STScI Logo

Space Telescope Science Institute
2010 Hubble Fellows Symposium Abstracts

From WFC3/IR: The Quenching Effect of AGN Feedback on Massive Host Galaxies at z ~ 1

S. Mark Ammons (University of Arizona)

AGN feedback may contribute to quenching of star formation by the expulsion or heating of cold gas, causing the host galaxy to evolve onto the red sequence (e.g., Di Matteo et al. 2005, Hopkins et al. 2006). We probe for the effects of feedback on the stellar populations of 100 X-ray-selected AGN hosts at z~1 in GOODS-South. Combining high spatial resolution optical imaging from HST ACS, and high spatial resolution near-IR imaging from Keck Laser Guide Star AO, HST NICMOS, and newly public WFC3/IR imaging, we test for the presence of young stars on sub-kiloparsec scales, independent of dust extinction. Correlations between near-ultraviolet /optical (NUV - R) colors and gradients and X-ray parameters such as hardness ratio and luminosity reveal the nature of AGN-driven feedback. Indeed, several QSO-mode feedback models (e.g., Hopkins et al. 2006, Menci et al. 2008) predict quenched stellar populations in unobscured AGN hosts relative to obscured AGN hosts. However, we find little (< 0.3 mags) difference between the mean NUV - R color of the obscured (hard in X-ray) hosts and the unobscured (soft in X-ray) hosts beyond ~2 kpc. This suggests that the unobscured sources are not increasingly quenched of star formation, which is inconsistent with some bright-mode feedback models. We compare the NUV - R colors of late-type galaxies that host AGN to non-active late-type galaxies and use full stellar populations modeling to measure stellar mass surface density profiles. The active hosts display strikingly similar mean stellar mass surface density profiles and mean NUV-R color gradient slopes to the nonactive hosts, but redder NUV-R colors (by ~0.9 mag). This suggests that the AGN is associated with a spatially uniform reddening of the stellar population in late-type hosts, not a centralized quenching or simple addition of a red bulge. In sum, these observations are more consistent with a strangulation or starvation quenching mode in intermediate-luminosity AGN.

Planet-Forming Disks Around Young Stars

Sean Andrews (Smithsonian Astrophysical Observatory)

Direct observations of the reservoirs of planet-building material - the disks around young stars - play a critical role in developing theoretical models of the planet formation process. I will highlight some exciting new radio observations that resolve large, low-density cavities in the centers of so-called protoplanetary "transition" disks and discuss how these may prove to be the signposts of very young (1 Myr) planetary systems. Along the way, I will comment on how this work foreshadows what we can expect from next-generation instruments in this field, in particular from ALMA beginning just 18 months from now.

Disk-Satellite Interaction, Then and Now

Pawel Artymowicz (University of Toronto)

20 years ago, I began working on interaction of objects with disks, with applications to planetary systems and active galactic nuclei. I still sometimes do that today. However, the field has changed considerably. The coupling mechanisms considered the sole important contributors then (Lindblad resonant torques) are now being superseded by other processes (corotational torques), which were known but considered unimportant. Gaps (in disks and in our knowledge) have been partially filled in recent times. While the analytical techniques have not progressed very far, the computers became thousands of times more capable in every respect. Despite that, we always tended to have (too) much trust in analytic calculations. Disk-planet interaction obtained a major unforseen observational boost 15 year ago: it explained the hot exo-jupiters. The theory predicted, vaguely rather than explicitly, their migration, as well as intermediate orbital eccentricity. I will speculate on the evolution of our theoretical tools and some discoveries to be made in the period between the present and the next round anniversary of the Hubble Fellowship program.

Detailed Studies of the Spatially-Unresolved Broad Line Region in AGNs

Misty Bentz (University of California, Irvine)

A defining spectral characteristic of bright AGNs is their extremely broad permitted emission lines. This emission is theorized to originate very close to the central supermassive black hole, on scales of ~0.001 pc (comparable in size to the Solar System). Even for the most nearby AGNs, this region is spatially unresolvable with current instrumentation. As such, the geometry and kinematics of this region of gas are still much debated. The technique known as reverberation mapping has been extremely successful in the past at using the variable nature of AGNs to measure the "echo" of continuum events in the broad line flux, thereby deducing a mean radius for the broad line region from the time delay. The full potential of reverberation mapping has not yet been achieved, however, in which the response of individual velocity packets of gas are fully mapped out across the emission line profile. The Lick AGN Monitoring Project (LAMP) is one of two recent reverberation-mapping campaigns where the data quality is now of the caliber to begin to achieve this goal. I will discuss LAMP and the main findings from the project thus far, and focus on the preliminary results and ongoing work to determine the detailed nature of the gas in the broad line region.

