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Space Telescope Science Institute
2010 May Symposium Talks

The Formation and Rapid Disruption of Young Clusters

Nate Bastian (IoA Cambridge / University of Exeter)

I will review the observational evidence and current models for the rapid dissolution of stellar clusters during the first few Myr after their formation. I will address the fraction of stars that are formed in "clusters", both locally and in extreme environments. Particular attention will be given to selection effects and biases that effect our understanding of clusters and cluster formation in the solar neighbourhood as well as extragalactic samples.

Galactic Archaeology: The Lowest Metallicity Stars

Timothy Beers (Michigan State University)

The lowest metallicity stars in the Galaxy preserve our best snapshots of the chemical elements formed in the Big Bang, and in the very first generations of (presumably) massive stars that theory tells us formed shortly thereafter. Over the past several decades, dedicated searches for these objects have identified at least a handful of stars with metallicities [Fe/H] less than one hundred thousandth the solar abundance, and approaching one millionth solar. Astronomers are also learning how better to identify and mine these fossils of creation in the future, based on the apparent association of the lowest metallicity stars with the outer-halo component of the newly recognized inner/outer structure of our Galaxy. In this review, I summarize the current state of affairs, consider current evidence for the multiple metallicity distribution function of the dual halo, and point toward the new survey efforts to begin in the near future.

Stellar Populations of High-Redshift Star-Forming Galaxies Using Rest-frame Optical and UV Imaging

Nicholas Bond (Rutgers University)

We present a rest-frame optical and rest-frame ultraviolet analysis of stellar populations in 160 2 < z < 3.5 Star-Forming Galaxies (SFGs) in the Extended Chandra Deep Field South, including 39 Lyman Alpha Emitters. Using Hubble Space Telescope images taken as part of the Hubble Ultra-Deep Field survey, the WFC3 Early Release Science program, and the Great Observatories Origins Deep Survey, we analyze the sizes and light profile shapes of SFGs in BVIzYJH images, comparing the rest-ultraviolet and rest-optical morphological properties on an object-by-object basis. The physical sizes of SFGs range from <1 kpc to ~5 kpc, while LAEs are typically < 1.5 kpc in size in all of the observed filters. We see evidence for a bimodality in the distribution of relative sizes between the rest-optical and rest-UV, with a minority of objects displaying considerably larger rest-frame optical sizes, suggesting the presence of dust or an older underlying stellar population. Furthermore, many objects display a characteristic "two-clump" structure, in which each clump is dominated by stellar populations of different ages, suggesting that these objects are mergers-in-progress.

Uncertainties in Population Synthesis Models: Applications of Population Synthesis Models to Local and Distant Galaxies

Gustavo Bruzual (CIDA)

I will present a summary of recent advances in the fields of stellar evolution, stellar model atmospheres, and stellar spectral libraries, which allow us to build more realistic stellar population synthesis models than those available up to now. Applications of these models to problems of current interest for local and distant galaxies will be discussed. Problems that need to be understood and data sets that need to be collected in order to solve issues present in these models will be mentioned.

Mapping Stellar Populations in the Milky Way with Gaia

Carla Cacciari (INAF, Osservatorio Astronomico di Bologna)

The Gaia space project, planned for launch in 2012, is one of the ESA Cornerstone missions, and will produce an all-sky astrometric, photometric and spectroscopic survey of very high quality of all sources brighter than V=20 (about one billion objects). This unprecedented level of information and knowledge will have a fundamental impact on several of the most important areas of Astrophysics, such as stellar classification and parameterization, complete study of the stellar populations in the Milky Way and, to a lesser degree, in the galaxies of the Local Group, the distance scale and age of the Universe, dark matter (potential tracers), reference frame (quasars, astrometry), planet detection, fundamental physics, Solar physics, Solar system science. I will present a description of the instrument and its characteristics and expected performance, and discuss how Gaia data promise to contribute a fundamental improvement to our knowledge of the Milky Way stellar populations.

