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Space Telescope Science Institute
2010 May Symposium Poster Abstracts

SED fitting with Markov Chain Monte Carlo: results for a new sample of Lyman Alpha Emitters at z=2.1

Viviana Acquaviva (Rutgers University)

We present a Monte Carlo Markov Chain (MCMC) algorithm for galaxy Spectral Energy Distribution (SED) fitting, and apply it to the sample of 250 Lyman Alpha Emitters (LAE) at z = 2.1 from Guaita et al. 2010 (arXiv:0910.2244). Our results indicate that even the 30% of z=2.1 LAEs that have sufficient stellar mass to be detected in deep Spitzer+IRAC images have a relatively young stellar population (Age ~160 Myr), with high Star Formation Rate (~30 M_sun/yr) and significant dust (A_V~0.8). The rest of the population (not detected in IRAC) looks similar to typical LAEs studied at higher redshifts, with Age ~50 Myr, SFR ~ 8 M_sun/yr, and A_V~0.4. Our MCMC algorithm allows for fast, efficient, and robust computation of best fit values and associated uncertainties, with a tremendous saving of CPU time with respect to traditional grid-based methods. For example, the above fit required 8 hours of CPU time on a standard desktop, with continuous sampling of parameter values, a factor of 50 times faster than a brute-force approach sampling each parameter at 100 different values. Furthermore, the higher the dimensionality of the sampled parameter space, the more dramatic the savings of CPU time are with respect to a grid-based method. This enables more detailed models to be fit, including variation in redshift versus a best-fit photo-z, multiple stellar populations, and realistically complex star formation histories. This approach will illuminate degeneracies missed by oversimplified models and therefore greatly reduce systematic errors on SED parameters (Acquaviva et al. 2010, in prep).

Stellar Populations in the Cosmological Context

Peter Behroozi (Stanford University)

We conduct a comprehensive analysis of the relationship between central galaxies and their host dark matter halos, as characterized by the stellar mass-halo mass (SM-HM) relation, with rigorous consideration of uncertainties. Our analysis focuses on results from the abundance matching technique, which assumes that every dark matter halo or subhalo above a specific mass threshold hosts one galaxy. We discuss the quantitative effects of uncertainties in observed galaxy stellar mass functions (GSMFs) (including stellar mass estimates and counting uncertainties), halo mass functions (including cosmology and uncertainties from substructure), and the abundance matching technique used to link galaxies to halos (including scatter in this connection). Our analysis results in a robust estimate of the SM-HM relation and its evolution from z=0 to z=4. The shape and evolution are well constrained for z < 1. The largest uncertainties at these redshifts are due to stellar mass estimates; however, failure to account for scatter in stellar masses at fixed halo mass can lead to errors of similar magnitude in the SM-HM relation for central galaxies in massive halos. We also investigate the SM-HM relation to z=4, although the shape of the relation at higher redshifts remains fairly unconstrained when uncertainties are taken into account. These results will provide a powerful tool to inform galaxy evolution models.

The Galactic Bulge and Thick Disk: An Intragalactic Connection Revealed by Microlensed Dwarf Stars

Thomas Bensby (European Southern Observatory)

The Galactic bulge and the Galactic thick disk are two major stellar components of the Milky Way galaxy. Recent high-resolution spectroscopic studies have shown that they might be intimately connected. Deciphering and understanding this possible connection will give many valuable pieces to the puzzle of galaxy formation in general. Here I will present and discuss recent results based on dwarf stars in the Bulge that were observed with high-resolution spectrographs while they were optically magnified during microlensing events. This sample of microlensed Bulge dwarfs will be compared to a new sample of 700 dwarfs in the Galactic disk, analysed with the exactly same methods, enabling truly differential comparisons between the different stellar populations.

Young Stellar Populations in the Local Group

Luciana Bianchi (Johns Hopkins University, Dept. of Physics and Astronomy)

Wide-field far-UV and near-UV GALEX imaging of Local Group galaxies affords a global, deep census and characterization of their recent star formation, across a wide variety of environmental conditions. The resolved stellar constituents of selected star-forming sites are studied in depth with HST multi-band imaging (Bianchi' treasury program 11079). The HST results provide the key for the interpretation of GALEX's data covering the whole extent of the Local Group galaxies, and of measurements available for hundreds of nearby galaxies beyond the Local Group, shedding new light on the process of star formation in differing conditions, and on the interplay between star formation and interstellar dust.

Are the Ultra-Faint Dwarfs Fossils of the First Galaxies?

Mia Bovill (University of Maryland)

We present the results from a new set of CDM simulations suggesting that a fraction of the recently discovered ultra-faint dwarf population consists of the fossil remnants of galaxies that formed the majority of their stars before reionization. Our new simulations allow us to map the distribution of fossil and non-fossils dwarfs in the Local group and compare the simulated maps to observations. We show good agreement between our simulations and observations of the ultra-faints in both their radial distribution within the Milky Way virial radius and their stellar properties. However, the properties of the faintest ultra-faint satellites found within 50 kpc of the Galactic center do not agree with being well preserved fossils of the first galaxies, perhaps because are modified by tidal effects. We also find some discrepancies with respect to the number of dwarfs in the regions outside the Milky Way virial radius.

Metallicity Gradients and the Milky Way Disk as Observed by the SEGUE Survey

Judy Cheng (University of California Santa Cruz)

Observations of old stars in the Milky Way's disk are ideal for providing detailed constraints on models of disk formation and evolution. Both secular processes and mechanisms within a cosmological context -- such as mergers and accretion -- may have important effects on disk growth at early times. Thus, the observed kinematics, spatial distribution, and compositions of old disk stars in the galaxy can serve as a testbed for models of disk formation and serve as a complement to studies of disks at high redshift. We study the radial and vertical metallicity distribution of the Milky Way disk using medium resolution spectra of 7712 main sequence turnoff stars from the Sloan Extension for Galactic Understand and Exploration (SEGUE). The sample consists of mostly old (> 8 Gyr) thin and thick disk stars, with a minimal contribution from the stellar halo. We present the radial metallicity gradient as a function of vertical height above the disk in the region 6 < r < 16 kpc, 0 < |z| < 1.5 kpc. The radial gradient becomes shallower as the distance from the galactic plane increases. We compare these results with previous radial metallicity gradient results from studies of open clusters. Because various mechanisms affect the metallicity distribution in the disk, these data may be used to distinguish between different formation models.

Effects of Super-Helium-Rich Populations on the Population Synthesis of Early Type Galaxies.

Chul Chung (Yonsei University)

The recent discoveries of multiple stellar populations in massive Milky Way globular clusters (GCs) have provided the evidence that Helium abundance anomalies are primordial in the GCs. In order to keep pace with the recent discoveries, we have constructed enhanced Helium abundance Yonsei Evolutionary Population Synthesis (YEPS) model based on most up-to-date Y2 isochrones and HB evolutionary tracks with enhanced Helium abundance. We present integrated spectro-photometric quantities of enhanced Helium abundance populations with special care in dealing with the morphologies of horizontal branch stars (HBs) with respect to metallicity, age and Helium contents. Our new models provide completely new insights not only on the strong FUV flux of extra galactic GCs but also on the enhanced Balmer-lines of local and intermediate redshift early type galaxies.

Pre-Main Sequence Turn-On as a Chronometer for Young Clusters: NGC346 as a Benchmark

Michele Cignoni (INAF - Astronomical Observatory of Bologna)

We present a novel approach to derive the age of very young star clusters, by using the Turn-On (TOn). The TOn is the point in the color-magnitude diagram (CMD) where the pre-main sequence (PMS) joins the main sequence (MS). In the MS luminosity function (LF) of the cluster, the TOn is identified as a peak followed by a dip. We propose that by combining the CMD analysis with the monitoring of the spatial distribution of MS stars it is possible to reliably identify the TOn in extragalactic star forming regions. Compared to alternative methods, this technique is complementary to the turn-off dating and avoids the systematic biases affecting the PMS phase. We describe the method and its uncertainties, and apply it to the star forming region NGC346, which has been extensively imaged with the Hubble Space Telescope (HST). This study extends the LF approach in crowded extragalactic regions and opens the way for future studies with HST/WFC3, JWST and from the ground with adaptive optics.