The Gemini NICI Planet-Finding Campaign: Deep Imaging of Stars in the Solar Neighborhood

Beth Biller (Institute for Astronomy, University of Hawaii)

The novel Near Infrared Coronagraphic Imager (NICI) on the 8-m Gemini South telescope combines a number of techniques to attenuate starlight and suppress superspeckles for direct detection of exoplanets: 1) Lyot coronagraphic imaging, 2) dual channel imaging for Spectral Differential Imaging (SDI) and 3) operation in a fixed Cassegrain rotator mode for Angular Differential Imaging (ADI). The combination of these techniques allows unprecedented contrasts of dmag > 14 (median value) at 1" in H band. We are currently conducting a 50 night planet-finding campaign using NICI. Although the majority of our survey targets are young (<300 Myr, so as to catch young planets while they are still self-luminous), we are also observing a subset of nearby stars in the immediate solar neighborhood. While these objects are old (and hence, planetary companions will be faint), they are still desirable targets for planet searches due to the excellent physical resolution possible around these stars. Additionally, their large proper motions also mean that planet candidates can be quickly confirmed. NICI is the first instrument with achieved star-planet contrasts high enough to image supergiant planets around these stars (M > 10 MJup). Here, we report on results from observations of 3 such nearby stars. At respective distances of 1.39 pc, 2.35 pc, and 3.8 pc, Proxima Centauri, Wolf 359, and Luyten's Star are the 1st, 3rd, and 22nd closest stars to the Sun. While our search yielded numerous candidate companions, especially around Proxima Centauri, all of these candidates were found to be background in second epoch observations.

The Dawn of 21 cm Cosmology with EDGES

Judd Bowman (Caltech)

For the first time, observations targeting the highly redshifted 21 cm line have yielded a meaningful probe of neutral hydrogen in the intergalactic medium during the epoch of reionization. The Experiment to Detect the Global Epoch of Reionization Signature (EDGES) has measured the all-sky radio spectrum between 100 and 200 MHz with better than 10 mK sensitivity. This is a factor of three below the maximum redshifted 21 cm contribution predicted for a fully neutral intergalactic medium above redshift 6. These measurements rule out instantaneous reionization histories between redshifts 6 and 13. The ability to detect reionization histories with long time scales is currently limited by the subtraction of a low-order polynomial to remove systematics that change slowly with frequency. The latest results will be presented.

Dynamite Diameters: Observations of main-sequence stars with long baseline optical/infrared interferometry

Tabetha Boyajian (Georgia State University)

Recent innovations in high-resolution astronomical techniques, in particular, long-baseline optical/infrared interferometry, have enabled us to resolve the disks of nearby, main-sequence stars. These direct measurements of the sizes of stars provide a crucial resource in constraining stellar atmosphere, structure, and evolutionary models. I will present the current status of observations underway of mid to late type main-sequence stars. These results indicate and continue to support the discrepancy between theory and observation: models have a tendency to over-predict temperatures and under-predict radii of these types of stars. I will also discuss how these empirical data provide the means to calibrate less direct relationships to predict fundamental properties of stars.

The Rise and Fall of Passive Disk Galaxies

Kevin Bundy (University of Calfornia, Berkeley)

The tranformation of blue, star-forming disk galaxies into red and dead spheroidals is one of the most important features of galaxy evolution since z=1-2. This mass-dependent growth of the "red sequence" has been the subject of many studies, and yet we still lack a convincing picture of the physical processes involved. New insight can be gained, however, by revisiting the assumed link between morphology and star formation. Specifically, I will show how large galaxy surveys are now revealing a significant population of quenched, red-sequence galaxies with surprising disk-like morphologies. These passive disks typically have large bulges but are not confined to dense environments. They represent as much as one-half of all red-sequence galaxies and dominate at lower masses (log Mstar < 10). Their evolving abundance compared to other populations suggests that as much as 60% of galaxies transitioning onto the red sequence evolve through a passive disk phase. Simple fading of blue disks is not able to explain their origin, and mergers (possibly inducing AGN feedback) should destroy the disk. I will discuss several other explanations, including environmental effects, internal stabilization, and disk regrowth during gas-rich mergers. Instead of red-sequence growth driven by a single transformation process, this work highlights the fact that galaxy evolution may actually be shaped by several processes proceeding through separate stages.

Wide Binaries: Opportunities at the Non-Violent End of the Semi-Major Axis Distribution

Julio Chaname (Carnegie Institution of Washington)

While stars in binary systems of all types have been always recognized to be of fundamental importance in astronomy, only in recent years has the population of binaries at the wide end of the distribution of orbital separations (semi-major axis of 100 AU and larger) been properly exploited. I will review the status of research on wide binaries and highlight the potential inherent to these objects for the study of the Galaxy, including their unique usefulness as probes of inhomogeneities of the gravitational potential.

Intermediate Redshift Galaxy Evolution with PRIMUS and AGES

Richard Cool (Princeton University)

I will present a description and early science results concerning intermediate redshift (z<1) galaxies from the PRIsm MUlti-object Survey (PRIMUS) and the AGN and Galaxy Evolution Survey (AGES). PRIMUS has surveyed 200,000 galaxies over 10k square degrees of deep multi-wavelength imaging and is the largest sample of intermediate-redshift galaxies with spectroscopic redshifts currently available. I will focus primarily on our latest measurements of the the galaxy luminosity function at intermediate redshifts and highlight other topics of on-going research.

A Universal Stellar Initial Mass Function?