Current Uncertainties in Stellar Evolution Models

Santi Cassisi (INAF, Osservatorio Astronomico di Collurania)

During this last decade our knowledge of the evolutionary properties of stars has significantly improved. This result has been achieved thanks to our improved understanding of the physical behavior of stellar matter in the thermal regimes characteristic of the different stellar mass ranges and/or evolutionary stages. This notwithstanding, the current generation of stellar models is still affected by several, not negligible, uncertainties related to our poor knowledge of some thermodynamical processes and nuclear reaction rates, as well as the efficiency of mixing processes. These drawbacks have to be properly taken into account when comparing theory with observations, to derive evolutionary properties of both resolved and unresolved stellar populations. We review the major sources of uncertainty along the main evolutionary stages, and emphasize their impact on the studies of both resolved and unresolved stellar populations.

The Life Cycle of Star Clusters

Rupali Chandar (Department of Physics and Astronomy)

There is now strong evidence that most stars form in clusters, rather than individually. This means that most stars that we observe in galaxies have, at some point, lived in a star cluster, and that the life cycle of star clusters tells us about the build up of galaxies. I will present a picture for the life cycles of star clusters, from their birth in molecular clouds to their dissolution in the (unclustered) field star population. This picture is motivated by recent observational studies of the mass and age distributions of star clusters in a number of nearby galaxies of different types and environments (e.g., (dwarf, giant, spiral, irregular, interacting, quiescent). Underlying the apparent diversity and complexity of these cluster systems, there are some intriguing regularities. This suggests that this simple picture for the formation and disruption of clusters in galaxies of different type is "universal" -- that it approximately describes the life cycles of most star clusters, whether they have been called open, populous, or super, in many galaxies.

Pushing Towards the Large Scale: Cluster Formation Simulations with N < 10^4

Cathie Clarke (Institute of Astronomy)

Recent years have seen the advent of simulations that, under the simplest assumption about the input physics, can model the formation of star clusters numbering many thousands of stars. Simulations on such scales thus have the dynamic range to capture star cluster formation as a hierarchical process. The detailed analysis of such simulations can be confronted with observations of clustering statistics in star forming regions and can thus point to the insufficiencies of the simulations (in terms of the physical processes included). Simulations that incorporate more realistic input physics (such as the inclusion of magnetic fields or stellar feedback) are in their relative infancy, but early results point out some qualitative trends that need to be borne in mind when interpreting simpler simulations. We will particularly discuss the form of the upper IMF and show that even the simplest (no feedback) simulations produce truncated power law IMFs, whose possible significance for the integrated galactic IMF (IGIMF) has been much discussed of late.

Stellar Population Synthesis: Which Uncertainties Matter?

Charlie Conroy (Princeton)

In this talk I will discuss the most important uncertainties in SPS modeling, including (time permitting) advanced stellar evolutionary phases, dust attenuation, the IMF, and the spectral libraries. I will explain in which contexts these uncertainties matter, and describe ongoing work that aims to minimize their impact where possible, and faithfully account for them elsewhere.

Galaxies as Collections of Individual Stars

Julianne Dalcanton (University of Washington)

The optical and infrared colors and luminosities of a galaxy depend sensitively on the exact mixture of its constituent stars. Unfortunately, broad-band integrated photometry and luminosity-weighted spectra provide only coarse indicators of the age and metallicity of the underlying stellar population. In nearby galaxies, however, we can dissect galaxies into their individual stars, and directly assess the contribution of different populations to the integrated light. I will discuss two large programs which harness the power of HST to resolve individual stars in large volume-limited samples of nearby galaxies. I will focus on the short-lived but luminous phases of stellar evolution that are revealed in optical and NIR color magnitude diagrams, and which can potentially contribute significantly to a galaxy's spectral energy distribution.

The Where, When, Why and How of Star Formation in the Local Group

Guido De Marchi (European Space Agency)