Mapping the Evolution of M31 - New Results From the Z-PAndAS Spectroscopic Survey

Michelle Collins (Institute of Astronomy)

Over the past 8 years, our team has been conducting a kinematic survey of individual Red Giant Branch (RGB) stars within the disk and halo of the M31 galaxy. We have discovered and analysed a range of interesting and unusual properties in this, our sister galaxy, including an extended thin stellar disk (Ibata et al., 2005), numerous stellar streams (Ibata et al., 2007; Chapman et al., 2008) and a number of faint dSphs (Collins et al., 2010). In this contribution, I shall present our ongoing survey and results, focusing in particular on the discovery of a kinematically hot component coupled to the thin stellar disk. I will discuss the chemical composition of this structure and comment on its possible origins and what it can tell us about the structure and formation of spiral galaxies.

Detailed Chemical Abundances of Extragalactic Globular Clusters Using High Resolution Spectroscopy

Janet Colucci (UC Santa Cruz)

We have developed a new technique that allows us to measure detailed abundances from high resolution spectra for globular clusters (GCs) in galaxies within 4 Mpc, using current telescopes and instruments. Our method is based on standard methods for analysis of single stars and has been tested with a "training set" of GCs with known properties in the Milky Way and LMC. We have already published results for an initial sample of 5 GCs in M31, as part of an ongoing project to measure abundances in 25-35 GCs. We are also collecting samples of GCs in other nearby galaxies, including the SMC, NGC 6822, NGC 205, NGC 5128, and NGC 1313. In this talk, we present abundances for preliminary GC samples (<5 clusters per galaxy) in M31 and several dwarf galaxies in the Local Group. Our results to date show that these samples have abundances consistent with the field stars in their respective galaxies, demonstrating that they are powerful tools for probing galaxy formation. In addition, our results show that low-resolution line indexes of GCs do not produce accurate results for alpha-element abundances in particular. Here we discuss our abundances for Fe, Fe-peak, alpha-, and light-elements in these clusters.

A Multi-Color Optical Survey of the Orion Nebula Cluster. The New H-R Diagram

Nicola Da Rio (Max-Planck Institute for Astronomy)

The Orion Nebula Cluster (ONC) is the closest site of active massive formation, and therefore regarded as a prototype of dense star forming regions. We present a new analysis of the ONC stellar population, based on the most complete and accurate set of optical photometric data in this region. Based on multi-band, simultaneous, optical photometry as well as new spectroscopy and spectra from the literature, we are able to accurately place ~1000 members in the H-R diagram. We find evidences that the intrinsic colors of ONC members differ from those of MS-dwarfs, while are in agreement with synthetic photometry results utilizing state of the art atmosphere models; we develop a method to accurately disentangle color excesses due to mass accretion and dust extinction. We determine an average age for the ONC higher than previous works, peaking at 2-3Myr with no evidence for mass-age correlations. We determine the IMF complete down to the H-burning limit, and find that it peaks and presents a turn-over at M~0.2Msun, while for intermediate masses we confirm a Salpeter slope.

A Maximum-Likelihood Approach to Constrain Ages and Age Spreads in Young Clusters

Nicola Da Rio (Max-Planck Institute for Astronomy)

An open question in the observational study of young stellar systems concerns the problems related to age estimation. In case of photometric data, the disagreement between different theoretical evolutionary models, together with the observational and physical uncertainties, make an accurate determination of ages and possible age spreads rather challenging. We present a maximum-likelihood method in the CMD to assign ages to PMS cluster members accounting simultaneously for photometric errors , unresolved binarity, source confusion, CTTS variability, differential reddening. This allows us to derive an unbiased estimate of the cluster age. Furthermore, the method allows to accurately disentangle the apparent spread in the CMD typically seen in PMS and the real age spread. We apply the method to the stellar association LH95 in the LMC, observed with HST/ACS to 0.2Msun.

Stars Formed Within HI Debris of Interacting Galaxies

Duilia De Mello (CUA/GSFC)

I will present an overview of our study searching for star formation within HI debris using a variety of diagnostics. Our sample contains 34 galaxies that are going through collisions and have extended HI gas and GALEX data. We have identified dozens of stellar clusters candidates which are located outside the galaxies but within the HI debris. Our follow up data, which includes VATT, WYIN and Gemini data, reveal masses, metallicities and SFR for a subsample of these objects. We have also searched the HST archive and found a few stellar associations, nicknamed blue blobs. Our results so far show that some of these young stellar populations were formed in situ from reprocessed gas which was ejected during the interaction. We are aiming at quantifying the amount of star formation that takes places in this type of environment which might be very common at high redshifts, when interactions were frequent and galaxies were gas rich, and might explain how the intergalactic medium got enriched at an early epoch.

Characterization of Andromeda's Disk Kinematics

Claire Dorman (University of California, Santa Cruz)

Trends in stellar kinematics in the inner regions of the Andromeda galaxy (M31) offer insight into the structure and formation of spiral galaxies. We present a spectroscopic study of the resolved red giant star population in the inner (R_proj<~7.5kpc) southern quadrant of M31 based on Keck/DEIMOS observations of nine multislit masks. Radial velocities are measured for about 2000 red giants and for the ambient ionized gas in M31 in a subset of the slits, the latter being an unexpected by-product of this study. The 2D pattern of stellar velocities appears to be quite regular and is well explained by the combination of two components: one with high v_rot/sigma associated with an exponential disk and one with low v_rot/sigma associated with a Sersic bulge. A detailed comparison is carried out between the kinematics of the stellar disk and that of the ionized gas, the latter plausibly associated with M31's star-forming thin disk. These data provide valuable constraints on models of disk formation and evolution, including the role of heating by/accretion of small satellite galaxies. In the fall, we plan to obtain DEIMOS spectra and radial velocities for about 3000 stars in the eastern quadrant of M31, mostly disk stars, along with kinematics of the ionized gas along these lines of sight. The spatial coverage of these proposed new DEIMOS observations corresponds to that of the recently approved HST Multi-Cycle Treasury Program to obtain deep UV to near-IR photometry of M31's resolved stellar population. Over the next few years, the synergy between these rich spectroscopic and photometric datasets will reveal the detailed assembly, star formation, and chemical enrichment histories of the disk of our large spiral galaxy neighbor.

Stellar Evolutionary Implications of Individual Abundance Variations Spanning a Wide Range of [Fe/H]

Aaron Dotter (University of Victoria - Dept. of Physics and Astronomy)

Stellar evolution and atmosphere models provide a framework for interpreting observations of resolved and unresolved stellar populations. Stellar evolution models typically assume a heavy element distribution based on the Sun or, for metal-poor stellar populations, an 'alpha-enhanced' mixture. However, spectroscopic studies clearly indicate that detailed abundances vary as a function of [Fe/H] and the environment in which stars form. It is crucial to understand the influence of individual heavy elements on stellar evolution but, to date, theoretical analyses have only scratched the surface. Preliminary results from a significant effort to examine the effect of individually varying the abundances of C, N, O, Ne, Na, Mg, Si, S, and Ca at fixed [Fe/H] values ranging from -3.0 to 0.0 and initial He abundances corresponding to Y=0.25, 0.29, and 0.33 will be reported. This will include a discussion of element-specific variations in stellar lifetimes and CMD morphology as well as implications for determining the ages of resolved stellar populations.

The Nature and Dynamics of Andromeda's Giant Southern Stream Debris

Mark Fardal (UMass)

M31 shows a giant stream of metal-rich stars extending 150 kpc into its southern halo, as well as many other other tidal features, several having stellar populations quite similar to the giant stream. Our dynamical models of a satellite galaxy colliding with M31 show good agreement with the morphology and kinematics of many of these tidal features. Bayesian sampling of our N-body models tightly constrains the model parameters, suggesting an incoming satellite of log M_s approx 9.5 and an M31 virial mass of log M_{200} approx 12.2. The initial satellite mass is comparable to that of the LMC, though the total luminosity is smaller. The satellite's orbital time is much smaller than the age of the stream population determined from deep field CMDs, suggesting an earlier episode of gas stripping. Our approach automatically generates a sample of models spanning the allowed region, enabling robust predictions to be tested against observations. We discuss new kinematic data in one of the tidal features that bolsters its identification as a radial shell in this model. The significant metallicity gradients observed within the stream call into question the use of stellar properties alone in identifying the stream material. This work shows how the cooperation of detailed stellar population observations and dynamical models helps to decode the complex patterns in a galaxy's halo.