Kevin Covey (Cornell University)

The Stellar Initial Mass Function (IMF) is a powerful tool for characterizing stellar populations, and IMF variations across environments could reveal important clues about the physics that govern the star formation process. We will review observations of the stellar IMF across a range of Galactic environments, including the field population of the Milky Way disk, active star forming regions and moderate age open clusters. Most studies present a consistent portrait of a `universal' Kroupa/Chabrier type IMF: a power-law of Salpeter index ($Gamma=1.35$) above a few solar masses, and a log normal or shallower power-law ($Gamma sim 0-0.25$) between a few tenths and a few solar masses (ignoring the effects of unresolved binaries). The shape and universality of the IMF at the stellar-substellar boundary is still under investigation and uncertainties remain large, but most observations are consistent with a IMF that declines ($Gamma < -0.5$) well below the hydrogen burning limit. We conclude by highlighting specific regions that present intriguing evidence for IMF variations, and summarize new directions, both observational and analytical, which could enable confident measurements of subtler Galactic IMF variations.

Cosmological Simulations of Galaxies in the Reionization Epoch

Kristian Finlator (University of California, Santa Barbara)

The new WFC3/IR camera on board the Hubble Space Telescope has recently granted us our first direct glimpse of the young galaxy population that had already formed by the time that cosmological reionization completed. These data anchor our understanding of galaxy evolution at lower redshifts, but their implications remain unclear. The observed galaxies are growing vigorously and seem relatively pristine chemically, yet their colors also indicate surprisingly old stellar populations for such early times. They have had relatively little time to relax dynamically following interactions, yet their colors are remarkably uniform, suggesting that the "main sequence" of galaxy evolution previously observed at z<=2 was already in place when the Universe was only 700 Myr old. I describe efforts to unravel these mysteries through three-dimensional numerical modeling of hierarchical structure formation in representative cosmological volumes. I discuss how recent observations yield new constraints on outflows and star formation histories at early times and remark on how additional observations will improve our understanding. I introduce a new generation of simulations that treats the growth of an inhomogeneous ionizing background self-consistently. I use early results from these simulations to explore how galaxies bring about cosmological reionization, and how photoheating feedback from the nascent background in turn regulates early galaxy growth.

New Ways to Explode Stars

Avishay Gal-Yam (Weizmann Institute of Science)

The study of exploding stars - supernovae - has important consequences on diverse physical and astrophysical problems: from detection of cosmic neutrinos and gravitational waves, through synthesis of the elements from which planets and people are made, driving star and galaxy formation and evolution, and all the way to cosmology, and the first stars. For many years two dominant processes of explosion were studied: the thermonuclear explosion of carbon-oxygen white dwarf stars (type Ia supernovae), and the gravitational collapse of the degenerate iron cores of massive stars. I will review progress in recent years that led to the recognition and study of at least two (and perhaps as many as four) additional distinct types of explosions. I will briefly discuss implications: from detection of the first stars to the formation of the elements of life.

Black Hole Accretion in the Nearby Universe: Evidence for Down-Sizing

Elena Gallo (MIT)

An issue of fundamental importance in understanding the galaxy-black hole connection is the duty cycle of accretion. If black holes are indeed ubiquitous in galactic nuclei, little is known about the frequency and intensity of their activity, the more so at the low-mass/low-luminosity end. I will present new results from AMUSE-Virgo, a Chandra survey of (formally) inactive early type galaxies in the Virgo cluster. Out of 100 objects, 32 show a nuclear X-ray source, including 6 hybrid nuclei which also host a massive nuclear cluster as visible from archival HST images. After carefully accounting for contamination from nuclear low-mass X-ray binaries based on the shape and normalization of their X-ray luminosity function, we conclude that between 24-34% of the galaxies in our sample host a X-ray active super-massive black hole. This sets a firm lower limit to the black hole occupation fraction in nearby bulges within a cluster environment. At face value, the active fraction is found to increase with host stellar mass down to our luminosity threshold. However, taking into account selection effects, we find that the average Eddington-scaled X-ray luminosity scales with black hole mass as M_BH^{-0.62}, with an intrinsic scatter of 0.46 dex. This represents the first observational evidence for `down-sizing' of black hole accretion in local early types, that is, the fraction of active galaxies, defined as those above a fixed X-ray Eddington ratio, decreases with increasing host galaxy mass.

First Results from the GALEX Time Domain Survey

Suvi Gezari (Johns Hopkins University)

Motivated by the successful discovery of UV/optical transients with serendipitous coincident observations between the GALEX Deep Imaging Survey and the optical CFHT Legacy Deep Survey, GALEX has begun a dedicated time domain survey that operates in parallel with the Pan-STARRS Medium Deep Survey. With simultaneous UV and optical coverage of 7 deg^2 of sky with a cadence of days, we have the unique ability to detect UV bright phenomena at early times, including shock breakout and hot ejecta in core-collapse SNe, as well as probe the simultaneous UV and optical properties of variable AGNs and flares from dormant supermassive black holes caught in the act of tidally disrupting a star. In this talk, I will present our transient detection and classification methods and the first results of the survey.