We have undertaken a systematic study of pre-main-sequence (PMS) stars spanning a wide range of masses (0.5 - 4 Msolar), metallicities (0.1 - 1 Zsolar) and ages (0.5 - 30 Myr). We have already used new WFC3 and archival ACS data to identify and characterise a large sample of PMS objects in three star forming regions in the local group, namely NGC 3603 in the Milky Way, 30 Doradus in the Large Magellanic Cloud and NGC 346 in the Small Magellanic Cloud. Thanks to a novel method that we have developed to combine broad-band (V,I) photometry with narrow-band Halpha imaging, we have determined the physical parameters (temperature, luminosity, age, mass and mass accretion rate) of more than 2000 bona-fide PMS stars still undergoing active mass accretion. This is presently the largest and most homogeneous sample of PMS objects with known physical properties. We will present the most important results of this research, including: (1) All regions exhibit multiple recent episodes of star formation with at least two populations of younger and older PMS stars separated by about 10 Myr from each other, suggesting a multi-generation pattern; (2) There is no correlation between the projected spatial distribution of young and old PMS stars and the younger population is systematically more concentrated, contrary to what one could expect in a triggered star formation scenario; (3) There is no correspondence between the positions of young PMS stars and those of massive OB stars of similar age, indicating that the conditions necessary for high- and low-mass star formations are different; (4) The mass distribution of stars with similar ages reveals large variations throughout the regions, showing that the concept of initial mass function is not meaningful for individual star forming units (~5pc in size); (5) The mass accretion rate appears to scale with the first power of the stellar mass, with the square root of the age, and approximately with the inverse of metallicity. These results are bound to have important consequences for, and constraints on our understanding of the star formation process.

Multiple Populations in Globular Clusters: Massive Binaries as the Source of Abundance Anomalies

Selma De Mink (Utrecht University)

Various features in the color magnitude diagrams of globular clusters indicate the presence of multiple stellar populations within one cluster differing in chemical composition and possibly age. It has been proposed that the more massive stars in the cluster enriched their surroundings with material processed by hydrogen burning. Two main sources have been suggested: asymptotic giant branch (AGB) stars and massive stars rotating near the break-up limit ("spin stars"). A challenge for all proposed scenarios is to provide the large amount of ejecta required to form subsequent stellar populations that are equally or even more numerous than the first population. We propose massive binaries as a promising additional source of enrichment and we argue why this source may be more important than AGB stars and "spin stars", at least in terms of the amount of ejected mass. To demonstrate the principle we compute the evolution of a typical 20 solar mass star in a close binary considering the effects of mass transfer, rotation and tidal interaction. We find that this system sheds about 10 solar masses of material, about 5 times than in the "spin star"-scenario. This material is probably ejected at low velocity. We expect that it remains inside the potential well of the cluster and becomes available for the formation or pollution of a second stellar generation. Given the high fraction of close binaries among massive stars in nearby loose OB associations, -- a fraction which may be even higher in the dense center of the young progenitor of massive globular clusters --, it is likely that the majority of massive stars interact with a companion and contribute to the self enrichment of the cluster. This scenario strongly relieves the need to adopt commonly made assumptions such as strong preferential loss of the first generation of stars form the cluster, a very high fraction of fast rotating stars among massive stars, external pollution or an anomalous initial mass function.

30 Doradus: The Quintessential Resolved 'Starburst'

Christopher Evans (UK ATC, Royal Observatory Edinburgh)

In this review I will highlight recent results from studies of 30 Doradus in terms of the feedback from stellar winds and ionizing fluxes, triggered star formation and ejected stars. These will be used to argue that 30 Dor provides us with an essential bridge between studies of spatially-resolved star formation and cluster evolution, and the intense starburst clumps seen at high redshift.

M31 and M33: Probing the Assembly of our Nearby Neighbours with Stars and Star Clusters

Annette Ferguson (Institute for Astronomy, Royal Observatory)

Unique insight into formation and evolution of large galaxies can be gained from studies of our nearest spiral neighbours, M31 and M33. I will discuss some recent results regarding the assembly history of these systems derived from deep photometric studies of their resolved stellar populations as well as from wide-field views of their globular cluster systems.

Direct Age Determination of Local Group Dwarf Elliptical Galaxies

Marla Geha (Yale University)

Dwarf elliptical (dE) galaxies account for more than 75% of objects brighter than MV < -14 in nearby galaxy clusters, yet the origin of this galaxy class remains controversial. Understanding the dE galaxy class will provide insight into the assembly of dense environments and is a necessary step toward answering general questions of dwarf galaxy formation and evolution. We present deep ACS/WFC imaging for the Local Group dE NGC 147 and NGC 185 which extend below the main sequence turnoff. These are the only two dE galaxies in which HST can cleanly resolve stars in the main sequence turnoff region and thus directly measure their star formation histories. These observations are a unique opportunity to test formation models for an entire galaxy class.