Using the Calcium Triplet as a Metallicity Indicator for Unresolved Stellar Populations: A New Puzzle

Caroline Foster (Swinburne University of Technology)

The Calcium Triplet (CaT) has been employed as a metallicity indicator for resolved Galactic and Local Group stars as well as for the integrated light of Milky Way globular clusters (GCs) and galaxies. Due to its prominence, the CaT index is easily measured, even for relatively faint populations making it a potentially useful feature for measuring stellar phase metallicities using NIR spectra for moderate redshift galaxies. Single stellar population models predict that the CaT should be an adequate metallicity indicator. On the other hand, its usefulness in determining metallicities for the “simplest” unresolved systems, i.e. extragalactic GCs, has not been tested. Using DEIMOS on Keck we obtained NIR spectra of 144 GCs around the giant elliptical galaxy NGC 1407 to test whether or not the CaT can be used as a metallicity indicator for extragalactic GCs. We use Armandroff & Zinn’s empirical conversion obtained from integrated light spectra of the Milky Way GCs to convert CaT to metallicity. The measured CaT/metallicity distributions show unexpected features, the most remarkable being that the brightest blue and red GCs have similar CaT index values suggesting they have a similar mean metallicity ([Fe/H]~-0.8 dex) despite their large difference in mean color. Current Single Stellar Population models do not predict this behavior. Several possibilities are explored.

Estimating Cluster Properties in a Stochastic Context

Morgan Fouesneau (Observatoire Astronomique Universite de Strasbourg)

Star clusters are probes of star formation histories, and of cluster formation and disruption processes. The vast majority of clusters in the Universe are small, and it is well known that the integrated fluxes and colors of all but the most massive ones have broad probability distributions, due to small numbers of bright stars. We developed a Bayesian approach, which provides posterior probability distributions in the age-mass-extinction space, using multi-wavelength photometric observations and a large collection of Monte-Carlo simulations of clusters of finite stellar masses. This poster goes beyond the description of predicted probability distributions, and presents results of the analysis of cluster energy distributions in an explicitly stochastic context. We first explain errors arising from the use of standard analysis methods, which are based on continuous population synthesis models, and then we compare to our Bayesian method. This contribution also presents effects on real observations and derived results.

Stellar Populations in Milky Way Progenitors at z=3

Eric Gawiser (Rutgers University)

We studied the multiwavelength spectral energy distributions (SEDs) of a complete sample of 154 z=3.1 Lyman Alpha Emitting (LAE) galaxies. The evolution of galaxy clustering with redshift implies that z=3.1 LAEs evolve into typical present-day galaxies with L~=L*, whereas other z>3 galaxy populations, including Lyman Break Galaxies, typically evolve into more massive galaxies. Only 30% of the LAEs have sufficient stellar mass (>3x10^9 M_sun) to yield detections in deep Spitzer-IRAC imaging. A two-population SED fit to the stacked UBVRIzJK+[3.6,4.5,5.6,8.0]micron fluxes of the IRAC-undetected objects finds that the typical LAE has a young (starburst) component age of 20 Myr, an old component age ~1 Gyr, low stellar mass (10^9 M_sun), and moderate star formation rate (2 M_sun/yr). The IRAC-detected LAEs are significantly more massive but have lower specific star formation rates, with stellar mass ~10^10 M_sun and SFR ~6 M_sun/yr. Both sets of LAEs reveal minimal dust (A_V<0.2), consistent with being in the early phases of a starburst. However, the presence of evolved stellar populations argues against the interpretation that most LAEs are experiencing their initial burst of star formation. References: Gawiser et al. 2007 (ApJ 671, 278), Lai et al. 2008 (ApJ 674, 70)

Tracers of Stellar Mass-Loss: Optical and IR Colors and Surface Brightness Fluctuations

Rosa Amelia Gonzalez-Lopez Lira (UNAM)

We present optical and IR integrated colours and SBF magnitudes, computed from stellar population synthesis models that include emission from the dusty envelopes surrounding TP-AGB stars undergoing mass-loss. We explore the effects of varying the mass-loss rate by one order of magnitude around the fiducial value, modifying accordingly both the stellar parameters and the output spectra of the TP-AGB stars plus their dusty envelopes. We compare these models to optical and IR data of single AGB stars and Magellanic star clusters. Neither broad-band colours nor SBF measurements in the optical or the near-IR can discern global changes in the mass-loss rate of a stellar population. However, we predict that mid-IR SBF measurements can pick out such changes, and actually resolve whether a relation between metallicity and mass-loss exists.

The SPLASH Survey and Progressive Stages of Galaxy Formation

Raja Guhathakurta (UCO Lick Observatory, Univ 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. These spectral data are complemented by wide-field ground-based images and deep Hubble Space Telescope images. 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.

Color and Stellar Population Gradients of Early-type Galaxies at z∼2 from HST/WFC3 Deep Imaging in the Hubble Ultra Deep Field

Yicheng Guo (University of Massachusetts)

We report the detection of color gradients in massive (stellar mass M>10^{10} Msun) galaxies with low specific star formation rate (SSFR<10{-11}/yr) at redshift z~2. The galaxies are selected by means of SED fitting to spectral population synthesis models using the Great Observatories Origins Deep Survey (GOODS) broad--band photometry, which spans the optical, near--IR and mid--IR windows, augmented by recent ultra--deep near--IR images obtained with HST WFC3. The estimated stellar mass and SSFR would place these galaxies among today's Hubble early types, while their rest--frame optical morphology, as shown by the WFC3 images, is consistent with that of spheroidal systems. The inner regions of these galaxies are found to have redder rest UV--optical colors than their outer parts. To study the physical implications of these color gradients we fit spatially resolved HST seven--bands photometry from ACS and WFC3 images (BVizYJH) in concentric shells across the light profile of each galaxies, sampling the color gradients. The fit shows that the bluer outer regions of the galaxies are up to about 1 Gyr younger than the redder inner regions and have up to 10^2 times higher SSFR. This can result from a differential mass assembly in these galaxies, in which star formation in the outer regions takes place at a later time or on a longer time scale than in the denser cores. Alternatively, the observed color gradients could result from a mechanism that shuts down star formation more abruptly and effectively in the central regions of the galaxies than in the outskirts.

Observing the First Stars in Luminous Red Galaxies

Sara Heap (NASA's Goddard Space Flight Center)

Modern cosmological simulations predict that the first stars are to be found today in luminous, red galaxies. Although observing such stars individually against a background of younger, metal-rich stars is impossible, the first stars should make their presence known by their strong, line-free ultraviolet flux. We have found evidence for a UV-bright stellar population in Sloan spectra of LRG’s at z=0.4-0.5. We present arguments for interpreting this UV-bright stellar population as the oldest stars rather than other types of stellar populations, e.g. young stars or blue straggler stars in the dominant, metal-rich stellar population.

A Study of M32's Internal Stellar Kinematics from a Combination of Integrated Light and Resolved Star Spectroscopy

Kirsten Howley (University of California, Santa Cruz)

We present new integrated light (r<100") and resolved star (100"<200") spectroscopic observations of the galaxy M32 (r_eff=28.5"=0.1 kpc) using the DEIMOS instrument on the Keck II telescope. M32, a satellite of the Andromeda (M31) galaxy, is our nearest example of the rare and enigmatic compact elliptical galaxy type. It has long been known that the outer isophotes of M32 are distorted, and this is presumed to be caused by its tidal interaction with M31. Previous kinematical studies of M32 have obtained a rotation curve and velocity dispersion profile out to r~30" --- by contrast, our new measurements of M32's stellar kinematics extend out to nearly r=200", just beyond the tidal distortion radius of r~150". Our kinematical study of the low surface brightness outlying region of M32 is made challenging by the fact that it is projected against the bright/complicated inner regions of M31. However, we use surface brightness and kinematical trends versus sky position to statistically account for contamination of this region by M31's disk and bulge stars. Unlike NGC 205, another close neighbor of M31, M32's kinematics appear to be regular and symmetric and do not show obvious sharp gradients across the tidal distortion region. This lack of a strong gradient will serve as a constraint on tidal interaction models. At present, the velocity and dispersion profiles obtained from these data are being used to measure the mass profile of M32 through Jeans modeling. This research is supported by the National Science Foundation, NASA and the Lawrence Scholars Program.