The SDSS Visual Arc Sample

Michael Gladders (University of Chicago)

The strong lensing rate in galaxy clusters has long been touted as a cosmological probe, and lensed sources have been a basis of discovery for many of the most distant galaxies ever studied. However, the true cosmological impact of strong lensing has been limited by small numbers of sources and heterogeneous sample selection, and the total number of bright high-z sources studied to date is similarly limited to only a few objects. We have recently completed a new survey, based on a detailed analysis of the SDSS DR7 imaging, which aims to change this. From a candidate pool of some 40000 potential lensing systems we have confirmed some 150 new cluster and group scale lenses within the SDSS. This new homogeneous sample offers an unprecedented opportunity to study strong lensing statistically; I will present details of the arc statistics in comparison to current predictions. This large sample also includes several new lensed LBGs brighter than the prototype object cB58, as well as several lensed LAEs at z~5 which are 5-10 brighter than any other LAEs previously studied at such redshifts. Time permitting I will also briefly highlight the imminent construction of a second similarly sized sample of lenses from the now-completed RCS2 project, which will extend the lensing statistics for clusters to z=1.

Our Changing Neighborhood - the Evolving View of the Local Group

Eva Grebel (ZAH, University of Heidelberg)

Our view of our immediate cosmic neighborhood, the Local Group of galaxies, has changed dramatically over the last 12 years. After a long hiatus, new dwarf galaxies were discovered in the Local Group, initially still on photographic plates, and then a few years later in modern, wide-field CCD surveys such as the Sloan Digital Sky Survey. Meanwhile the galaxy census of the Local Group has more than doubled, including the least luminous galaxies ever detected - truly ultra-faint dwarfs. Deep HST imaging has changed our understanding of the evolution of these nearby, low-mass galaxies, revealing ubiquitous old stellar populations and complex, often extended star formation histories. HST is also permitting us to extend these kinds of studies to nearby groups of galaxies, revealing how common or how unique our own group is. Low- and high-resolution spectroscopy with large ground-based telescopes has revolutionized our understanding of the chemistry and chemical evolution of our neighbors and our own Galaxy, increasingly providing clues on to what extent dwarf galaxies akin to the present-day survivors contributed to the build-up of the Milky Way. Spectroscopy also helps us to constrain the dark-matter content of the low-mass Local Group members, revealing highly dark-matter-dominated galaxies. The evolutionary history of the Local Group was my Hubble Fellowship theme. In my proposed talk I would like to highlight the amazingly rapid evolution and changing views that this intriguing field has experienced over the past 12 years, and what the implications are for our understanding of galaxy evolution and cosmology.

The SPLASH Survey and Progressive Stages of Galaxy Formation

Raja Guhathakurta (UCO/Lick Observatory, University of California, Santa Cruz)

Our SPLASH (Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo) collaboration has been conducting a spectroscopic survey of tens of thousands of red giant stars in our neighbor, the Andromeda galaxy (M31), and its dwarf satellite galaxies. The Keck/DEIMOS stellar spectra yield information on membership, radial velocity, and chemical abundance patterns. The M31 system is an excellent testbed for studying the interplay among the dynamical, assembly, star-formation, and chemical-enrichment histories of the different structural subcomponents of M31 --- its outer halo, inner spheroid, disk, and the past and present satellite populations --- in the context of the Lambda-CDM hierarchical galaxy formation paradigm. I will present the latest results from our ongoing SPLASH survey and then briefly discuss the planned capabilities of the Thirty-Meter Telescope and the contributions it is expected to make in this area of astrophysics.

Mapping Galactic Structure with Large Photometric Surveys

Mario Juric (Harvard College Observatory)

The distributions of stars (and their properties) in our Galaxy contain a treasure trove of information about the formation, evolution, and present day structure of the Milky Way. Large surveys such as SDSS and Pan-STARRS allow us to mine this rich resource to unprecedented scale and accuracy by directly mapping the distributions of stellar number density, metallicity, and kinematics in a representative volume of the Galaxy. The maps enable us to measure the shape of the Galaxy, observe the relationships between dynamical and physical properties of its stars, and identify and characterize streams and satellites in the Galactic halo. In this talk, I will discuss the recent and upcoming results of photometric studies of the Milky Way with the SDSS, including comprehensive maps of stellar kinematics, maps and discussion of disk asymmetries, as well as an improved measurement of luminosity functions of disk and the halo. All of these provide valuable insights in the present day state of the Milky Way, as well as the process of its assembly.

Magnetic Field-Decay-Induced Electron Captures: a Strong Heat Source in Magnetar Crusts

David Kaplan (University of California, Santa Barbara)

We propose a new heating mechanism in magnetar crusts. Magnetars' crustal magnetic fields are much stronger than their surface fields; therefore, magnetic pressure partially supports the crust against gravity. The crust loses magnetic pressure support as the field decays and must compensate by increasing the electron degeneracy pressure; the accompanying increase in the electron Fermi energy induces nonequilibrium, exothermic electron captures. The total heat released via field-decay electron captures is comparable to the total magnetic energy in the crust. Thus, field-decay electron captures are an important, if not the primary, mechanism powering magnetars' soft X-ray emission.