Formation of Globular Clusters in LCDM Cosmology

Oleg Gnedin (University of Michigan, Department of Astronomy)

Modern hydrodynamic simulations of galaxy formation are able to predict accurately the rates and locations of the assembly of giant molecular clouds in early galaxies. These clouds could host star clusters with the masses and sizes of real globular clusters. I will describe current state-of-the-art simulations aimed at understanding the origin of the cluster mass function and the age and metallicity distributions. Metallicity bimodality appears to be a natural outcome of hierarchical formation and gradually declining fraction of cold gas in galaxies. Globular cluster formation was most prominent at redshifts z>3, when massive star clusters contributed as much as 20% of all galactic star formation.

The 2D Galaxy Population: Mass-Dependent Star Formation, Stochastic Assembly-Driven Quenching

Genevieve Graves (UC Berkeley)

To zeroth order, galaxies form a one-dimensional family in which their observed properties---morphology, color, metallicity, environment, central black hole masses---all scale with their mass. This makes it difficult to disentangle the critical properties that determine their evolutionary history because everything correlates with everything else. Examining where this 1D sequence breaks down can provide essential clues to galaxy evolution. I present observational evidence that galaxy star formation histories form a 2D family, such that galaxies of the same mass today start out with similar star formation histories but shut down star formation ("quench") at different times. A parallel study using semi-analytic models of galaxy evolution suggests that these differences are due to their underlying halo mass assembly histories. Indeed, the observed 2D family of galaxy star formation histories seems to be a generic result of a mass-threshold for quenching (whether the process is driven by stellar mass, halo mass, or central black hole mass), given a paradigm of mass-dependent evolution coupled with stochastic hierarchical assembly.

Luminosity Functions and Photometric Properties of Lyman-alpha Emitters at 2 < z < 3

Caryl Gronwall (Pennsylvania State University)

We have used the Mosaic camera of the CTIO 4-m telescope to conduct deep, narrow-band surveys of Ly-alpha Emitting Galaxies (LAEs) in the Extended Chandra Deep Field-South over the redshift ranges 3.08 < z < 3.15 and 2.04 < z < 2.08. Our survey covers 0.28 square degrees down to a limiting line flux of about 1.5E-17 ergs/cm2/s at z=3.1. Our LAE sample now consists of several hundred candidate line-emitters. By combining our narrow-band data with archival broadband photometry from MUSYC (the Multiwavelength Survey by Yale-Chile), we have been able to determine the photometric properties of these samples. We will present results including the Ly-alpha and rest frame-UV luminosity functions of both redshifts, and discuss the evolution of this population of galaxies "in the act of formation." We will also discuss the equivalent widths, colors, star-formation rates, and dust properties of these objects.

Hugging Hubble

John Grunsfeld (STScI)

In May 2009, a team of astronauts flew to the Hubble Space Telescope on space shuttle Atlantis. On their 13 day missionand over the course of 5 spacewalks they completed an extreme makeover of the orbiting observatory. They installed the Wide Field Camera-3, the Cosmic Origins Spectrograph, repaired the Advanced Cameria for Surveys and the Space Telescope Imaging Spectrograph, as well as a number of maintenance activities. The Hubble Space Telescope story has been a fascinating study in public policy, engineering, ethics, and science. For the first time on orbit the Hubble has a full complement of instruments capable of performing state-of-the-art observations from the near infra-red to ultraviolet end of the spectrum. The early results of the new and repaired instruments hint at a bright scientific future for Hubble and will be presented in the talk as well as a narrative of the adventures on orbit.

A high resolution panoramic study of the structure and substructure of the Milky Way analogue NGC891

Rodrigo Ibata (Observatoire Astronomique Universite de Strasbourg)

I will present an analysis of the structure and substructure of the nearby edge-on galaxy NGC 891, based on deep HST/ACS images and an ultra-deep ground-based survey with Subaru/SuprimeCam. These observations resolve stars in the upper two magnitudes of the red giant branch of this Milky Way analogue over a vast area of 90x90kpc, providing the first panoramic view of a spiral galaxy beyond the Local Group. We find evidence for subtle, but very significant, small-scale variations in the median colour and density over the halo area surveyed with ACS. These colour variations are unlikely to be due to internal extinction or foreground extinction, and reflect instead variations in the stellar metallicity. Their presence suggests a startling conclusion: that the halo of this galaxy is composed of a large number of incompletely-mixed sub-populations, testifying to its origin in a deluge of small accretions. Indeed, NGC891 has been caught in the act of accreting a former companion in the SuprimeCam survey: we detect a giant stellar stream that wraps several times around the galaxy. I will also discuss how we are using this stream to understand the large-scale dark matter content in that galaxy.