Resolved Stellar Populations With VLT MUSE

Tim-Oliver Husser (Institute for Astrophysics Goettingen)

In 2012 a consortium of seven major European research institutes will deliver a new instrument for ESO's Very Large Telescope (VLT) located on the Cerro Paranal in Chile: MUSE (Multi Unit Spectroscopic Explorer). With its 1'x1' Field of View, a spatial sampling of 0.2" and a spectral sampling of 1.3 Angstrom ranging from 4650 to 9300 Angstroms, this Integral Field Unit (IFU) is an ideal playground for studying resolved stellar populations in our own galaxy and beyond. We are going to present first analyses of the potential that comes with the new instrument in this field of research. Currently we are working on a data pipeline covering all steps from extracting the spectra of single stars from the data cube produced by MUSE using PSF fitting and deconvolution techniques, to an automatic classification of those spectra. This becomes necessary, since a single MUSE exposure will yield hundreds or even thousands of spectra. There are already tools available for simulating MUSE's behaviour in both the spatial and the spectral domain. For the spectral classification we have also created a large grid of synthetic spectra in order to determine parameters like effective temperature, surface gravity, metallicity or even single element abundances for observed spectra.

Star Clusters as Tracers of Galaxy Evolution

Iraklis Konstantopoulos (Penn State University)

Star cluster formation is not limited to 'classical' environments such as galactic spiral arms. Observations find them in all types of settings, supporting the existence of a single 'mode' of sub-galactic structure formation: clustered star formation. In that way, clusters maintain a close link to the overall activity in a system and as such can be used to trace the star formation history of a galaxy. Furthermore, their brightness enables observations of clusters in faraway systems. This talk will present studies of clusters in a variety of environments, from starburst cores to intergalactic space. I will discuss our thorough spectroscopic and photometric measurements of age, metallicity, extinction and mass of stellar populations in the disk and nucleus of M82, the tidal tails of NGC 6872 and the Antennae, as well as two very different galaxy groups, Stefan's Quintet and Hickson Compact Group 7. Star clusters provide important information about each of these host systems - information that is not easily discernible using conventional methods. Thus, we learn much about the state of each host, with the added bonus of enhancing our understanding of cluster formation and evolution.

Period Changes and Evolution Similarities Between of the RR Lyrae in Oosterhoff I and Oosterhoff II Globular Systems

Andrea Kunder (Cerro Tololo Inter-American Observatory)

RR Lyrae in Milky Way globular clusters are known to divide into two distinct groups, now known as Oosterhoff groups I and II (OoI and OoII), based on the average periods of the fundamental mode RR Lyrae ( ~ 0.55 d for OoI and ~ 0.65 d for OoII globular clusters, with very few clusters in the range 0.58 < < 0.62, termed the “Oosterhoff gap”). Oosterhoff group I tend to be more metal-rich than those of Oosterhoff group II, and both Oosterhoff groups occupy different areas on the Period-Amplitude plane. However, the dwarf galaxies surrounding the Milky Way and their globular clusters, do not exhibit the Oosterhoff dichotomy. The current formation scenario of the Galactic halo is that the halo was built-up from the accretion of smaller “protogalactic fragments” similar to the present-day Milky Way dSph satellite galaxies. If this were the case, their should be no Oosterhoff dichotomy in the Milky Way, since the dSph galaxies and their globular clusters are predominantly intermediate between the two Oosterhoff classes. I will discuss the implications of the Oosterhoff dichotomy and the Oosterhoff gap for stellar evolution and for the formation and evolution of the Galactic halo. In particular, new period change rates of the RR Lyrae in IC 4499 are calculated, which give insight to the horizontal branch evolution for this Milky Way globular cluster. A reassessment of period change rates of RR Lyrae in different Oosterhoff systems is performed, to provide insights of the stellar evolution on the Horizontal Branch.

Panchromatic Estimation of Star Formation Rates in Galaxies at 1<3

Peter Kurczynski (Rutgers University)

We utilized the rest-UV-through-radio Spectral Energy Distributions (SEDs) of a sample of BzK galaxies to determine their average star formation rates and dust temperatures. Galaxies were separated into bins of photometric redshift. In order to obtain panchromatic SEDs for each set of galaxies, we performed stacking analyses in Spitzer-MIPS (24, 70 micron), sub-millimeter (250, 350, 500 microns from BLAST, 870 microns from the Laboca ECDF-S Sub-mm Survey, LESS), and radio (VLA 1.4 GHz and GMRT 610 MHz) data. Our estimates of bolometric IR luminosity utilized photometric data that spanned the entire peak of the dust emission spectrum, and thus improved upon previous single band estimators of IR luminosity. We combined UV continuum and bolometric IR star formation rate estimators and found redshift binned, average star formation rates of 29-130 M_sun yr^-1, increasing with median redshift from 1.3-2.7. Our results imply substantially higher star formation at high redshift, and also somewhat diminished dust obscuration at high redshift.

Spitzer Analysis of HII Region Complexes in the Magellanic Clouds: Determining a Suitable Monochromatic Obscured Star Formation Indicator

Brandon Lawton (STScI)

HII complexes are the birth places of stars, and as such they provide the best measures of current star formation rates in galaxies. The close proximity of the Magellanic Clouds allows us to probe the nature of these star forming regions at small spatial scales (~10pc). We present the spatial analysis, via aperture/annulus photometry, of 16 LMC and 16 SMC HII complexes using the Spitzer IRAC and MIPS bands. The results are compared with the bolometric luminosities, L(TIR), where L(TIR) is integrated from 8um to 500um. We find that the HII complex sizes depend on the dust component measured. The warm dust probed by 24um extends out to ~50pc. The cold dust probed by 160um extends out to ~90pc. The sizes measured via 70um are nearly identical to the sizes measured via L(TIR) (~70pc) as a result of the SEDs of the star forming regions all peaking near 70um at all radii. Consequently, the 70um band remains relatively constant when normalized by the L(TIR) at all radii (L70 ~0.45 L(TIR)). Contrarily, the normalized 24um band is highly peaked in the center of the HII complexes, and the normalized 160um band is highly peaked in the outer radii of the HII complexes. The normalized 8um and 24um bands also show a significant dependence on the host galaxy in that they are weaker at all radii in the SMC relative to the LMC. The 1s standard deviation of the normalized 70um luminosities is a lower fraction of the mean (0.05 to 0.12 depending on radii), relative to the normalized 8um, 24um, and 160um luminosities (0.15 to 0.52). From these findings we argue that 70um is the most suitable IR band to use as a monochromatic obscured star formation rate indicator. SFR calculations from the literature, modified to use 70um luminosities, are included.

Understanding the Stellar Population Properties of High-Redshift Star-forming Galaxies : Challenges and Lessons

Seong-Kook Lee (Johns Hopkins University)

Powerful facilities -- such as Hubble Space Telescope/ACS and new WFC3 -- provide us unprecedented opportunity for probing and investigating high redshift galaxies. This progress is expected to advance with the upcoming powerful JWST. One significant difference between the studies of local or low-redshift galaxies and those of high-redshift galaxies is that we cannot see the resolved individual stars or stellar clusters, but only the integrated light from entire stellar and ISM component of a given galaxy. Thus, for an investigation of these high-redshift galaxies, we need a tool for estimating the stellar populations of these galaxies from their integrated light. One widely used technique is to fit models to the spectral energy distributions as measured via broadband photometry. A problem that is not widely appreciated is that the inferences for stellar masses, ages, star-formation rates, and dust contents thus derived are not only uncertain, but also tend to be highly biased. We show the patterns of biases in SED-fitting analysis with various forms of SFHs used and demonstrate that increasing SFHs are more relevant forms of SFHs than exponentially declining ones in SED-fitting analysis of high-redshift star-forming galaxies. We also present the stellar population properties and their evolution of the high-redshift ($3 leq z leq 6$) star-forming galaxies observed in the GOODS-S field -- such as stellar masses and star-formation rates.