Constraints on Black Hole Growth, Quasar Lifetimes, and Eddington Ratios from the SDSS Quasar Black Hole Mass Function

Brandon Kelly (Harvard-Smithsonian Center for Astrophysics)

I will present an estimate of the evolution of quasar black hole mass number density based on a sample of ~10,000 of broad line quasars at 1 < z < 4.5, drawn from the SDSS. We find evidence for 'cosmic downsizing' of black holes, with the most massive black holes growing first. We use our estimated mass distribution to estimate the lifetime of broad line quasars, their Eddington ratio distribution, and the maximum black hole mass. We find that most broad line quasars are not radiating at the Eddington limit. I will discuss our results within the context of models for black hole growth; in particular, we find that at least part of the growth of these black holes must have occurred in an earlier obscured phase of near-Eddington accretion.

The Elemental Abundance Distributions of Milky Way Satellite Galaxies

Evan Kirby (Caltech)

The proximity of the satellite galaxies of the Milky Way (MW) renders them the best targets for studying the stellar populations of small galaxies. The chemical compositions of their stars reveals information about the history of gas flows and star formation intensity. This talk presents a Keck/DEIMOS spectroscopic survey of the Fe, Mg, Si, Ca, and Ti abundances of 3006 stars in eight MW dwarf satellites. The metallicity and alpha-to-iron ratio distributions obey the following trends with the total luminosity of the galaxy: (1) The more luminous galaxies are more metal-rich, indicating that they retained gas more efficiently than the less luminous galaxies. (2) The shapes of the metallicity distributions of the more luminous galaxies require gas infall during their star formation lifetimes. (3) At [Fe/H] < -1.5, [alpha/Fe] falls monotonically with increasing [Fe/H] in all MW satellites. In the more luminous satellites, [alpha/Fe] becomes constant at [Fe/H] > -1.5. (4) The average [Mg/Fe] and [Si/Fe] in the satellites is roughly the same as the MW halo, but [Ca/Fe] and [Ti/Fe] in the satellites lie below the MW average at all metallicities. One interpretation of these trends is that the more luminous galaxies truly are more massive. They are able to retain existing gas and accrete new gas to power bursts of star formation. The star formation timescale in all MW satellites is long enough that Type Ia supernovae contribute to the chemical compositions of stars at all times, but in the more luminous/massive satellites, the burstiness of the star formation rate achieves an equilibrium between the contributions of Types Ia and II supernovae.

The Role of Multiplicity in Disk Evolution and Planet Formation

Adam Kraus (University of Hawaii, Institute of Astronomy)

Interactions with close stellar or planetary companions can significantly influence the evolution and lifetime of protoplanetary disks. It is now possible to search for these companions, directly studying the role of multiplicity in disk evolution and planet formation. I will describe a survey to directly detect these stellar and planetary companions around young stars in several nearby star-forming regions. My program uses adaptive optics and sparse aperture mask interferometry to achieve typical contrast limits of 6-7 magnitudes at the diffraction limit (5-10 MJup at >5-10 AU), while also detecting similar-flux binary companions at separations as low as 15 mas (2.5 AU). In most cases, my survey has found no evidence of companions (planetary or binary) among the well-known ``transitional disk'' systems with large inner gaps. If these gaps are signposts of ongoing planet formation, as has been previously suggested, then this paucity places an upper limit on the mass of any resulting planet. My survey also has uncovered many new binary systems, with the majority falling among the diskless (WTTS) population; this disparity suggests that disk evolution for close (5-40 AU) binary systems is very different than for single stars. The majority of circumbinary disks are cleared within <1 Myr, while almost all disks around single stars survive for at least 2-3 Myr. This difference in disk lifetimes has significant implications for planet formation around both single stars and binary systems.

Galaxy Formation During the Epoch of Reionization

Ivo Labbe (Carnegie Observatories)

The arrival of WFC3 aboard HST has enabled a leap in our knowledge of the galaxies populating the earliest universe (z>7). These galaxies are interesting for many reasons: as beacons of the earliest sites of star formation, as testbeds to constrain galaxy formation models, and as probes of reionization. Using the most recent data taken with WFC3/IR, over the Hubble Ultra Deep Field (HUDF09) and part of GOODS-South (ERS), I will present the results of these recent searches. Combining with ultradeep Spitzer/IRAC data, I will also discuss clues about their stellar populations, such as stellar mass, age, and star formation history, correlations between M* and SFR, and implications for reionization.

Evidence for a Triaxial Galactic Dark Matter Halo from the Sagittarius Stellar Tidal Stream

David Law (UCLA)

Observations of the lengthy tidal streams produced by the destruction of the Sagittarius dwarf spheroidal galaxy (Sgr dSph) are capable of providing strong constraints on the shape of the Galactic gravitational potential. However, previous work has yielded conflicting results: While the angular position of the Sgr leading arm is most consistent with a spherical or slightly oblate Galactic dark matter halo, the radial velocities and distances of stars in this arm are best matched by prolate halo models. I demonstrate that this apparent paradox can be resolved by adopting a fully triaxial halo model, in which it is possible to simultaneously satisfy all major observational constraints on the orbital path of the Sgr stream. In this model, the minor axis of the dark halo is approximately coincident with the Galactic X axis connecting the Sun and the Galactic Center.

