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Hubble Fellows Symposium 2013
Talk Abstracts

Listing of Talk Abstracts

Numerical Modeling of Accretion Disks: Local Simulations of the Magnetorotational Instability and Disk Outflow
Dr.  Xuening Bai (Smithsonian Astrophysical Observatory)
The magnetorotational instability (MRI) is believed to be the primary mechanism for driving accretion in a wide range of astrophysical disks. The non-linear saturation of the MRI is best studied by shearing-box simulations, which represents a local patch of a thin accretion disk. The properties of the MRI sensitively depend on the geometry of large-scale magnetic field, which is represented by the net vertical magnetic flux in the shearing-box approach. I will describe our most recent local simulations of the MRI that for the first time, include relatively strong vertical magnetic field (with plasma beta of the net field at midplane up to 100). Simulation results reveal that the Shakura-Sunyaev alpha increases from ~0.01 for the zero net-flux case to order unity with sufficiently strong net vertical flux. In addition, the disk always launches an outflow in the presence of net vertical flux. Although the fate of the outflow can not be determined from local studies, we speculate that it is unlikely to be directly connected to a magnetocentrifugal wind.
Bridges and Tails of Interacting Dwarf Galaxies
Dr.  Gurtina Besla (Columbia University)
I will outline ongoing work to systematically quantify the rate of tidal interactions between dwarf galaxies in the local volume and the contribution of such interactions to modes of star formation in low metallicity environments and the baryon cycle of dwarf galaxies. This work involves multi-wavelength studies of a sample of interacting dwarf pairs extracted from SDSS and statistical comparisons to dwarf pairs identified within large-scale Nbody cosmological simulations.
New measurements of the vertical mass distribution near the Sun
Dr.  Jo Bovy (Institute for Advanced Study)
Measurements of the vertical mass distribution near the Sun can be used to constrain the relative contributions of stellar, gaseous, and dark matter to the local gravitational potential, and they provide the most direct constraints on the local density of dark matter. I will present results from a number of recent analyses of the vertical kinematics of disk stars near the Sun.
Far-infrared views of Cosmic Star Formation
Dr.  Caitlin Casey (Hawaii)
I will present recent results in far-infrared observations (from Herschel and SCUBA2) which shed new light on the origins of cosmic star formation and discuss the relation between these observations and more commonly studied optical and rest-frame ultraviolet studied high-z galaxy populations.
Three-Dimensional Simulations of Core-Collapse Supernovae
Dr.  Sean Couch (University of Chicago)
I will discuss my recent work on 1D, 2D, and 3D hydrodynamical simulations of core-collapse supernovae including a parameterized neutrino heating and cooling scheme. In contrast to some previous works, I find that 3D simulations explode later than 2D simulations. I find, however, that in 2D the explosion delay time increases as the numerical resolution of the simulation increases. The balance between neutrino-driven-buoyancy and drag forces is a critical ingredient in determining explosion delay time. In particular, I will show that the drag force is proportional to a buoyant plume’s surface area while the buoyant force is proportional to a plume’s volume and, therefore, plumes with greater volume-to-surface area ratios will rise more quickly. My work shows that buoyant plumes in 2D are inherently larger, with greater volume-to-surface area ratios, than plumes in 3D. In the scenario that the supernova shock expansion is dominated by neutrino-driven buoyancy, this balance between buoyancy and drag forces may explain why 3D simulations explode later than 2D simulations and why explosion delay time increases with resolution. I will also compare my results with other calculations in the literature and discuss the relevance of semi-analytic explosion indicators.
Live fast, die young - The Evolution of Massive Stars towards their Death: Rotation, Binarity and Mergers
Dr.  Selma E. de Mink (STScI / JHU)
Although they are rare and short-lived, massive stars play a major role in Universe since the formation of the very first stars. With their large luminosities, strong stellar winds and spectacular explosions they act as cosmic engines, heating and enriching their surroundings, where the next generation of stars and their planets are forming. I will highlight recent developments in the massive star community, triggered by new surveys and theoretical modeling, that are changing our general picture of how massive stars live their lives and die. In particular, I will discuss new insight in impact of rotation and the importance of binarity and the frequency of mergers. A wide variety of astrophysical problems rely on the predictions of classic stellar models which do not account for these effects. I will argue that these developments call for a critical reconsideration of the impact on the role of massive stars as cosmic probes and cosmic engines, nearby and at high redshift.
The Cold and Broken Milky Way Stellar Halo
Dr.  Alis Deason (UCSC)
The phase-space structure of the Milky Way stellar halo is intimately linked to its accretion history. I will discuss the connection between the (broken) density profile of the stellar halo and its formation via the accretion of dwarf galaxies. I will also present a new measure of the velocity anisotropy of halo stars out to 30 kpc based on accurate proper motions from multi-epoch HST imaging. Intriguingly, the location of the 'break' in the stellar halo density profile is coincident with a tangential dip in the velocity anisotropy profile of the halo, which may suggest that there is a shell-type structure in this radial regime. Finally, I will discuss the surprisingly cold radial velocity dispersion of the most distant halo stars (beyond 100 kpc), and the implications this has for the total mass of our Galaxy.