Galaxy Buildup in the First Gyr: The Nature of Galaxies in the Epoch of Reionization

Garth Illingworth (University of California, Santa Cruz)

The recent extraordinarily deep WFC3/IR data from the HUDF09 program has dramatically expanded our insight into the galaxy population at z~7-8-9, and given us constraints on the population at z~10. Over 100 z~7-8 galaxies have now been detected. These larger samples have enabled us to undertake a thorough analysis of the luminosity functions to much lower luminosities than previous studies. We have used the very deep field data (the HUDF, HUDF09 and the HUDF05 datasets) to derive the contribution to the luminosity density and the star formation rate from lower luminosity galaxies to as faint as ~0.05L* from redshift 4-7. We have incorporated recent results on the contribution of ULIRGS and sub-mm star-forming galaxies to indicate that in the first 1-2 Gyr the dominant contributors to the star formation rate density in the universe are the lower luminosity, lower mass objects, that are fully represented in the Lyman Break population at z~4-7 and at earlier times. Evidence from the HST WFC3/IR and Spitzer IRAC data over the HUDF suggests that these z~7-8 galaxies include earlier populations that reach back to z>10. I will contrast the striking evolution from z~7-8-9 to z~6 and later, including the stellar population characteristics and the results from recent structural analyses. I will discuss how the broad nature of the galaxy population changes during the period 0.5 billion years to 1-2 billion years, based on the new HUDF09 WFC3/IR data.

An Observatinal Testbed for the Formation of Massive Stars: 30 Doradus

Remy Indebetouw (University of Virginia, Department of Astronomy)

Massive stars are the main agents responsible for evolution of the interstellar medium of galazies, and their subsequent star formation history. We would like to know how molecular clouds turn into stellar clusters, with what efficiency massive stars from in those clusters, and once the massive stars form, the efficacy with which they affect their natal material and beyond. Infrared observations with Spitzer and Herschel allow us to search for the imprint of "precluster" molecular cloud conditions reflected by the distribution in time and space of young massive protostars. Dust-penetrating long-wavelength spectroscopic mapping reveals the spatial extent of radiative and sometimes mechanical feedback, even on the very short timescales relevant to the most energetic stars. Nearby objects like 30 Doradus can be used as templates for the spatially unresolved physics in galaxies throughout the low-redshift universe, and even for insight into more distant systems.

How dSph Galaxies Can Bring Light on the General Physical Processes at Play in Galaxy Formation

Pascale Jablonka (Ecole Polytechnique Federale de Lausanne (EPFL))

I will present new results from high resolution spectroscopy of individual stars in classical dwarf spheroidal galaxies (dSph) and from chemo-dynamical simulations. I will show how the first generations of stars in dSphs compare to ones in the Milky Way halo. We have recently established that all dSphs contain extremely metal-poor stars ([Fe/H] from -3 to -4). The detailed analysis of their abundance ratios provide crucial clues on the impact of the explosion of supernovae in different galaxy mass range, from ultra faint dwarfs to massive galaxies. I will point out how stellar nucleosynthesis depend on the galaxy properties and present how chemo-dynamical simulations help the interpretation of the observations and discuss the next developments to be undertaken.

Signatures of Hierarchical Structure Formation in Stellar Populations

Kathryn Johnston (Columbia University)

Stars that make galaxies are expected to form within dark matter halos that are themselves growing through gravitational collapse and mergers. This scenario implies a rich interplay between the simple collisionless dynamics of the dark matter and the complex baryonic physics that shapes the galaxies. In this talk I will review known characteristics of abundance patterns in the Local Group that are believed to reflect this interplay, and look ahead to future prospects for this work.