Comparison of Alpha-Element-Enhanced Simple Stellar Population Models with Milky Way Globular Clusters

Hyun-chul Lee (University of Texas-Pan American)

We present simple stellar population (SSP) models with scaled-solar and α-element-enhanced abundances. The SSP models are based on the Dartmouth Stellar Evolution Database, our library of synthetic stellar spectra (Lee et al. 2009, ApJ, 694, 902), and a detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity. In order to test the relative importance of a variety of SSP model ingredients, we compare our SSP models with integrated spectra of 41 Milky Way globular clusters (MWGCs) from Schiavon et al. (2005). Using the Mg b and Ca4227 indices, we confirm that Mg and Ca are enhanced by about +0.4 and +0.2 dex, respectively, in agreement with results from high-resolution spectra of individual stars in MWGCs. Balmer lines, particularly Hγ and Hδ, of MWGCs are reproduced by our α-enhanced SSP models not only because of the combination of isochrone and spectral effects but also because of our reasonable HB treatment. Moreover, it is shown that the Mg abundance significantly influences Balmer and iron line indices. Finally, the investigation of power-law initial mass function (IMF) variations suggests that an IMF much shallower than Salpeter is unrealistic because the Balmer lines are too strong on the metal-poor side to be compatible with observations.

The Formation of Spiral Spheroids and Their Globular Cluster Systems

Aparna Maybhate (STScI)

Two main scenarios for building bulges in spiral galaxies are currently popular: Merger-driven formation by dissipative merging and secular evolution of inner disk stars. The former process involves the formation of massive metal-rich star clusters, while the latter does not. In order to discriminate between these two processes of bulge formation, we use deep, HST images taken the the ACS Wide Field Camera in the F475W and F814W filters to detect and study the globular cluster systems in four edge-on Sa galaxies: NGC 5475, NGC 4710, NGC 5308, and NGC 4866. These cover a factor of 4 in luminosity, and hence in mass. We see a clear trend of increasing red cluster fraction with increasing bulge luminosity in these galaxies. This indicates that merging processes played a major role in the formation of the bulges in these early-type spirals. We also find that the specific frequency of blue clusters is consistent with typical specific frequency values for later-type spirals.

The Stellar Populations of M32: Resolving the nearest elliptical with HST ACS/HRC

Antonela Monachesi (Kapteyn Astronomical Institute)

The Local Group galaxy M32 is the only {em bona fide} elliptical close enough such that both spectroscopy and photometry of its resolved stars can be applied to study this galaxy. It is therefore a vital template for models used to decipher the stellar populations of all other elliptical galaxies, which can only be studied by the spectra of their integrated light, given their greater distances. We present here the deepest optical color-magnitude diagram (CMD) to date of M32, obtained from deep $F435W$, $F555W$ photometry based on HST ACS/HRC images. The CMD of M32 displays a wide color distribution of red giant branch stars indicating an intrinsic spread in metallicity with a peak at [Fe/H] $sim -0.2$, a strong red clump -- expected for metal-rich populations -- and asymptotic giant branch stars rising to $mathrm{M}_{F555W}sim -2.0$. We detect for the first time a red giant branch bump in M32 which, together with the asymptotic giant branch bump, allows us to constrain the age and metallicity of the M32 dominant population. There is not a noticeable presence of blue horizontal branch stars, suggesting that an old population with $mathrm{[Fe/H]} < -1.3$ does not significantly contribute to the light or mass of M32 in our observed fields. The detection of a blue component of stars (blue plume) reveals for the first time the presence of a very young stellar population, with ages of the order of 0.7 Gyr, in this galaxy.

Stellar Population Fitting in the ALHAMBRA-Survey

Antonio Montero-Dorta (IAA-CSIC)

The ALHAMBRA-Survey is an ongoing photometric survey primarily designed for the study of the evolution of the galaxy properties from z~1. To this purpose, it utilizes the 3.5 m Telescope at Calar Alto Observatory to cover a region of 4 sq. deg, down to magnitude I(AB)~25. Observations have been carried out in 20 contiguous, equal-width filters covering the entire optical range and in the NIR J, H, K bands. Featuring a combination of depth and spectral resolution, ALHAMBRA has the potential to shed light on the average evolution of stellar populations from the time when the cosmic star formation rate is known to peak. In order to characterize stellar populations, we use the Flexible Stellar Population Synthesis code (FSPS, Conroy et al. 2009) to generate a grid of models representative of the ALHAMBRA population of galaxies at different redshift ranges. In my talk (or poster) I would introduce the ALHAMBRA-Survey and present some preliminary results on stellar population fitting of ALHAMBRA galaxies. I would focus on the estimation of the stellar masses and the Specific Star Formation Rates.

Star Clusters in the Tidal Tails of Interacting Galaxies: Photometric Properties and the Tail Environment

Brendan Mullan (Penn State University)

We have searched for compact stellar structures within the tidal tails of twelve different interacting galaxies using F606W- and F814W- band images from the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST). The tidal tails of these galaxies span a physical parameter space of HI and stellar mass, pressure, and density from their varied interactions, and refine our understanding of the conditions in which star cluster formation occurs. The sample of seventeen tails of twelve interacting galaxies includes a diverse population of tail lengths, optical brightness, merging mass ratios, Hi column densities, and stage on the Toomre sequence. We have found that clustered star formation appears to occur frequently in galaxy interactions, regardless of many gross properties of the interaction and tail environment. However, there is a significant difference in the populations, with some tails having very few luminous clusters. This could be due to certain inhospitable merger dynamics (e.g. high speed, V 1000 km/s encounters), and/or the timing of the most recent star formation episodes in the tail. We contend that star cluster formation in tidal tails is likely a ubiquitous consequence of galaxy interactions where there is strong evidence of in situ star formation. Statistical diagnostics also indicate that these clusters arise from initial mass functions similar to those in quiescent galaxies and mergers, but possibly modulated by different peak and exponential parameters related to differences in turbulent pressures and ambient ISM density of the tail environment.

Revealing the Large Stellar Disk of the Small Magellanic Cloud

David Nidever (University of Virginia)

The Magellanic Clouds are our local laboratory for understanding the evolution and properties of dwarf galaxies. In order to reveal the interaction history of the Magellanic Clouds, it is vital to undertake a large-scale photometric and spectroscopic study of their stellar periphery (The MAgellanic Periphery Survey, MAPS). We present our first results for the Small Magellanic Cloud (SMC); Washington M, T2 + DDO51 photometry reveal metal-poor red giant branch stars in the SMC that extend out to large radii (8.8 kpc), are nearly azimuthally symmetric (ellipticity=0.1), and are well-fit by an exponential profile. These results suggest that the SMC has a large, inclined exponential stellar disk. This structure is in contrast to an inner stellar distribution that is more elliptical (0.3) and offset from the outer disk by 0.5 kpc, suggestive of an off-center bar (similar to that of the LMC). Our enhanced understanding of the stellar structure and populations of the SMC will help shed light on the evolution of dwarf galaxies that are interacting with their larger neighbors.

Stellar Populations in the Super Star Clusters NGC3603 and 30 Doradus

Francesco Paresce (INAF/IASF Bologna)

ERS observations of the super star clusters NGC3603 and 30 Doradus in the UV, optical and near IR with WFC3 show for the first time clear evidence of multiple stellar populations in both clusters with an age spread of 1 to 20-30 Myrs. The spatial distributions of the cluster stars indicate that the older population is more widely and uniformly distributed over the cluster field than the much clumpier younger population. A reasonable separation of the main components of these populations can now be made by the use of their Halpha excess, a good indicator of their PMS status, allowing a better understanding of the stellar mass function and its evolution in time.

Stellar Populations in M33: Focus on Cepheid Variables

Anne Pellerin (Texas A&M University)

We present the preliminary results of our long-term photometric survey of variable stars in the nearby galaxy M33. The ultimate goal of the survey is to better constrain the calibration of the Cepheid Period-Luminosity relation. This work combines previously-obtained data from the DIRECT project with new observations acquired at the WIYN 3.5m telescope. The entire data set spans over 7 years with excellent synoptic coverage which enables the discovery and characterization of stars displaying variability over a wide range of timescales (days, weeks, months, years). Here we focus on the Cepheid population of M33. We show representative light curves, color-magnitude diagrams, and optical Cepheid Period-Luminosity relations for M33. More particularly, we discuss the effect of blending and crowding on the search for a metallicity dependance of Cepheid luminosity and ultimately on the distance modulus determination.