Feeding the supermassive black hole in the Milky Way
Dr.  Nathalie Degenaar (University of Michigan)
Accretion onto supermassive black holes, and the associated feedback to their surroundings, lie at the foundation of their formation, growth and evolution, the chemical enrichment of the interstellar medium, galaxy evolution and the formation of large scale structures in the Universe. Sagittarius A*, the dynamical center of the Milky Way, is the most nearby supermassive black hole and allows for an unparalleled study of the fueling process for such objects. Although our Galactic nucleus is relatively inactive at present, it was recently discovered that a cloud of gas of ~3 Earth masses is on a collision course with Sagittarius A* and is projected to make a close encounter in the fall of 2013. The cloud is expected to become disrupted by gravitational interaction, feeding gas to the supermassive black hole. This presents an unique and exciting opportunity to closely study how accretion onto Sagittarius A* proceeds. I'll discuss the results of a daily X-ray monitoring campaign using the Swift satellite. This program has offered a unique view of the long-term accretion behavior of Sagittarius A* and provides an excellent setup to closely monitor the interaction of the supermassive black hole with the dense cold gas cloud later this year.
Probing AGN/Galaxy Co-Evolution with Spitzer-Selected AGN
Dr.  Jennifer Donley (Los Alamos National Laboratory)
Spitzer IRAC selection is a powerful tool for identifying luminous AGN. The current set of IRAC AGN selection wedges, however, were defined for use in shallow surveys and are heavily contaminated by star-forming galaxies in the deep IRAC data now available across most extragalactic deep fields. Using the large samples of luminous AGN and star-forming galaxies in COSMOS, we redefine the IRAC AGN selection criteria for use in deep surveys. The new criteria are both highly complete and reliable, and recover a sample of luminous, high-redshift, and heavily obscured AGN. As such, IRAC AGN represent the ideal sample with which to test the role of major mergers in triggering SMBH growth and driving AGN/galaxy co-evolution. Using deep NIR imaging from the CANDELS survey, we constrain the rest-frame optical morphologies of the hosts of IRAC and X-ray selected AGN in CANDELS/COSMOS. While mergers and/or interactions appear to be equally common among IRAC and X-ray selected AGN, the hosts of luminous IRAC AGN are significantly more disturbed than their X-ray detected counterparts. This indicates that while secular evolution remains important at lower redshifts/luminosities, mergers likely play a significant role in triggering the growth of luminous, high redshift AGN.
Warm Spitzer Parallaxes of Cool Y Dwarfs
Trent Dupuy (Smithsonian Astrophysical Observatory)
Members of the new spectral class of Y dwarfs are estimated to have temperatures of ~300-400 K, colder than any previously known free-floating brown dwarfs. However, direct distances are needed to determine model-independent temperatures for this sample and to test their observed properties against current theoretical models in this new physical frontier. I will present results from our recently completed DDT program using Spitzer to measure parallaxes for the coldest brown dwarfs discovered in WISE data. In spite of its large pixels, Spitzer is the ideal tool for obtaining the astrometric precision needed for this sample, as it is capable of obtaining many high SNR images for our targets in mere minutes. The astrometric analysis takes advantage of our new distortion solution for IRAC, which results in an rms of ~10-20 milliarcsec (~1% of a pixel) in position measurements that is suitable for measuring parallaxes of objects within ~15 pc. The >2-year time baseline achieved by combining our new Spitzer data from 2012 with WISE+Spitzer data from 2010 enables us to robustly break the degeneracy between parallax and proper motion, resulting in distance errors of 6%-20% and temperature uncertainties of 30-50 K. Our distances also enable fundamental tests of theoretical models by placing these coldest brown dwarfs on color-magnitude diagrams, where they lie up to ~5 mag below previously known objects at near-IR wavelengths.
Lessons from cosmic history - The evolution of the star formation rate density
Robert  Feldmann  (UC Berkeley )
Observations show that star formation in galaxies is closely correlated with the abundance of molecular hydrogen. However, systematics in the observational measurements and challenges in the theoretical modeling complicate a direct determination of the functional form of this relationship. I will discuss how the star formation -- gas relation can be constrained from measurements of the cosmic star formation history and the evolution of global galaxy properties. Specifically, I will show that a linear relation with an H_2 depletion time of ~2.5 Gyr, as suggested by studies of nearby galaxies, results in good agreement with current observations including the evolution of the cosmic star formation rate density out to z~10, the z~4-9 UV luminosity function, the evolution of the mass -- metallicity relation, the relation between stellar and halo mass, and the gas-to-stellar mass ratios of galaxies. In contrast, short depletion times that result from adopting a highly super-linear star formation -- gas relation would imply large star formation rates, substantial metal enrichment (~0.1 solar), and low gas-to-stellar mass ratios already at z~10, in disagreement with observations. These results can be understood in terms of an equilibrium picture of galaxy evolution in which gas inflows, outflows, and star formation drive the metallicities and gas fractions toward equilibrium values that are determined by the ratio of the gas accretion time to the gas depletion time. In this picture, the cosmic modulation of the accretion rate is the primary process that drives the evolution of stellar masses, gas masses, and metallicities of galaxies from high redshift until today.