Galaxy Hydroponics: Growing Galaxies from Their Satellite Seeds

Evan Kirby (Caltech)

Large spiral galaxies have to come from somewhere, and the currently prevailing winds prefer the accretion of satellite galaxies to monolithic collapse. Although the accreted galaxies are now dissolved, some of their counterparts still exist today in various states of dissolution. Images and spectroscopy of the resolved stellar populations of surviving satellite galaxies of the Milky Way and other large, nearby galaxies are windows into the seeds of galaxy formation. I review the state of knowledge of the ages, metallicities, detailed chemical abundances, and kinematics of surviving satellite galaxies. I compare these to the stellar populations of galactic halos, mostly of the Milky Way and M31. The comparison leads to the conclusion that the stellar populations of large galaxy halos are consistent with the accretion of smaller galaxies.

The Hubble Sequence Beyond z=2

Mariska Kriek (Princeton University, Department of Astrophysical Sciences)

Similar to the situation at low redshift, the properties of galaxies beyond z=2 are strongly correlated. Massive galaxies at this epoch clearly separate into two classes: the large star-forming galaxies that form the blue cloud, and the smaller quiescent galaxies on the red sequence. Thus, it is evident that a Hubble sequence with strongly correlated galaxy properties is already in place beyond z=2. Nonetheless, this sequence does not resemble the local Hubble sequence, as the structures and morphologies of the z>2 galaxies are different from their local analogs. While quiescent galaxies are much more compact than nearby early-type galaxies, massive star-forming galaxies have irregular and clumpy structures. I will discuss the origin of the z=2 Hubble sequence, and its evolutionary connection to the local Hubble sequence.

The IMF of Simple and Composite Populations

Pavel Kroupa (Argelander Institute for Astronomy, University of Bonn)

The stellar IMF is the distribution function of stellar masses born together in one causally connected event within a spatial region of not more than a few pc in extend. It cannot be measured in any system, but statistical methods combined with corrections for dynamical bias allows one to infer the existence of a universal canonical IMF as the parent distribution from which the various simple stellar populations are drawn. There is no evidence for variation of the IMF except at the highest star-burst cluster masses above 106 Msun. The IMF in a whole galaxy is, in contrast, that of a composite population such that the IMFs of the individual simple populations must be added. This leads to an integral over the currently forming star cluster population and implies the integrated galactic IMF (IGIMF) to be top light. The steepness of the IGIMF for massive stars can be shown to depend on the star formation rate of the galaxy. This leads to an entirely new avenue of understanding galaxy evolution with potentially deep insights into fundamental physics issues.

The Predictive Power of Galaxy SED Models

Danilo Marchesini (Tufts University)

Our knowledge of the properties of galaxies at intermediate redshifts (z=1-4) is mostly based on the modeling of the observed spectral energy distributions (SEDs) of galaxies from broad-band photometry using stellar population synthesis models. This technique has allowed us to study large samples of galaxies, to derive their stellar population properties, such as stellar mass and star formation rate, and to study the evolution of these properties as a function of cosmic time. I will review some of these results, highlighting the power of SED modeling and its current limitations when applied to distant galaxies.

A Survey of Stellar Tidal Stream in Nearby Spiral Galaxies

David Martinez-Delgado (Max-Planck Institut fur Astronomie)

Within the hierarchical framework for galaxy formation, minor merging and tidal interactions are expected to shape large galaxies to this day. As part of a pilot survey, we have carried out ultra-deep, wide-field imaging of some isolated spiral galaxies in the Local Volume with data taken at small (0.1 to 0.5-meter diameter),robotic telescopes that provide exquisite surface brightness sensitivity. Our observational effort has led to the discovery of previously undetected giant stellar structures in the halos of these galaxies, likely associated with debris from tidally disrupted satellites. In addition, we confirm several enormous stellar over-densities previously reported in the literature, but never before interpreted as tidal streams. Our collection of galaxies presents an assortment of tidal phenomena exhibiting strikingly diverse morphological characteristics. In addition to identifying great circles-like features that resemble the Sagittarius stream surrounding the Milky Way, our observations have uncovered enormous structures that extend tens of kiloparsecs into the halos of the central spiral. We have also found remote shells, giant clouds of debris within galactic halos, jet-like features emerging from galactic disks and large-scale, diffuse structures that are almost certainly related to the remnants of ancient, already thoroughly disrupted satellites. Together with these remains of possibly long-defunct companions, our observations also capture surviving satellites caught in the act of tidal disruption. Some of these display long tails extending away from the progenitor satellite as seen in cosmological simulations. Our comparison with available stellar halo simulations set in a Lambda-Cold Dark Matter cosmology suggests that this extraordinary variety of morphological specimens detected in our survey could represent one of the first comprehensive pieces of evidence to support that the hierarchical formation scenarios predicted by these theoretical models apply generally to galaxies similar to the Milky Way in the Local Volume. Finally, I discuss as the detailed study of the stellar population of these tidal streams via deep Hubble Space Telescope photometry data can render important constraints on the formation of the galactic halos.