Understanding the most Massive Galaxies using Globular Clusters

Eric Peng (Peking University)

Globular clusters are our most easily accessible local window onto the high redshift Universe. While the ability to resolve field stellar populations in local galaxies is limited to the Milky Way and nearby galaxies, globular clusters are visible out to distances of hundreds of Mpc. They are the "tip of the iceberg" for the oldest stars -- populations which are otherwise extremely difficult to study in distant galaxies -- and it is our ability to study local GCs in detail that make them such useful markers. HST and ground-based surveys of nearby galaxy clusters are transforming our view of these halo populations. I will present recent results from the HST/ACS Coma Cluster Treasury Survey and the Next Generation Virgo Survey showing new evidence for intergalactic globular clusters, and describe what this tells us about galaxy evolution.

Chemodynamics of Compact Stellar Systems in the Nearest Massive Elliptical NGC 5128

Thomas H. Puzia (Herzberg Institute of Astrophysics)

We will present velocity dispersion measurements for luminous GCs in NGC 5128 derived from high-resolution VLT/UVES spectra. Combining the measured velocity dispersions with surface photometry and structural parameter data from HST enables both dynamical masses and M/L ratios to be derived. It is found that the fundamental plane relations of these massive stellar systems match those of nuclear clusters in dwarf elliptical galaxies and UCDs better than those of Local Group GCs, and that all objects share similarly old (>8 Gyr) ages, suggesting a possible link between the formation and evolution of dE,Ns, UCDs and massive GCs. We find a very steep correlation between dynamical (M/L) ratio and dynamical mass of the form (M/L)_dyn ~ M_dyn^(0.24+/-0.02) above M_dyn = 2x10^6 Msol. Formation scenarios are investigated with a chemical abundance analysis using absorption line strengths calibrated to the Lick/IDS index system. The results lend support to two scenarios contained within a single general formation scheme. Old, massive, super-solar [alpha/Fe] systems are formed on short (<100 Myr) timescales through the merging of single-collapse GCs which themselves are formed within single, giant molecular clouds. More intermediate- and old-aged (~3-10 Gyr), solar- to sub-solar [alpha/Fe] systems are formed on much longer (~Gyr) timescales through the stripping of dE,Ns in the 10^13-10^15 Msol potential wells of massive galaxies and galaxy clusters.

The GHOSTS Survey - Probing the Outskirts of Disk Galaxies

David Radburn-Smith (University of Washington)

The GHOSTS survey is to date the largest study of the resolved stellar populations in the outskirts of nearby disk galaxies. With the ACS and WFC3 instruments onboard HST, we are studying stars down to 1.5 magnitudes below the tip of the Red Giant Branch both in the disk and in the stellar halo. This allows us to probe stellar densities down to extremely low surface brightness levels, equivalent to ~32 V-mag per square arcsecond. The observations thus offer a detailed insight into both past and recent formation in each galaxy. This talk details the survey and presents some of our initial findings, including a study of the profile and structure of the stellar halos as well as a look at the phenomenon of disk truncations and their implications for stellar migration rates.

Young Stellar Populations Near and Far: The Importance of Nebular Emission

Amy Reines (University of Virginia)

In the early universe, the cosmic star formation rate was high and the formation of massive star clusters was common. Therefore, the study of young massive clusters (YMCs) in local starburst environments plays a crucial role in our understanding of galaxies at high redshifts. In both nearby YMCs and distant galaxies, broadband photometry is commonly used to estimate physical properties such as masses, ages, and extinctions; however, contamination from ionized gas emission is often neglected. By comparing spectroscopic and photometric observations of young massive star clusters to evolutionary synthesis models, we assess the impact of nebular continuum and line emission on broadband photometry of YMCs (Reines et al. 2010). We find that nebular continuum can actually rival the stellar continuum at optical wavelengths and that contamination from line emission is severe in many commonly used broadband filters. Neglecting the effects of nebular emission in observations of YMCs can lead to gross overestimates of stellar masses, older ages, and erroneous extinctions, clouding our understanding of the birth and evolution of massive clusters. Similarly, nebular emission should not be readily dismissed when interpreting observations of starburst galaxies throughout the universe.

Ancient Stars in Dwarf Galaxies Beyond the Local Group

Marina Rejkuba (European Southern Observatory)

We report the discovery of blue horizontal branch stars and numerous RR Lyrae variables in two Sculptor group dwarf galaxies. These stars are the first direct observational confirmation of the presence of ancient stellar populations beyond the Local Group. The lower density of Sculptor group with respect to our Local Group probes a different environment. These results thus suggest that gas can condense and form stars in low mass halos at the earliest epochs even in relatively low-density dwarfs, providing new constraints to star formation and galaxy formation models at early cosmological epochs.

Evidence that the Milky Way bulge is a dynamical bar

R. Michael Rich (Dept of Physics and Astronomy, UCLA)

We have fit velocity data covering much of the bulge, from the Bulge Radial Velocity Assay, to an N-body model derived from a disk that buckles and evolves into a bar. The fit is robust, exhibiting cylindrical rotation 1000 pc from the plane. We are also able to show that no more than 8% of the mass of the bar can be in the form of a classical bulge component. We argue that the bulge is dominated by the bar, and does not have multiple dynamical subcomponents; thus, a predominance of classical spheroid population at high Galactic latitudes cannot be invoked to explain the observed abundance gradient. The evidence that the bulge is an old secularly evolved bar challenges the LCDM scenario, which requires that the bulge grow via mergers.

Blue Hook and Beyond: The Deepest Color-Magnitude Diagram of NGC 6397

R. Michael Rich (Dept of Physics and Astronomy, UCLA)

We present a new color-magnitude diagram reaching ~1 mag fainter than prior work, for the halo globular cluster NGC 6397. We employ proper motion separation over a 5-year time baseline, with the first epoch being 9 orbits, just concluded in 2010 March. The cleaned CMD reveals a clean, well defined, faint "blue hook" in the white dwarf population, as well as a population of WD fainter than the blue hook, these stars detected for the first time. We report improved age constraints based on the fitting cooling models to the white dwarf luminosity function. We conclude that the predicted blue hook structure (from application of collision induced absorption) is a clear, real feature in the color-magnitude diagram.

The Morphologies of the Star Forming Regions in LIRGs and ULIRGs

Javier Rodriguez (Instituto de la Estructura de la Materia (CSIC))

Luminous and Ultraluminous infrared galaxies (LIRGs and ULIRGs) are much more numerous at higher redshifts than locally, dominating the star formation rate density at redshifts ~1 - 2. Therefore, they are important objects in order to understand how galaxies form and evolve through cosmic time. Local samples provide a unique opportunity to study these objects in detail. With that in mind we have undertaken a project based on integral field spectroscopic (IFS) observations of a large sample of LIRGs and ULIRGs. Among other topics, we have studied the star formation histories of these objects as well as to the morphologies and kinematics of the spatially resolved HII regions. For example, the Hα 2D maps show clumpy extended structures, such as tidal tails, bridges or rings that are not visible in continuum emission. In addition, we have used spectral synthesis modeling and found that the star formation histories of this objects are complex, with at least two epochs of star formation in some cases. These results will be presented in a poster.

Constraining the Evolution of Virgo Cluster Galaxies with Stellar Populations

Joel Roediger (Queen's University)

Star formation and chemical enrichment are two fundamental aspects of the duty cycle of baryons in galaxies, such that accurate descriptions of these phenomena represent critical tests for any model of galaxy formation and evolution. In order to improve our understanding of these phenomena, we derive the boundary conditions on stellar populations using the most extensive database of resolved, optical and near-infrared photometry to date for a complete sample of 300 Virgo cluster galaxies. We compare the ages, metallicities and gradients for these galaxies against morphological, structural and environmental diagnostics. Our analysis shows that galaxies' star formation histories are dictated by their morphology and gas removal processes, whereas their chemical evolution (while also affected by morphology) is also strongly connected to both their stellar mass and the depth of their potential wells. These results conflict with similar previous studies and highlight the need for more extensive modelling and for well-sampled multi-wavelength galaxy light profiles. We also discuss the tentative implications of our results in the context of popular galaxy formation/evolution theories.