The Origin of Globular Clusters with Multiple Populations

Young-Wook Lee (Yonsei University)

In the current LCDM hierarchical merging paradigm, a galaxy like the Milky Way formed by numerous mergers of ancient subsystems. Most of the relics of these galaxy building blocks, however, are yet to be discovered or identified. Here we show that about 25% of the Milky Way globular clusters have characteristics of the remaining cores of these early building blocks rather than genuine star clusters. They are clearly distinct from other normal globular clusters in (1) the presence of multiple stellar populations, (2) mass, (3) metallicity distribution function, and (4) most importantly in orbital kinematics. Based on this result, a three-stage formation picture of the Milky Way is suggested, which includes early mergers, collapse, and later accretion.

Turning Gas Into Stars in Nearby Galaxies

Adam Leroy (NRAO)

I will present evidence from a combining CO, HI, IR, and UV data that the ability to form molecular gas (traced by CO-to-HI ratio) plays a key role in regulating the star formation rate over large areas of disk galaxies. Comparison to a variety of proposed physical drivers suggest that a mixture of dust abundance and average gas density in turn regulates the H2-to-HI ratio on large scales.

Ground-based Exoplanet Atmospheres Characterization: Progress Since the 2009 Breakthrough

Mercedes Lopez-Morales (Carnegie Institution of Washington)

In early 2009 we witnessed the first detections of atmospheric emission from transiting exoplanets using ground-based telescopes. One year later, the atmospheres of at least another five planets have already been detected, and several new detections are underway. Like in any new field, many of the results that we are obtaining are startling and challenge the most current exoplanet atmospheric models. I will detail in this talk the results that we have so far and the problems they pose for state-of-the-art models.

Probing Reionization with Lyman Alpha Galaxies

Sangeeta Malhotra (Arizona State University)

Lyman-alpha line is resonantly scattered by neutral Hydrogen, and is therefore a good, local probe of reionization. We have used Lyman-alpha emitting galaxies to show that the IGM was ionized at redshift 6.5, and now we extend this test to higher redshifts: z=6.9 and 7.7. Despite the challenges of searching for lyman-alpha galaxies in the near-infrared, we see promising candidates. These should provide exciting science opportunities for JWST.

The Sunyaev-Zel'dovich Effect, In Detail

Daniel Marrone (University of Chicago)

Observations of galaxy clusters through the Sunyaev-Zel'dovich (SZ) effect have matured quickly in the last few years, proceeding from samples of tens of objects to surveys designed to constrain dark energy with thousands of new cluster detections. Nevertheless, very little observational work has been done to understand the effects of astrophysics and the systematics of the SZ effect in clusters. In this talk I will present two approaches to understanding the cluster SZ signal. I will show the first calibration of the SZ-mass relation incorporating weak lensing, this relation is a key ingredient for cosmological measurements with SZ surveys and for calibration of cluster simulations. The use of weak lensing avoids the bias that is normally introduced when deriving cluster masses under the assumption of hydrostatic equilibrium, and allows us to investigate merger effects on the SZ signal. Second, I will show first results from our high-resolution imaging of the SZ effect signal with the merged CARMA and SZA interferometers. These observations show the complementarity of X-ray and detailed SZ measurements in revealing the state of the intracluster medium.

Was Reionization Complete By z ~ 5-6?

Andrei Mesinger (Princeton University)

It is generally taken for granted that reionization has completed by z=6, due to the detection of flux in the Lyman alpha forest at redshifts z<6. However, since reionization is expected to be highly inhomogeneous, much of the spectra pass through just the ionized component of the intergalactic medium (IGM) even for non-negligible values of the volume-weighted mean neutral hydrogen fraction, x_HI. We study the expected signature of an incomplete reionization at z ~ 5--6, using very large-scale (2 Gpc) seminumeric simulations. We find that ruling out an incomplete reionization is difficult at these redshifts since: (1) quasars reside in biased regions of the ionization field, with fewer surrounding HI patches than implied by the global mean, x_HI; this bias extends tens of comoving megaparsecs for x_HI < 0.1; (2) absorption from the residual neutral hydrogen inside the ionized IGM generally dominates over the absorption from the remaining HI regions, allowing them to effectively "hide" among the many dark spectral patches; (3) modeling the Lyman alpha forest and its redshift evolution even in just the ionized IGM is very difficult, and nearly impossible to do a priori. We propose using the fraction of pixels which are dark as a simple, nearly model-independent upper limit on x_HI. Alternately, the size distribution of regions with no detectable flux (dark gaps) can be used to place a more model dependent constraint. Either way, the current sample of quasars is statistically insufficient to constrain x_HI at z~6 to even the 10 per cent level. At z~5, where there are more available sightlines and the forest is less dark, constraining x_HI < 0.1 might be possible given a large dynamic range from very deep spectra and/or the Lyman beta forest. We conclude with the caution against over-interpreting the observations. There is currently no direct evidence that reionization was complete by z ~ 5--6.