The Pan-Andromeda Archaeological Survey

Alan McConnachie (Dominion Astrophysical Observatory)

I will present the latest results from the Pan-Andromeda Archaeological Survey (PAndAS) that is contiguously surveying >300 square degrees surrounding M31 and M33, reaching to a maximum projected radius of 150kpc and 50kpc from the centers of these galaxies. Our photometry reaches approximately 4 magnitudes below the tip of the RGB, enabling the identification of RGB stars (and their classification by photometric metallicity), AGB stars and young stellar populations (in the disks of our two targets). After a few years of data, the main body of the survey is now complete and the northern and southern hemispheres have been mapped to comparable depth over comparable areas. I will discuss the nature of a large scale structure surrounding M33 and show how this implies an interaction between M33 and M31 approximately 2 - 3 Gyrs ago and helps constrain M31's transverse motion. I will discuss the overall properties of the dSph satellite system (now up to AndXXVI+), and review new discoveries of numerous large stellar streams, some of which have identifiable progenitors (such as AndI and some of the dwarf elliptical companions), and some with no clear progenitor. I will also present follow-up of some of these structures obtained with Keck/DEIMOS and Subaru/SuprimeCam. Finally, I will show how this unique panoramic view of an L* galaxy system is being quantified from a global perspective, specifically relating to measurement of the 2D shape of the stellar halo out to very large radius, and an analysis of the statistics of the substructure distribution for comparison to galaxy formation models.

Stellar-Dynamical Evaporation of Globular Star Clusters

Dean McLaughlin (Keele University)

Star clusters in tidal fields dissolve on Gyr timescales as a result of stellar escape driven by internal two-body relaxation. This evaporation process can have consequences for the internal density and velocity profiles of individual clusters, it has been suggested to affect their mass-to-light ratios, and --- because evaporation lifetimes depend on initial cluster mass --- it substantially alters the mass functions and the structural-parameter correlations of entire cluster systems. I will review the theoretical characterization of evaporation as it relates to these topics and discuss the observational implications.

Star Formation Near & Far: The WFC3 Early Science Program

Robert O'Connell (University of Virginia)

The Wide Field Camera 3 was installed in the Hubble Space Telescope during the May 2009 servicing mission. I will describe some of the early studies that have taken advantage of its extended wavelength coverage (0.2-1.7um), large suite of specialized filters, and improved "discovery efficiency." These include the star formation histories of nearby disk and early-type galaxies (from both resolved sources and integrated light), discovery of multi-age pre-main-sequence populations in massive Local Group star clusters, and the evolutionary state of galaxies at intermediate redshifts (z ~ 0.5-3) in the GOODS field.

The High-Mass Stellar Population and Resulting Feedback Effects

Sally Oey (University of Michigan)

Massive stars are responsible for energetic feedback effects that drive evolutionary processes in star-forming galaxies. Radiative feedback is especially important because it is thought to be responsible for cosmic reionization, as well as ionization of the diffuse, warm ionized medium. Mechanical and chemical feedback are also fundamental in driving galactic chemical evolution, star formation, and other major evolutionary processes. I first describe parameterization of massive stars as a population, to then quantify feedback effects in terms of simple, analytic relations. This review then focuses on our current understanding of radiative feedback as studied in the local universe.

Multiple Stellar Populations in Star Clusters: The Observational Scenario

Giampaolo Piotto (Universite di Padova, Depto di Astronomia)

For half a century it had been astronomical dogma that a globular cluster (GC) consists of stars born at the same time out of the same material, and this doctrine has borne rich fruit. In recent years, however, discoveries made largely with HST shattered this paradigm, and the study of GC populations has acquired a new life that is now moving it in new directions. By pushing HST to the highest level of photometric and astrometric accuracy, multiple sequences have been identified in the color-magnitude diagrams of several Galactic and Magellanic Cloud GCs where they had never been imagined before. In addition, all GCs with spectroscopic studies show a spread in light-element content, including Na/O and Mg/Al anti-correlations. Multiple populations in GCs are no longer confined to some fascinating objects, but are now widespread. I will summarize the main observational evidence of multiple stellar populations in Gcs, and highlight next observational steps we need to compose what still remains an open puzzle.