Origins of the Thick Disk as Traced by the Alpha-Elements of Metal-Poor Stars

Gregory Ruchti (Johns Hopkins University)

Theories of thick disk formation can be differentiated by measurements of stellar elemental abundances. We have undertaken a study of metal-poor stars selected from the RAVE spectroscopic survey of bright stars to establish whether or not there is a significant population of metal-poor thick-disk stars ([Fe/H] < -1.0) and to measure their elemental abundances. Here, we present abundances of four alpha-elements (Mg, Si, Ca, Ti) for our sample stars from this study. We find that the alpha-to-iron ratios imply that enrichment by purely core-collapse SNe requires that star formation had a short duration. The relative lack of scatter in the ratios implies good mixing in the ISM prior to star formation. In addition, the ratios are similar to that of the halo, indicating that the halo and thick disk share a similar massive star IMF.

Star Formation History of the Small Magellanic Cloud

Elena Sabbi (STScI)

We observed six fields of the Small Magellanic Cloud (SMC) with the Advanced Camera for Survey (ACS) on board of the Hubble Space Telescope (HST). The selected fields sample regions characterized by different gas and stellar densities. We find evidence of a radial variation in chemical enrichment, with the SMC outskirts characterized by a lower metallicity than the central zones. Even if the lack of a clear horizontal branch suggest a modest star forming activity in the first few billion years, we find that the SMC was already forming stars ~12 Gyr ago. The SMC formed stars over a long interval of time until ~2-3 Gyr ago. In the SMC bar and wing star formation increased again in the last ~500 Myr.

SAGE Studies of Oxygen-Rich Evolved Star Mass Loss and Stellar Populations in the Large Magellanic Cloud

Benjamin Sargent (Space Telescope Science Institute)

The Surveying the Agents of a Galaxy's Evolution (SAGE) Spitzer Space Telescope Legacy project has obtained photometric data at mid-infrared wavelengths (3.6, 4.5, 5.8, 8, and 24 microns) for over 6 million stars in the Large Magellanic Cloud. Amongst this sample are thousands of asymptotic giant branch (AGB), red supergiant (RSG) and other evolved stars, which have circumstellar dust shells emitting in these Spitzer bands. Combined with optical and near-infrared photometry from other sources, we have assembled Spectral Energy Distributions (SEDs; emitted flux versus wavelength) for these evolved stars. To determine the dust mass-loss from the oxygen-rich (O-rich) evolved stars, we have constructed a grid of radiative transfer models using 2Dust. Each model assumes that mass is lost from the star at a constant rate and that the resulting dust shell from this mass loss is spherically symmetric. For each model in the grid, the stellar radiation field assumed is a detailed stellar photosphere spectral model. The grid explores different values of stellar effective temperature, stellar luminosity, dust shell inner radius, and dust shell optical depth at 10 microns wavelength. We then seek for each O-rich evolved star candidate from the SAGE sample the best fit of model to data in terms of SED photometry. From this, we obtain best-fit values and their uncertainties for the four parameters mentioned, in addition to dust mass-loss rate (itself output from 2Dust). From the best-fit values of stellar effective temperature and luminosity for each O-rich evolved star, we determine mass and age by assuming stellar evolutionary tracks. We then explore mass-loss in the context of stellar populations in the Large Magellanic Cloud.

A Comparative Analysis of Stellar Populations of Galactic and M31 Globular Clusters

Ricardo Schiavon (Gemini Observatory)

I will present a comparative analysis of high S/N spectra of hundreds of M31 clusters, obtained with MMT/Hectospec, and 40 Galactic clusters observed with the Blanco/R-C spectrograph. The data are used to revisit previous claims that the spectra of the two cluster families differ in terms of their Balmer-line and CN-band strengths. Single stellar population models were used to determine the abundances of Fe, Mg, C, N, and Ca. These results will also be presented, and their implications for our understanding of the formation of the Andromeda Halo will be briefly discussed.

Milky Way Structure and Formation as Revealed by Cold Halo Substructure

Kevin Schlaufman (UC Santa Cruz Astronomy and Astrophysics Department)

The accretion history and current state of the Milky Way is partially encoded in its halo substructure. I describe a systematic statistical search for elements of cold halo substructure (ECHOS) in the radial velocity distribution of stars in the inner and outer halo of the Milky Way observed during the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey. Radial velocity substructure is systematically older than surface brightness substructure (e.g. tidal streams), and therefore provides a direct measure of the accretion history of the Milky Way in a region and time interval that has yet to be fully explored. Likewise, I observe the Via Lactea II halo as SEGUE observed the Milky Way, and I compare the radial velocity substructure properties of both halos. In addition, I combine the spectra of individual stars in each inner halo ECHOS as well as in the kinematically smooth inner halo population to measure average chemical abundance properties in both populations. Accordingly, I use that information to better determine the characteristics of their progenitors. Finally, I describe how outer halo ECHOS may be able to constrain the shape of the Milky Way's dark matter halo.

The Star Formation History of the Leo I Dwarf Spheroidal Galaxy

Tammy Smecker-Hane (University of California, Irvine)

Dwarf spheroidal galaxies (dSphs) have had surprisingly complex star formation histories despite their low mass. The isolated Leo I dSph had previously been thought to be a dSph that was surprisingly young (Gallart et al. 1999, Dolphin 2000), and possibly a galaxy that began forming must later than other Local Group galaxies. However we our results from deep imaging obtained with the Hubble Space Telescope (HST) Advance Camera for Surveys (ACS) that shows Leo I is much older than previously thought. We derived its star formation rate as a function of time by modeling the observed density of stars in our observed color-magnitude diagrams (CMD) using the latest Padova stellar evolutionary models (Marigo, et al. 2008). Approximately 50% of the Leo I dSph’s stars formed over 10 Gyr ago, but star formation continued in it until only a few 100 million years ago. Our new imaging goes much fainter than previous data, which allows us to unambiguously identify the main sequence turnoffs of the ancient population and constrain the star formation rate at the epoch of the formation of the "first stars” in this galaxy. In addition, the chemical abundance distribution we derived from CMD analysis agrees extremely well that the one we derived independently by measuring chemical abundances from spectra of red giant stars in the Leo I dSph. Like other dSphs, Leo I has a very highly peaked metallicity distribution, which argues for infall of pristine gas over time and the outflow of metal enriched winds playing a significant role in its evolution.

The 6dFGS Fundamental Plane

Christopher Springob (Anglo-Australian Observatory)

The 6dF Galaxy Survey (6dFGS) is an all southern sky galaxy survey, including 125,000 redshifts and more than 10,000 peculiar velocities, making it the largest peculiar velocity sample to date. In combination with 2MASS surface brightnesses and effective radii, 6dFGS yields the near-infrared Fundamental Plane (FP) for a large and uniform sample. We have fit the FP relation for the galaxies in the peculiar velocity sample using a maximum likelihood method which allows us to precisely account for selection effects and observational errors. We have investigated the effects of varying stellar populations and environments on the FP, and find clear trends of stellar population parameters across and through the FP. We discuss the implications of these results for our understanding of the origin of the FP for early-type galaxies and bulges. Finally, we describe how we can correct for these stellar population trends, to reduce the scatter in the FP and derive more accurate distances and peculiar velocities in the local universe.

Evaporation and the Mass Function of Globular Clusters in Early-Type Virgo Galaxies

Kevin Thomas (Keele University)

It has recently been shown that the peak masses of the globular cluster mass functions (GCMFs) of the Milky Way and the Sombrero galaxy (M104) globular cluster systems show a strong dependence on the internal densities of the clusters themselves. This dependence is an expected result of evaporation driven by two-body relaxation, preferentially destroying low mass clusters over Gyr timescales. This explains the difference in shape of these evolved GCMFs from the mass function of young star clusters in nearby galaxies, such as starbursts, which continue to rise towards low masses. Here, we investigate the density dependence of the GCMF in early type Virgo Cluster galaxies. Using data from the 26 most populous globular cluster systems in the HST/ACS Virgo Cluster Survey, we find GCMF density dependencies that are analogous to those in the Milky Way and Sombrero. We fit these dependencies with models that incorporate density-dependent cluster evaporation rates, as was also done to fit the Milky Way and Sombrero GCMFs.