Origins of Chemical Complexity

Karin Oberg (Harvard-Smithsonian Center for Astrophysics)

Complex organic molecules (>6 atoms) have been detected toward low- and high-mass protostars, galactic center clouds, protostellar outflows and comets, demonstrating the existence of efficient astrophysical pathways to chemical complexity. The detected molecules all reside in the gas. Yet they probably form on interstellar grains, in ices that evolve with their environment and finally evaporate as the grains are heated by new-born stars or by shocks. I will explore this ice evolution by combining Spitzer spectra of the first, simple ices with laboratory simulations of UV induced ice photochemistry and millimeter observations tracing complex ice evaporation. The experiments show that UV irradiation of protostellar ices is efficient enough to explain the complex molecule observations. Moreover, the experiments predict that before the onset of thermal evaporation, small fractions of the complex ice will continuously evaporate non-thermally due to photodesorption, resulting in gas-phase fingerprints of the ice composition as it evolves. To test this prediction and thus the ice origins of complex molecules in space, we searched for a number of gas-phase complex organic molecules toward an ice-rich cloud core, irradiated, but not heated, by a nearby protostar.

A New Model for Emission from Microquasar Jets

Asaf Pe'er (STScI)

There are strong evidence for powerful jets in the low/hard state of black-hole candidates X-ray binaries (BHXRBs). I present a model in which electrons are accelerated once at the base of the jet, and are cooled by synchrotron emission and possible adiabatic energy losses. The accelerated electrons assume a Maxwellian distribution at low energies and possible energetic power law tail. These characteristic energies, combined with the decay of the magnetic field along the jet, introduce a wealth of spectra. I will identify critical values of the magnetic field, and show that: (I) the decay of the magnetic field along the jet enables, for wide jets, production of flat radio spectra without the need for electrons re-acceleration along the jet. (II) An increase of the magnetic field above a critical value of ~105 G leads to a sharp decrease in the flux at the radio band, while the flux at higher frequencies saturates to a constant value. (III) For strong magnetic field, the flux decays in the optical/UV band as F_ u ~ u^{-1/2}, irrespective of the electrons initial distribution. (IV) For B_0 ~ 104 G, the flux at the X-ray band gradually steepens. (V) With adiabatic energy losses, flat spectrum can be obtained only at a limited frequency range, and under certain conditions (VI) For narrow jets, r(x) ~ x^{alpha} with alpha < 1/2, flat radio spectrum cannot be obtained. References: Pe'er & Casella, 2009, ApJ, 699, 1919 Casella & Pe'er, 2009, ApJ, 703, L63

An All-Sky Automated Survey for the Brightest Supernovae

Jose Prieto (Carnegie Observatories)

I will present an ongoing all-sky search for bright supernovae. The search is using the All-Sky Automated Survey for variable stars (ASAS), which has two small telescopes with wide-field cameras in sites in Chile and Hawaii and is obtaining images of the entire sky every ~3 days down to V = 14-15 mag. The high cadence, large area, and magnitude limit of this survey are optimal for making early discoveries of bright supernovae in nearby galaxies. I will discuss the science opportunities that will be opened by this project, early results, and plans.

Constraining the Global Reionization History

Jonathan Pritchard (Harvard-Smithsonian Center for Astrophysics)

One of the major unanswered questions surrounding the period of the first sources is how the Universe became reionized. In this talk, I will discuss the extent to which existing observations of the cosmic microwave background and the lyman alpha forest constrain the reionization history. Building upon this I will address the question of what observations of the redshifted 21 cm signal have to add to our understanding of reionization and the first luminous sources.

Dust Obscuration and Metallicity at High Redshift: New Inferences from UV, H-alpha, and 8 Micron Observations of z~2 Galaxies

Naveen Reddy (NOAO)

I will discuss some recent work to constrain the relationship between 8 micron emission and the star formation rates of z~2 galaxies. Using this relation, I show that the rest-frame UV slope can be used to recover the dust attenuation of L* galaxies at z~2 to within 0.4 dex scatter, but that the youngest galaxies at these redshifts appear to follow a steeper extinction curve than what is typically assumed. Our data imply that dust obscuration is likely the dominant mechanism that modulates the bright-end of the UV luminosity function. Finally, a comparison between the metallicities of z~2 galaxies and local starbursts suggests a strong evolution in the luminosity-metallicity and luminosity-obscuration relations; this evolution has direct consequences for our ability to obtain a complete census of galaxies at high redshift using dropout selection techniques.

The Physical Conditions for Star Formation in the High Redshift Tail of Submillimeter-Luminous Starburst Galaxies

Dominik Riechers (Caltech)

We have performed a detailed investigation of the z=4.055 submillimeter galaxy (SMG) GN20, the first spectroscopically confirmed member of the high redshift tail of SMGs, over the full wavelength range from X-ray to radio. GN20 is the brightest and most distant submm source in the GOODS-N field, and lives in a unique protocluster environment at z>4. It hosts an intense, heavily obscured starburst of 2000 M_sun/yr, fueled by a massive molecular gas reservoir of >10^11 M_sun. The gas and star forming regions are distributed over 10 kpc, yielding gas and star formation rate surface densities consistent with the Schmidt law for nearby starburst galaxies. Its star formation reservoirs also exhibit bright emission from polycyclic aromatic hydrocarbons (PAH) in the mid-IR, but the underlying rest-frame 6um continuum emission appears to be partially powered by a Compton-thick AGN. By comparison to 'typical' SMGs at z~2, we explore how the unusual overdense environment in which GN20 grows may play a role for the formation of massive, present day cluster galaxies. GN20 is a showcase example of how the detailed, synergetic studies that will become possible in the age of ALMA and JWST will impact our picture of galaxy formation and evolution in the early universe.