The abundance of optically thick hydrogen in the high redshift universe
Michele Fumagalli (Carnegie )
Mapping the redshift distribution of the denser gas clouds in the distant universe - the so called Lyman limit systems - has important implications in diverse fields of cosmology, including studies of the UV extra-galactic background, of the cosmic metal budget, and, more recently, of the gas cycle in galaxies. Several past efforts have provided the census of Lyman limit systems across most of the cosmic history, leaving however largely unexplored the redshift interval between 2.5-3.5, right at the peak of the cosmic star formation history. To fill in this gap, we have recently completed a blind survey of ~100 quasars using the blue sensitive MagE spectrograph at the Magellan telescope. With these data, we have established the number density of Lyman limit systems and calculated the mean free path of ionizing photons in the z~2.5-3 universe. Owing to the moderate signal-to-noise and resolution of these spectra, we have further investigated the typical metal content of these systems. This study adds new evidence in support of the hypothesis that Lyman limit systems are associated to the halos of galaxies.
Reconstructing Andromeda's Past: Properties of the Stellar Halo
Karoline Gilbert (University of Washington)
Diffuse, extended stellar populations encode information about the earliest stages of a galaxy’s formation. The Andromeda galaxy is distant enough to facilitate global studies of its stellar halo, while being close enough to allow spectroscopic observations of individual stars. The SPLASH survey utilizes photometric and spectroscopic observations of red giant branch stars to study Andromeda's extended stellar halo and dwarf satellite population. We present the surface brightness profile and metallicity distribution of Andromeda's stellar halo, based on spectroscopically selected samples of Andromeda stars. We trace Andromeda's power-law profile halo out to 2/3 of Andromeda's virial radius, identify a gradient in the metallicity distribution of halo stars, and investigate the effect of substructure on the measured properties of Andromeda's halo.
Main-belt comets as tracers of ice in the inner solar system
Dr.  Henry Hsieh (University of Hawaii)
As a recently recognized class of objects exhibiting apparently cometary (sublimation-driven) activity yet orbiting completely within the main asteroid belt, main-belt comets (MBCs) have revealed the existence of present-day ice in small bodies in the inner solar system and offer an opportunity to better understand the thermal and compositional history of our solar system, and by extension, those of other planetary systems as well. Achieving these overall goals, how- ever, will require meeting various intermediate research objectives, including discovering many more MBCs than the currently known seven objects in order to ascertain the population’s true abundance and distribution, confirming that water ice sublimation is in fact the driver of activity in these objects, and improving our understanding of the physical, dynamical, and thermal evolutionary processes that have acted on this population over the age of the solar system. I will review the latest progress on these objectives and outline research that remains to be done.
Circumstellar Studies with SEEDS
Dr.  Markus Janson (Princeton University)
Direct imaging is playing an increasingly important role in the study of exoplanets, and allows to study planets in environments that are unattainable by other means, such as on wide orbits, or in their young natal environments. SEEDS (Strategic Exploration of Exoplanets and Disks with Subaru) is a high-contrast imaging survey using the AO-assisted HiCIAO camera on the Subaru telescope, with the aim of detecting and characterizing disks and planets, in order to improve the understanding of planet formation and disk-planet interaction. In this talk, I will describe the nature and progress of SEEDS, and show recent results on disk and planet detections.
Physical Properties of Spectroscopically Confirmed Galaxies at z >= 6
Dr.  Linhua Jiang (Arizona State University)
I will present the rest-frame UV properties and morphology of the largest sample of spectroscopically confirmed galaxies at z >= 6. The sample consists of nearly 70 Lyman-alpha emitters (LAEs) and Lyman-break galaxies (LBGs). They have extremely deep optical images in a series of broad and narrow bands. They also have deep imaging data in the near-IR and mid-IR from our HST and Spitzer programs. The wealth of the multi-band data and secure redshifts provide great advantages to measure their physical properties and morphology. I will present many interesting results for this unique sample, including their steep UV continuum slopes, a variety of SEDs and morphology, positional misalignment between UV continuum and Lyman-alpha emission, etc. For example, a small fraction of the galaxies exhibit very steep slopes of beta ~ -3, suggesting the existence of very young stellar populations with extremely low metallicity and no dust content. Another example is that roughly 50% of the brightest galaxies in this sample appear to be merging systems (at z >= 6!). These merging galaxies show a range of Lyman-alpha position offsets relative to their UV continuum positions, including significant positional differences. The LAEs and LBGs in this sample share many common properties, implying that LAEs represent a subset of LBGs with strong Lyman-alpha emission.
