoes cascade annihilations, i.e. the annihilation products are 4-body final states. In the context of model-independent cascade annihilation models, we review the compatibility of the dark matter interpretation of the Fermi-LAT Galactic center gamma-ray emission with null detections from dwarf spheroidal galaxies using six years of Fermi-LAT data. In addition, we present the analysis of data from 20 Dwarf Spheroidal galaxies and derivation from a stacked analysis, robust 95% confidence level upper limits on the dark matter lifetime for several decay channels and dark matter masses between 10 GeV and 10 TeV.

# Condensed Dark Matter: An Axion Story

11/9/15
Chanda Prescod-Weinstein
(MIT)

Recently there has been significant interest in the claim that axions, a popular dark matter candidate, form a Bose-Einstein condensate on a scale that has consequences for structure formation. I will describe my work examining this claim. It is clear that while Bose-Einstein condensates of axions form, they don't have the structure that was initially expected. Yet again, it matters whether particles are attracted or repulsed by one another.

# Single and Double Degenerate Pathways towards Accretion-Induced Collapse

11/10/15
Josiah Schwab
(UC Berkeley)

A white dwarf in a binary system can collapse to a neutron star due to stable accretion from a non-degenerate companion or due to a merger with a white dwarf companion. In this talk, I will discuss the evolution of these systems prior to collapse. I will present simulations of accreting oxygen-neon WDs performed using the the state-of-the-art MESA stellar evolution code. These include previously neglected effects such as Urca-process cooling and are able to reach length-scales that directly connect full-star simulations to past studies of the onset of the collapse process. I will also discuss work exploring the long-term outcome of the merger of two carbon-oxygen WDs. Beginning with simulations of the short-lived viscous disk initially present in these remnants and then following the subsequent thermal evolution with MESA, I will outline the interesting path that leads to their final fate.

# The Milky Way in Stereo: Constraints on the Galactic Gravitational Potential from Multiple Stellar Streams

11/17/15
Ana Bonaca
(Yale)

Stellar streams are powerful constraints of the Galactic gravitational potential, but because the true potential form is unknown, individual streams can produce very biased results. Most potential recoverymethods rely on full, observationally expensive, 6D information for the stream member stars. Consequently, current constraints of the Milky Way potential are based on individual streams, with some tension between different streams.

These discrepancies can be resolved by simultaneously modeling multiple stellar streams that have been discovered in the Galactic halo. We use two most prominent cold streams in the Milky Way, tidal tails of the Palomar 5 globular cluster and GD-1 stream, to measure the global properties of our dark matter halo. Based on the analysis of synthetic streams on similar orbits in a realistic dark matter-only simulation, we discuss which new data would most improve our understanding of the Galactic gravitational potential.

# Cosmic Ray Anisotropy with Partial Sky Exposure

11/20/15
Peter Denton
(Vanderbilt)

UHECRs are the highest energy particles in the universe, yet very little is known about them. Their composition, sources, acceleration, and propagation details are all wholly unknown. The first step to addressing this problem is determining the sources, which requires measuring an anisotropy. Above $E\sim55$ EeV, anisotropies are expected to appear due to the GZK horizon, yet no definitive signal has been seen. Here I overview the current experimental status and present a discussion of anisotropy reconstruction techniques, along with their strengths and weaknesses. I use spherical harmonics as a general tool to detect large scale anisotropies in a low statistics environment. I compare the benefits of a full sky experiment such as JEM-EUSO to ground based partial sky experiments such as the Pierre Auger Observatory and Telescope Array. I show that while Auger can reconstruct a quadrupole without a partial sky penalty, partial sky exposure generally leads to a loss of precision beyond that just from lower statistics compared to a full sky experiment.

# Disentangling Fundamental Physics from Subtle Systematics in Cosmic Shear and Large-Scale Structure

11/24/15
Michael Troxel
(Manchester)

We now have the potential to produce constraints on fundamental physics from measurements of weak gravitational lensing and large-scale galaxy clustering with unprecedented precision. Even with its first year of data, the Dark Energy Survey (DES) will deliver weak lensing measurements covering an area that is more than five times that of previous state-of-the-art measurements. To take full advantage of the statistical power of DES and future surveys, however, we must control the impact of subtle, yet very large systematics like the intrinsic alignment of galaxies. To isolate these systematics from the lensing signal requires an equally precise knowledge and validation of photometric redshifts. I will discuss these challenges and the work we've done to address them in the DES Science Verification data. I will summarise the first measurements of cosmic shear from DES and resulting cosmological constraints, and I will comment on how we can improve methods for dealing with these potentially catastrophic systematics.

# Cosmic neutrinos - what we know, what we suspect

12/1/15
Markus Ackermann

I will review the latest IceCube results on the properties of the cosmic neutrino flux and their implications. While the origin of the neutrinos remains unknown, certain scenarios about their origin seem to be disfavored. There are first indications that the high-energy neutrino sky is remarkably different to the high-energy gamma-ray sky, even though both particles are produced in the same processes.

# Revolutionizing our understanding of galaxy evolution through simulations of the CGM

12/8/15
Cameron Hummels
(Caltech)

Cosmological hydrodynamics simulations are increasingly able to reproduce galaxies like those we observe in nature. However, the detailed structure of the circumgalactic medium (CGM), the tenuous gas around galaxies, remains a very difficult observable for simulations to accurately model. As the interface between star formation, feedback, and galactic gas accretion, the CGM is the key to understanding what drives galactic evolution. I will discuss the primary challenges and solutions in modeling and understanding the CGM including new feedback prescriptions, new simulation characteristics, and increased halo resolution. Lastly, I will present an open-source code for generating synthetic spectra to compare simulations with observations, as well as a new paradigm for sharing simulated data products between members of the community.

# 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.

# 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.

# 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.

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