Estimating Luminosity Function Constraints From High-Redshift Galaxy Surveys

Brant Robertson (Caltech)

The installation of the Wide Field Camera 3 on the Hubble Space Telescope will revolutionize the study of high-redshift galaxy populations, but the information obtained on those galaxy populations will depend strongly on the depth and area of future surveys. Using a Fisher matrix approach to fully account for Poisson and cosmic sample variance, as well as covariances in the data, we estimate the uncertainties on luminosity function parameters achieved by high-redshift galaxy surveys of a given depth and area. We also discuss related measurements of halo bias from cosmological N-body simulations that enable new estimates of the expected cosmic variance of high-redshift galaxies.

Clouds in the Tropics of Titan

Emily Schaller (University of Arizona)

Saturn's moon Titan is the only body in the solar system besides Earth with a thick nitrogen atmosphere and a hydrological cycle. Methane clouds, lakes and most fluvial features on Titan have been observed in the moist high latitudes, while the tropics have been nearly devoid of clouds and show an abundance of wind-carved surface features like dunes. Using the NASA Infrared Telescope Facility (IRTF), we frequently observe Titan to monitor its cloud activity. We couple the IRTF dataset with Gemini adaptive optics observations of Titan to determine cloud locations and morphologies. Recently, we observed an unexpected large outburst of cloud activity in Titan's tropical latitudes that may have been associated with a significant amount of methane rainout. These observations may explain the presence of small-scale channels and dry riverbeds seen near the equator by the Huygens probe on its decent through Titan's atmosphere. We found that the initial pulse of cloud activity in the tropics generated Rossby waves that instigated cloud activity at other latitudes across Titan that had been cloud-free for at least several years. Understanding Titan's methane-based hydrological cycle is vital for interpreting the variety and distribution of fluvial surface features seen by the Cassini/Huygens Spacecraft.

The Inner Structure of Seyfert Galaxies

Joseph Shields (Ohio University)

The Hubble Space Telescope continues to provide important insights into the nature of active galactic nuclei in terms of the physics of accretion sources, their impact on their surroundings, and the stellar environment in which they reside. This presentation will describe recent results from HST and other facilities that bear on each of these topics.

Mass Loss from Red Giants

Jay Strader (Harvard-Smithsonian, Center for Astrophysics)

The interpretation of color-magnitude diagrams for old stellar populations demands that low-mass stars lose a significant amount of mass on the red giant branch and in subsequent phases of stellar evolution. I present the initial results of a near-IR spectroscopic survey for mass loss among giants in the field and in globular clusters.

Milky Way Satellites and Lambda-Cold Dark Matter

Louis Strigari (Stanford University/KIPAC)

The observed properties of dark matter on Galactic scales provide a powerful test of structure formation and on the theory of Lambda-Cold Dark Matter (LCDM). In this talk I will present new results on the mapping of the observed Milky Way satellite population onto the dark matter halo population in LCDM-based theoretical models using the highest resolution numerical simulations. I will discuss how these results provide important information on the properties of the least luminous objects in the Universe and on the population of dark subhalos in our Milky Way.

A Multi-wavelength Study of Outflows in Local, Hard X-ray Selected Seyfert 1s

Lisa Winter (University of Colorado)

While mass outflows from AGN are believed to affect their host galaxy's evolution - enriching their local ISM and potentially IGM with metals from supernova explosions and quenching epochs of star formation - the properties of these outflows are poorly understood. Even the fraction of outflows is not known, since previous studies were based on sources selected for their optical/X-ray brightness. With the Swift Burst Alert Telescope (BAT) AGN survey, we have identified an unbiased sample of 51 nearby ( = 0.03) Seyfert 1-1.5s, selected in the 14-195 keV band. In addition to many well-known bright optical/soft X-ray sources, this sample includes lower luminosity Seyferts that were detected as AGN for the first time with Swift. We detail our preliminary results in studying the X-ray (through warm absorber signatures), UV (through absorption features in the broad AGN emission lines), and optical (through our high resolution ground based spectra of the Na ID doublet) signatures of outflows in our unbiased sample.

The Submillimeter View of Galaxy Formation

Josh Younger (Institute for Advanced Study)

This talk will look at the process of galaxy formation viewed at submillimeter wavelengths. We begin by presenting a physical model for the origin of the cosmic diffuse infrared background. This semi-empirical model -- which includes star-forming disks, merger-driven starbursts, and obscured AGN -- predicts that the vast majority of the observed emission is generated principally by moderate luminosity, steady-state star formation; mergers and obscured AGN (and high-luminosity objects more generally) do not contribute more than a few percent. However, the highest luminosity objects do provide an interesting laboratory for studying star formation in extreme environments. We thus change gears to look at submillimeter galaxies: high-redshift, hyper-luminous infrared sources selected directly via their far-infrared thermal dust emission. These objects are thought to be formed via one of two mechanisms: very large star-forming disks, or merger-driven bursts. New submillimeter interferometric imaging, which offers the opportunity to measure the physical scale of the star-forming region in the far-infrared directly, provides compelling evidence that submillimeter-selected galaxies represent major mergers of gas-rich disk galaxies, and may be radiating at or close to their Eddington limit.