The Role of Galaxy Mergers and AGN Among Ultraluminous Infrared Galaxies
Jeyhan Kartaltepe (NOAO)
In the local universe, Ultraluminous Infrared Galaxies (ULIRGs, L_IR>10^12 L_sun) are all interacting and merging galaxies. To date, studies of ULIRGs at high redshift have found a variety of results due to their varying selection effects and small sample sizes. Some studies have found that mergers still dominate the galaxy morphology while others have found a high fraction of morphologically normal or clumpy star forming disks. Near-infrared imaging is crucial for interpreting galaxy structure at high redshift since it probes the rest frame optical light of a galaxy and thus we can compare directly to studies in the local universe. Here, we present the results of a morphological analysis of a sample of high redshift (z~1-3) ULIRGs and compare these results to those at lower redshift in order to constrain how the role of mergers has evolved over cosmic time. These galaxies are selected based on their infrared luminosities measured using 100 and 160 micron data from the GOODS-Herschel and CANDELS-Herschel surveys. We visually classified all of the ULIRGs as well as a comparison sample with the same redshift and H band magnitude range using ACS and WFC3 data. In addition, we present the results of a near-infrared spectroscopic campaign for a large number of high redshift (z>1) ULIRGs in the COSMOS and GOODS fields, with well constrained luminosities from Spitzer and Herschel measurements. We use standard optical diagnostics to study the relative contribution of star formation and AGN activity in this sample and compare our results using several other commonly used diagnostics at these redshifts. We then combine this information with the morphologies to investigate the interconnected role that galaxy mergers and AGN may play among these systems.
Measuring the Relative Velocities of Galaxy Clusters Separated by 100 Mpc
Ryan Keisler (University of Chicago)
After very briefly reviewing the latest measurement of the CMB power spectrum from the South Pole Telescope (SPT), I will describe a measurement of the relative velocities of galaxy clusters that will soon be made possible by combining mm-wave data from the SPT with a catalog of galaxy clusters discovered in the Dark Energy Survey (DES), which recently came online. Assuming a standard model of structure formation, these measurements will yield detailed information on the gas content of the galaxy clusters. Alternatively, if the gas can be sufficiently characterized by other means (e.g. using tSZ, X-ray, or weak lensing), then the relative velocities of the galaxy clusters can be isolated, thereby providing a precision measurement of gravity on 100 Mpc scales.
The Architecture of Circumbinary Planetary Systems
Dr.  Kaitlin Kratter (JILA / CU Boulder)
In the past year, Kepler has unveiled a new class of planetary systems: planets in orbit about main-sequence close binary star systems. These systems provide a unique probe of multiple processes in star and planet formation. I will describe how the existence of such systems can inform our understanding of binary star formation. I will discuss how the system architecture can inform our models of disk evolution and planet formation.
The accretion and evolution of massive black hole pairs and binaries in hierarchical cosmologies
Dr.  Xin Liu (UCLA)
The observed growth of galaxies suggests that massive black hole pairs and binaries on their way to merger should be common, yet direct evidence remains scant. Over the past few years, systematic searches, and in particular those using large spectroscopic surveys, together with multi-wavelength follow-up observations, have enabled statistical investigation of the frequency and demographics of accreting black hole pairs down to separations of a few hundred parsecs. I will review recent progress and discuss ongoing efforts and future prospects to identify and characterize close black hole pairs and binaries at uncharted stages across cosmic time, towards building a complete picture of the life cycles of merging black holes.
The Stellar and Interstellar Properties of Low-Mass Star-Forming Galaxies
Chun  Ly  (STScI )
Star formation directly affects the surrounding interstellar medium (ISM) by replenishing it with gas, metals, and ionizing photons. As such, observables that explore the properties of the ISM and connect them with the stellar properties of the galaxies are key for a more comprehensive study of galaxy evolution. In particular, nebular emission-line measurements of ionized hydrogen and metals are effective at understanding the interstellar properties. I will discuss our on-going efforts to understand the nature of low-mass star-forming galaxies with rest-frame optical spectroscopy and imaging. Using a spectroscopic sample of several hundreds emission-line galaxies at z ≤ 0.85 from the Subaru Deep Field (SDF), we find that (1) the majority of our galaxies are photo-ionized by stars; (2) their emission-line flux ratios suggests a higher ionization state in low-mass galaxies, which is starkly different from the local SDSS sample; and (3) that a subset of our galaxies show higher electron densities of ~100 cm^3 or more. These measurements on the physical properties of the interstellar medium for emission-line galaxies are all similar to those seen in high-redshift galaxies, suggesting that our low-mass galaxies may in fact be low-redshift analogs to the high-redshift population. I will then describe new efforts to conduct an optical and near-infrared spectroscopic study aimed at measuring the metallicity, ionization, and electron density of the ISM for thousands of well-studied galaxies over the past 80% of cosmic time. These observations will allow us to answer a host of questions, which includes an understanding of how stellar feedback from massive stars regulates star formation and enriches the interstellar and circumgalactic media.