The Origin of Multiple Populations in Star Clusters

Alvio Renzini (INAF-Osservatorio Astronomico di Padova)

The discovery of widespread multiple stellar populations in globular clusters has forced us to change dramatically our vision of these objects and of their formation. Especially intribuing has been the realization that some of these populations are strongly enriched in helium. The many suggestions so far advanced on the origin of the multiple populations will be reviewed and confronted to updated observational constraints in the attempt to identify those which may remain viable.

Deep Multi-Color Imaging in 47 Tuc: Some Early Results

Harvey Richer (University of British Columbia)

47 Tuc is the nearest of the bulge/disk globular clusters and is also a template for extragalactic work on resolved stellar populations. Hence its age and stellar content is important in understanding how our galaxy formed and in understanding our nearest resolved galaxies. GO 11677 is an ambitious imaging program with HST to explore these and other facets of this cluster. The program was allocated 121 orbits in cycle 17 and uses both ACS and WFC3. A primary goal is to obtain the white dwarf cooling age of the cluster, compare this with the turnoff age determined with numerous filter combinations and explore for the end of the hydrogen-burning main sequence in both optical and near IR bands. Data collection began in January 2010 and will take almost a full year. In this talk I will present both the preliminary ACS CMD in F606W and F814W and the IR CMD in F110W and F160W obtained with WFC3. The age of the cluster based on these data will be discussed even though it is still extremely preliminary.

Stellar Populations in Galaxies Hosting Gamma-Ray Bursts

Sandra Savaglio (Max-Planck Institute for Extraterrestrial Physics)

Many galaxies hosting gamma-ray bursts (GRBs) have high star formation rate and low stellar mass. However, the sample might be biased, as a large fraction of GRBs is dark in the optical (no precise locationzation and unique identification of the host) or at high redshift, where SFR or stellar mass is hard to measure. It is possible that dark-GRB hosts are related to high-z massive galaxies with very high SFR and dust extinction (e.g., similar to sub-mm galaxies). Indeed, we know that some dark GRBs occur in dusty regions. There are also hints suggesting that galaxy mergers are common in the GRB host population, another feature reminding sub-mm galaxies. Although a key ingredient triggering the GRB event, and a population of galaxies hosting GRBs, over the whole history of the universe have not been uniquely identified yet, it seems that high star formation is a common feature in all GRB environments.

The Formation of Stars

Jonathan Tan (Department of Astronomy, University of Florida)

I review our current understanding of the star formation process. I focus on the birth of massive stars, star clusters, the initial mass function, and the connection to kiloparsec and galactic-scale star formation laws.

Modeling the Stellar Populations of High-Redshift Galaxies: Is What You See What You Get?

Scott Trager (Kapteyn Astronomical Institute, University of Groningen)

Using a powerful combination of stellar population models and hierarchical galaxy formation models, I examine what stellar population parameters (ages and star formation histories, metallicity, mass-to-light ratios, etc.) can be recovered from current observations of high-redshift (z>1.5) galaxies. I also make a few predictions about what we might learn about the stellar populations of high-redshift galaxies using the next generation of telescopes such as JWST and 30-40m telescopes.

Multiple Stellar Populations in Globular Clusters

Enrico Vesperini (Drexel University)

I will present the results of a study of the formation and dynamical evolution of multiple stellar populations in globular clusters. The results presented, based on an extensive set of hydrodynamical and N-body simulations, will include a discussion of the dependence of the amount of second generation stars formed on a cluster's initial properties and will show how the structure and the relative number of first and second generation stars evolve during a cluster early and long-term dynamical evolution. Finally I will discuss the results of an investigation aimed at calculating the fraction of the mass of the Galactic stellar halo composed of second generation stars that originated in globular clusters.

Conference Summary

Rosie Wyse (Johns Hopkins University, Physics and Astronomy Department)

Conference Summary