Extragalactic Star Clusters in Intermediate-Age Galaxy Merger Remnants

Gelys Trancho (Gemini Observatory)

We present preliminary results of combining HST optical photometry with ground-based K-band photometry from NIRI and Flamingos-I on Gemini to study the star cluster systems of four intermediate-age merger remnants (NGC 2865, NGC 1700, NGC 7727 and NGC 4382). The galaxies were chosen based on blue colors and fine structure such as shells and ripples that are indicative of past interactions. We find evidence for intermediate-age star clusters with ages consistent with the estimated merger ages.

Gemini Spectroscopic Survey of Young Star Clusters in Merging/Interacting Galaxies: Stephan's Quintet

Gelys Trancho (Gemini Observatory)

The goal of the Gemini Spectroscopic Survey of Young Star Clusters in Merging/Interacting Galaxies (Trancho et al. 2007a) is to use clusters as tracers of the evolution of their host galaxies. Using optical spectroscopy we have derived the ages and metallicities of dozens of massive clusters in nearby ongoing mergers. While spectroscopy is much more expensive than photometry in terms of telescope time, it allows for precise (non-degenerate) age/metallicity derivations and also offers the advantage of kinematical information that can be used to identify sub-populations within the merger/interaction. Thus far, we have focused on ongoing major mergers between spiral galaxies, namely NGC 3256 (Trancho et al. 2007a,b) and the Antennae galaxies (NGC 4038/39; Bastian et al. 2009). For the Antennae, we compared our results, and those of Whitmore et al. (2007), on the age distribution of the cluster to numerical models of the galaxy merger and found evidence for an increasing star-formation history over the past few hundred Myr. Additionally, we found clusters in the (projected) outer disks of the participating spiral galaxies that have “halo” type kinematics, showing the randomization of stellar orbits and the formation of a stellar halo around the future merger remnant. Clusters formed in the progenitor disks that are subsequently removed into the halo have been observed in galaxy merger simulations (Kruijssen et al. 2010). In the current study, we shift our focus to the much more complicated environment: Stephan’s Quintet, a compact group of interacting galaxies. In particular, we concentrate on the clusters that populate the intra-group medium (IGM; mostly in tidal structures), with the aim of using the clusters’ ages and kinematics to constrain the dynamical history of the interactions.

Understanding the Assembly of Galaxies in the Era of JWST and ELTs

Elysse Voyer (CUA/GSFC)

The Number counts of far-UV (FUV) galaxies as a function of magnitude provide a direct statistical measure of the density and evolution of star-forming galaxies. We report on the results of measurements of the rest-frame FUV (1500) number counts computed from data of several fields including the HUDF, the HDF-N, and the GOODS-N and -S fields. These data were obtained from the Hubble Space Telescope Solar Blind Channel Camera of the Advance Camera for Surveys and cover more area and more lines of sight than previous studies. The number counts cover an AB magnitude range from 20-29 magnitudes and probe galaxies at z < 1. We compare with local FUV counts from GALEX (Xu et al. 2004, Hammer et al. 2010) and the latest semi-analytical models (Gilmore et al. 2009). This project works towards understanding how star-forming galaxies at z < 1 evolve, but, a major scientific question still being explored is how star-formation has evolved in galaxies at redshifts higher than z=1? How did these earlier galaxies assemble into todays Hubble sequence? During this decade, the infrared observational capabilities of JWST will allow us to probe even deeper into the assembly history of star forming galaxies. As you look towards higher redshifts, the rest-frame UV band-shifts into the infrared, making this spectral region a primary target for detecting the earliest star-forming galaxies. The future work of our current project is to determine morphologies and other characteristics of the FUV sources in our number counts sample which would be valuable to compare with future JWST observations of higher-z sources.

Using H-Alpha Morphology to Age-Date Star Clusters in M83

Brad Whitmore (STScI)

The morphology of H-alpha line emission is used to age-date very young (t < 10 Myr) star clusters in the nearby grand-design spiral galaxy M83, based on our new WFC3 observations. The basic premise is that the distribution of H-alpha is largely coincident with the distribution of cluster light in the youngest clusters (i.e., < few Myr), is in a small ring-like structure around the cluster in slightly older clusters where stellar winds from the massive stars have had time to blow a bubble (i.e., 5 Myr), and is in a larger ring-like bubble for still older clusters (i.e., 5 - 10 Myr). Comparisons are made with other methods of age-dating the clusters, namely SED fitting of the integrated UBVI light from the clusters, and isochrone fitting of individual stars in the outskirts of clusters. A comparison is made with our similar study in M51. This project is based on Early Release Science observations made by the WFC3 Scientific Oversight Committee. We are grateful to the Director of the Space Telescope Science Institute for awarding Director's Discretionary time for this program.

Searches for Resolved Ultra-Faint Galaxies and Star Clusters in the Next Decade

Beth Willman (Haverford College)

The last five years has provided a wild ride in the field of ultra-faint galaxies. Since 2005, we have learned of dozens of previously unseen galaxies within the Local Group. Many of these are less luminous than any galaxy previously known to, or even thought possible to, exist. Not merely "missing satellites", the ultra-faint dwarf galaxies have been shown to include the most dark matter dominated ( (M/L)_{1/2} ~ 1000) and the least chemically evolved ([Fe/H] < -2.5) galaxies yet known. Their resolved stellar populations provide a unique opportunity to reconstruct the formation of the objects at the very bottom of the galaxy formation hierarchy. The post-SDSS landscape of this field will provide the opportunity for us to go from the excitement of discovery to the statistical samples of objects needed to truly learn about galaxy formation and dark matter at the extremes. I will review the observational biases still present in our census of ultra-faint systems, and then look ahead to how these observational biases may be overcome in the next decade. I will highlight our current search of the RCS2 dataset, our upcoming search of the imminent Southern Sky Survey, and the role that the future Large Synoptic Survey Telescope may play in this field.

Circumnuclear Clusters in M83

Aida Wofford (Space Telescope Science Institute)

It is not yet clear whether the star formation in circumnuclear starbursts occurs in a single burst, or whether it is due to the propagation of star formation from one region to another. The high precision age-dating possible from UV spectra of young (a few Myr old) star clusters provides a means of differentiating between these two scenarios. We analyze archival HST/STIS FUV imaging and spectroscopy of compact star clusters within a few x100 pc of the optical nucleus of M83 (NGC 5236, 4.6 Mpc). M83 hosts the nearest example of a circumnuclear starburst in a barred spiral galaxy, and it is nearly face-on. We compare the observed spectra with two sets of Starburst99 models, semi-empirical models based on an empirical library of Galactic stars (Robert et al. 1993), and fully theoretical models based on a synthetic library of high-resolution UV spectra for hot massive stars Leitherer et al. (2010). We generate single stellar population models at various ages, and we then use the model that best fits the data for determining the intrinsic reddenings, ages, and masses of the brightest clusters in M83's starburst. We compare the above properties with those derived from HST/WFC3 optical photometry. The latter is more sensitive to the masses than the UV spectroscopy because i) it captures light from stars in a lower mass regime, and ii) the total light can be better estimated from the photometry.

Connecting the Dual Origin of Simulated Halos with Observable Trends in Stellar Populations

Adi Zolotov (New York University)

Observational results have shown that the stellar halo of the Milky Way was likely assembled through a combination of both hierarchical accretions and in-situ formation. Disentangling the relative contributions of such different formation mechanisms has proven to be difficult in the inner halo, where large surveys, like SDSS, have amassed a considerable data set of stars. In this talk I will describe the formation of the stellar halos of four simulated Milky Way-mass disk galaxies using high resolution cosmological SPH + N-Body simulations. All four galaxies are surrounded by a kinematically distinct stellar halo, whose inner regions (r < 20 kpc) contain accreted stars, as well as stars formed in-situ. The relative contribution of the in-situ population to each stellar halo is a function of the merging history and ranges between 10 and 50%. While a galaxy with recent mergers, like M31, may host relatively fewer in-situ stars, the Milky Way, with its more quiescent recent history, will likely have a larger population of such stars. I present predicted trends in the metallicity and alpha abundances of in-situ and accreted halo stars that may be used to disentangle the two populations in observational data sets.