The Phoenix Cluster: A New Lease on Life for Cooling Flows
Dr.  Michael McDonald (MIT)
In the cores of galaxy clusters, the intracluster medium, if left undisturbed, should cool and condense onto the central most-massive galaxy, fueling substantial amounts of star formation. The fact that this is not observed in the local Universe is prime evidence that some form of energy input is offsetting cooling, the most promising theory of which is mechanical feedback from the central AGN. In this talk, I will present ongoing research which focuses on tracking this balance between heating and cooling, which is remarkably well-tuned at low redshift, to increasingly higher redshift. In particular, I will highlight the recently-discovered "Phoenix Cluster", which appears to contain a runaway cooling flow fueling a massive starburst of over 700 Msun/yr. This remarkable cluster is providing a unique laboratory to test our theories of AGN feedback, ICM cooling, and galaxy formation.
On using angular cross-correlations to determine source redshift distributions
Dr.  Matthew McQuinn (Berkeley)
I will discuss the use of cross-correlations between populations with poorly known redshifts and better known redshifts to improve the determination of the population redshift distribution of the former. This technique has a wide range of potential applications, from learning the redshifts of anisotropies in a diffuse background map to calibrating the source redshift distribution for weak lensing studies. I will quantify what populations are best suited for redshift distribution reconstruction to a specified level of precision.
Nonthermal Transients from Supernovae in Massive Circumstellar Material
Dr.  Kohta Murase (Institute for Advanced Study)
Recent optical/infrared surveys have suggested that massive stars sometimes experience violent eruptions of circumstellar material before their supernova explosions. We consider collisions of the supernova ejecta with the massive circumstellar material as potential cosmic-ray accelerators, and discuss theoretical aspects of their nonthermal emissions.
Probing the Dawn of Galaxies at z~9-12
Dr.  Pascal Oesch (UC Santa Cruz)
Over the last few years, the advent of the WFC3/IR camera on HST has completely revolutionized galaxy science in the reionization epoch, as it pushed the observational frontier out to z~9-12, only ~450 Myr from the Big Bang. From several large HST programs such as the HUDF09, CANDELS, or CLASH, we were able to identify large samples of more than 200 galaxies at z~7-8, and we are now starting to build up the sample sizes of z~9-11 galaxy candidates. In particular, the recent HUDF12 campaign further increased the depth of the WFC3/IR dataset over the Hubble Ultra-Deep Field (HUDF), and added new F140W filter imaging data. These new data reach to almost 30 AB mag in several NIR filters, which enabled us to detect a sample of nine z>8 galaxy candidates in the HUDF. Using these sources, combined with a search for z~10 galaxies in all the WFC3/IR data around the GOODS-South field, we infer that the cosmic star-formation rate density in galaxies with SFR>0.7Msol/yr decreases very rapidly at z>8, dropping by an order of magnitude from z~8 to z~10. In this talk I will highlight recent progress in exploring the high redshift frontier and in understanding the growth of galaxies out to the reionization epoch.
Stellar Feedback and Its Impact on Molecular Clouds
Dr.  Stella Offner (Yale University)
The most fundamental property of molecular clouds is that they are supersonically turbulent. The origin of the turbulence is unknown, although it is suspected that kinematic feedback from stars may be partially responsible. In order to investigate the origin of the observed turbulence I present multi-physics, adaptive mesh refinement simulations modeling stellar feedback processes acting on molecular clouds. I focus especially on the influence of protostellar outflows, stellar winds, and supernova explosions. I produce synthetic observations of the simulations in molecular line emission in order to contrast these different processes and compare directly to observations.
Core formation: the importance of small particles
Chris Ormel (UC Berkeley)
Traditionally, it has been thought that bodies in the 100m to 100km size range -- customarily referred to as planetesimals -- are the building blocks out of which rocky planets and the cores of gas giants form. In this classical model of planet formation, seeds (protoplanetary embryos) form out of the planetesimals through a process referred to as runaway growth, whereafter they accrete the remainder planetesimals at large gravitational focusing factors. However, several observational and theoretical arguments may invalidate the planetesimal-domined picture. For example, collisions among the planetesimal population are more likely to fragment rather than accrete. Because of these and other reasons I argue that not planetesimals, but much smaller, pebble-size particles are most likely the builiding blocks for planets. However, because small particles couple much stronger to the gas, one needs to reassess the role of the gas during encounters. Recent modeling efforts that address the interactions between small solids (size<~meter) and protoplanets embedded in the nebula, are presented. I will show that a pebble-dominated accretion scenario offers avenues for rapid core formation, and sketch the conditions that must be fulfilled to make this happen.
Neutral Halo Gas in the Cosmic Context
Dr.  Joshua Peek (Columbia University)
Historically, early observations of neutral halo gas represented our first glimpse into our Galactic halo, but the significance of these high velocity clouds has remained elusive in the cosmic context. I will review the observational data we have collected since around our Galaxy and nearby galaxies, as well as some of the work that has been done connecting these neutral halo structures to models of ongoing galaxy formation in the present universe. I'll highlight some new work we've been doing both in synthesizing these data, and in comparing them to cosmological simulations of galaxy formation designed to study gas flows in detail. I will present a new model in which HVCs do not significantly contribute to the growth of galaxy disks in the present universe, but are key indicators of much larger fueling flows in galactic halos.
Quantifying High-Redshift Star Formation with Gamma-Ray Bursts - Promises and Perils
Dr.  Daniel Perley (Caltech)
One of the most exciting broader applications of the study of gamma-ray bursts is in better understanding the star-formation history of the Universe, especially at the highest redshifts. Long-duration GRBs are produced exclusively by massive, young stars and can be detected beyond z>6 (and potentially even z>9) with small telescopes, pinpointing a SFR-selected sample of galaxies at each redshift unimpeded (in principle) by issues of gas and dust attenuation and without regard to host luminosity. On the other hand, the GRB rate may depend on complex factors other than just the SFR itself, and achieving the goal of an unbiased host-galaxy sample is much more complicated in practice - in particular with regard to the effects of dust attenuation, which can prevent accurate localization of the host. I will discuss recent efforts to shine light upon the population of heavily dust-obscured GRBs and their host galaxies, and their contributions to the broader population, using a combination of carefully-selected surveys of both optically-obscured and ordinary GRBs. Even with the effects of dust extinction accounted for, GRBs represent very poor star-formation tracers at z~1 (but much better tracers at z~2). A strong dependency on the GRB rate with host stellar mass points toward metallicity as the primary factor influencing the GRB rate. Despite a trend towards convergence of the GRB and star-formation rates at higher redshifts, caution is therefore warranted before relying on GRBs to constrain questions of cosmic history at any epoch.
What controls galaxy masses and star formation rates?
Dr.  Ryan Quadri (Carnegie Observatories)
With current redshift surveys we are able to study the properties of nearly complete samples of galaxies across a wide range in redshift using uniform techniques. Measurements of how galaxies grow in mass due to star formation and mergers, and of how these processes are regulated by environment, are particularly important and can be related to expectations from dark matter-based models. However recent observations have raised a number of challenges for our understanding of galaxy evolution, providing motivation to improve both the observations and the models.
Weather on Substellar Worlds: observations of cloud and weather phenomena in the atmospheres of cool brown dwarfs
Jacqueline  Radigan  (STScI )
Recent observations of cool brown dwarfs in the time-domain have revealed large-amplitude variability at near-infrared wavelengths for a subset of objects spanning the transition between cloudy L-dwarf and clear T-dwarf spectral types (~1200 K). This quasi-periodic variability of late type brown dwarfs is indicative of heterogeneous cloud features and evolving weather patterns in their atmospheres. I will share results from the largest and most sensitive variability survey to date, and discuss how the newfound population of variable brown dwarfs with patchy clouds provides an unprecedented opportunity to probe cloud structure and atmospheric dynamics in the non-irradiated, rapidly rotating regime.
Motion in the universe: large-scale flows and virialized motions of galaxies
Dr.  Beth Reid (Lawrence Berkeley National Lab)
Redshift space distortions in spectroscopic galaxy maps provide a statistical measure of cosmic flows. The largest systematic uncertainty in measuring the amplitude of the peculiar velocity field are small-scale, virialized motions, the ``Fingers-of-God.'' I will present progress in using anisotropic galaxy clustering from SDSS-III BOSS on Mpc scales to directly infer the impact of small-scale motions on large-scale clustering.
Chasing Type Ia Supernovae in the Early Universe
Dr.  Steven Rodney (Johns Hopkins University)
Type Ia Supernovae (SNIa) are a cornerstone of modern cosmology. They provided the first direct evidence for the accelerating expansion of the universe, and are among the most promising tools for unveiling the nature of the "dark energy" driving that acceleration. It is widely accepted that SNIa are generated by the thermonuclear explosion of a white dwarf in a binary system. Beyond that basic picture, however, there is very little certainty about their progenitor star systems. An important test of SNIa progenitor models is measurement of the time between star formation and supernova explosion: the Delay Time Distribution (DTD). The DTD can be derived indirectly from measurements of the cosmic SNIa rate. This test has the most leverage at high redshift, where progenitor models significantly diverge. I will present early results from two ongoing HST treasury programs, CANDELS and CLASH, in which we have for the first time extended SNIa rate measurements to z~2, providing new and unique constraints on SNIa progenitor models.
Which Sub-Neptune Exoplanets could have Liquid Water Oceans?
Dr.  Leslie Rogers (Caltech)
The presence of liquid water has been hypothesized as an important ingredient for planet habitability. A sub-Neptune mass planet with a liquid water ocean below a hydrogen-rich envelope is an intriguing prospect. If the planet transits, its atmosphere could be amenable to characterization with transmission spectroscopy. A practical method to assess whether a planet with measured mass and radius could potentially harbor a liquid water ocean is needed. Using a one-dimensional radiative-convective model of energy transport through water-saturated hydrogen-rich envelopes, we constrain the combinations of planet properties (mass, radius, equilibrium temperature, intrinsic luminosity) that are conducive to liquid water oceans. We find that sub-Neptune mass planets with radii exceeding 3.25 Earth-radii, or equilibrium temperature exceeding 370 K cannot have liquid water oceans. Kepler-22b need not be rock-dominated, but must have a mass of at least 7 Earth-masses to support a liquid water ocean.
The Search for small Kuiper Belt Objects using the Hubble Space Telescope
Dr.  Hilke Schlichting (UCLA)
The Kuiper Belt is a remnant of the primordial solar system located just beyond the orbit of Neptune. Studying the small body population in the Kuiper Belt is crucial for gaining an understanding of the physical processes underlying debris disk formation and for probing the formation and dynamical history of the outer solar system. Unfortunately, objects smaller than about 10 km in radius are too faint to be in detected in reflected light. They can, however, be detected indirectly by stellar occultations. For over 14 years, the Fine Guidance Sensors (FGS) on board the Hubble Space Telescope have been collecting a large number of photometric measurements of stars with 40 Hz time resolution. The archival FGS data are ideal for KBO occultation searches since they offer the stability of a space-based platform, they consist of more than 100,000 star hours of observations and they have an excellent time resolution that allows for the detection of the occultation diffraction pattern rather than a simple decrease in the photon counts. In my talk, I will present the final results of our analysis of 14 years of FGS data. I will discuss what our findings imply for abundance of small sub-km sized KBOs, the initial planetesimal sizes that Kuiper Belt Objects grew from and the collisional history of the Kuiper Belt.
Assessing Galactic Rotation and Structure
Ralph Schonrich (Ohio State University )
I will discuss different estimators to measure the rotation of Galactic components and show how they can be used together to control systematic biases in these measurements. Distance systematics must be kept at a minimum for an accurate determination of kinematic parameters, which we achieve by a new statistical approach. This yields the consistent picture of a non- or at the most weakly rotating Galactic halo both for stellar samples in the vicinity of the Sun and more remote Blue Horizontal Branch stars.
A New Channel for Low-Luminosity GRBs: Tidal Disruptions of White Dwarfs by Intermediate Mass Black Holes
Dr.  Roman Shcherbakov (University of Maryland)
Low-luminosity GRBs or X-ray flashes (XRFs), which often accompany supernovae, are typically ascribed to either the supernova shock breakout or weak GRBs powered by the central engine of stellar mass. We propose the tidal disruption of a white dwarf (WD) by an intermediate-mass black hole (IMBH) as another channel for XRFs. Such disruptions last for 100-5000 seconds. The release of gravitational energy over short time generates a powerful flare. The magnetic field is quickly amplified in the fallback material, and then the BH launches a slow uncollimated jet. The emission from jet photosphere dominates X-rays with Comptonized thermal spectrum, while the expanding jet shell produces most of IR/optical. The prompt flare may be followed by an underluminous fast supernova, resulting from a tidal compression and thermonuclear ignition of a WD. High event rate in dwarf galaxies warrants searches among the known and future transients observed with Swift satellite. We perform detailed dynamical and spectral modeling of a candidate disruption source GRB060218/SN2006aj. The BH mass is independently estimated to be 20,000 solar masses based on (1) the event duration, (2) the jet base radius from the thermal X-ray component, and (3) the properties of a host galaxy. The supernova position is consistent with a center of a dwarf host galaxy. Other potential candidates are the flashes with very weak/absent supernovae such as XRF040701.
Quasar Demography in Two-Dimension
Dr.  Yue Shen (Carnegie Observatories)
I will present a demographic study of quasars in the BH mass-luminosity plane, using the largest and most homogenous quasar sample from the SDSS and our dedicated forward models with Bayesian inference. This approach takes into account the selection effect of the sample flux limit, and more importantly, the uncertainties in the BH mass estimates. There is tentative evidence that downsizing also manifests itself in the quasar black hole mass function, and that the mean Eddington ratio (at fixed BH mass) increases with redshift. I will also discuss the uncertainties in BH mass estimates and some of their implications in quasar studies.
Galaxies in the Reionization Era: New Insight from Gravitational Lensing
Dr.  Dan Stark (Arizona)
Over the last several years, deep HST imaging campaigns have provided our first census of star formation activity in the reionization era. I will describe how detailed studies of highly-magnified gravitationally-lensed galaxies are complementing these programs, providing unique constraints on the ionizing contribution of early star forming galaxies and guiding future spectroscopic efforts targeting the most distant known galaxies.
Accretion Lags in Black-Hole Binary Systems
Dr.  James Steiner (Harvard-Smithsonian Center for Astrophysics)
Accreting black hole binaries show strong correlation between X-ray and optical variability. In two systems, we find that X-rays lag behind the optical with a characteristic delay of weeks. This behavior is most readily attributed to viscous delay as inflowing gas traverses the disk from outer to inner annuli. By applying a model primarily comprised of a slowly-varying alpha disk to describe the luminosity fluctuations, we are able to successfully map between the X-ray and the optical. Using this model, we measure alpha and explore the possibility of its dependence on luminosity. Additionally, we discover a strong dependence of X-ray heating upon the geometry of the binary system. Meanwhile, by removing the optical variance tied to the X-rays, this technique may be useful in recovering dynamical information from outbursting black holes, a feature of particular importance for those exceptionally X-ray bright systems which balk traditional methods.
Observing and Characterizing Disks around the Youngest Protostars
Dr.  John Tobin (NRAO)
The formation of proto-planetary disks begins during the earliest phase of the star formation process, while the nascent protostar is still surrounded by a dense envelope of gas and dust. I will present high resolution observations of Class 0 protostars at sub/millimeter wavelengths from the SMA and CARMA. As a shining example, we detected an edge-on R ~ 150 AU proto-planetary disk around the Class 0 protostar L1527 in Taurus. Simultaneous observations of the 13CO (J=2-1) transition are found to trace the disk rotation curve and a protostellar mass of 0.19 +/- 0.04 M_sun is found. Building on these results we have conducted a small survey with CARMA at 1.3 mm toward 9 protostars in Perseus finding a few examples of possible disk-like structures and rotation. Using the JVLA, we have observed a 3 protostars at a resolution of 0.06", finding two to be 100 AU binaries. The small separation of the binaries is a likely indication that they formed via disk fragmentation. This lays the ground work for ALMA, which will be needed to make significant gains in the area of disk formation with vastly improved resolution and sensitivity. Most importantly, ALMA's ability to detect faint molecular lines will enable masses of a large number of Class 0 protostars to be measured for the first time.
What Are Local Group Satellites Trying to Tell Us about Cosmology?
Dr.  Erik Tollerud (Yale University)
Local Group satellite galaxies are a unique testbed for both galaxy formation and LCDM. They allow us to probe the the lowest-luminosity extremes of star formation, and allow more detail than is possible for more distant extragalactic objects. As a result, they harbor interesting puzzles when placed in a cosmological context. I will describe how the LMC is a particularly valuable object because it serves as a bridge between Local Group satellites and larger-scale observations. Moving to lower masses, I will also describe kinematical studies of the M31 satellite system, which is only now being characterized in a way comparable to the Milky Way. These results serve to strengthen many of the puzzles pointed out about the Milky Way satellites, as well as revealing some new ones.
Moving Cosmological Simulations Forward
Dr.  Mark Vogelsberger (CfA/Harvard)
Over the last decades cosmological simulations of galaxy formation have significantly advanced and played a crucial role in enabling an understanding of the complicated processes underlying structure formation in the Universe. However, many relevant aspects remain poorly understood or uncertain due to limitations in computational modelling. In my talk, I will present new results based on a completely novel computational approach to follow the details of structure formation. I will emphasize that this leads to more reliable insights into galaxy formation and demonstrate that many previously hypothesized aspects of galaxy formation need to be revised. Specifically, I will show that gas accretion onto galaxies proceeds in a dramatically different manner than believed up to now, motivating a revision of the conventional picture of how galaxies acquire baryons.
Follow the Energy
Dr.  Matthew Walker (Harvard College Observatory)
Observations indicate that galactic dark matter halos differ structurally from the halos produced in dissipationless (Cold Dark Matter; 'CDM') cosmological N-body simulations. Either dissipatative processes (i.e., baryon physics) can significantly alter the internal structure of CDM halos, or the dark matter is not cold and collisionless. I will place limits on the former scenario by estimating the required halo transformations and energetic requirements thereof, then comparing to the amount of supernova energy that can be associated with stellar populations observed in the Milky Way's dwarf satellites. I will argue that a definitive conclusion awaits measurement of the luminosity function of streams in the Galactic halo.