arXiv subjects

Simona Mei

Publications and source records attributed to Simona Mei.

At least 91 records · Page 5Linked to original sources

arXiv2009

The ACS Fornax Cluster Survey. V. Measurement and Recalibration of Surface Brightness Fluctuations and a Precise Value of the Fornax--Virgo Relative Distance

We present g-z color and z-band surface brightness fluctuations (SBF) measurements for 43 early-type galaxies in the Fornax cluster imaged with HST/ACS. These are combined with measurements for Virgo cluster galaxies to derive a revised, nonlinear calibration of the z-band SBF absolute magnitude as a function of g-z. In all, we tabulate recalibrated SBF distances for 134 galaxies in Virgo, Fornax, the Virgo W' group, and NGC4697 in the Virgo Southern Extension. The calibration procedure yields a precise relative distance modulus for Fornax with respect to Virgo of 0.42+/-0.03 mag, or a distance ratio 1.214 +/- 0.017. The resulting Fornax distance modulus is 31.51 +/-0.03 +/-0.15 mag, 20.0 +/-0.3 +/-1.4 Mpc, where the second set of error bars reflects systematic uncertainty from our assumed Virgo distance of 16.5 Mpc. The rms distance scatter for the early-type Fornax galaxies is 0.49^{+0.11}_{-0.15} Mpc, consistent with its compact appearance on the sky. This translates to a depth scatter smaller than the intrinsic, or "cosmic", scatter sigma_cosmic in the SBF calibration, unlike in Virgo. As a result, we are able to place the first tight constraints on the value of sigma_cosmic, finding 0.06 +/- 0.01 mag, with a firm upper limit of sigma_cosmic<0.08 mag, at least for galaxies with g-z>1.02. We also present an alternative SBF calibration based on the `fluctuation count' parameter Nbar, a proxy for galaxy mass, which gives a consistent relative distance. Finally, we find no evidence for systematic trends of the galaxy distances with position or velocity (e.g., no current infall); the Fornax cluster appears both compact and well virialized.[abridged]

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arXiv2008

Evolution of the Color-Magnitude Relation in Galaxy Clusters at z ~1 from the ACS Intermediate Redshift Cluster Survey

We apply detailed observations of the Color-Magnitude Relation (CMR) with the ACS/HST to study galaxy evolution in eight clusters at z~1. The early-type red sequence is well defined and elliptical and lenticular galaxies lie on similar CMRs. We analyze CMR parameters as a function of redshift, galaxy properties and cluster mass. For bright galaxies (M_B < -21mag), the CMR scatter of the elliptical population in cluster cores is smaller than that of the S0 population, although the two become similar at faint magnitudes. While the bright S0 population consistently shows larger scatter than the ellipticals, the scatter of the latter increases in the peripheral cluster regions. If we interpret these results as due to age differences, bright elliptical galaxies in cluster cores are on average older than S0 galaxies and peripheral elliptical galaxies (by about 0.5Gyr). CMR zero point, slope, and scatter in the (U-B)_z=0 rest-frame show no significant evolution out to redshift z~1.3 nor significant dependence on cluster mass. Two of our clusters display CMR zero points that are redder (by ~2sigma) than the average (U-B)_z=0 of our sample. We also analyze the fraction of morphological early-type and late-type galaxies on the red sequence. We find that, while in the majority of the clusters most (80% to 90%) of the CMR population is composed of early-type galaxies, in the highest redshift, low mass cluster of our sample, the CMR late-type/early-type fractions are similar (~50%), with most of the late-type population composed of galaxies classified as S0/a. This trend is not correlated with the cluster's X-ray luminosity, nor with its velocity dispersion, and could be a real evolution with redshift.

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arXiv2008

The ACS Virgo Cluster Survey XV. The Formation Efficiencies of Globular Clusters in Early-Type Galaxies: The Effects of Mass and Environment

The fraction of stellar mass contained in globular clusters (GCs), also measured by number as the specific frequency, is a fundamental quantity that reflects both a galaxy's early star formation and its entire merging history. We present specific frequencies, luminosities, and mass fractions for the globular cluster systems of 100 early-type galaxies in the ACS Virgo Cluster Survey, the largest homogeneous catalog of its kind. We find that 1) GC mass fractions can be high in both giants and dwarfs, but are universally low in galaxies with intermediate luminosities. 2) The behavior of specific frequency across galaxy mass is dominated by the blue GCs. 3) The GC fractions of low-mass galaxies exhibit a dependence on environment. Nearly all dwarf galaxies with high GC fractions are within 1 Mpc of the cD galaxy M87, presenting the first strong evidence that GC formation in dwarfs is biased toward dense environments. 4) GC formation in central dwarfs is biased because their stars form earliest and most intensely. Comparisons to the Millennium Simulation show that central dwarfs have older stellar populations and form more stars at higher star formation rates (SFRs) and SFR surface densities. The SFR surface density in simulated dwarfs peaks before the total SFR, naturally producing GC populations that are older and more metal-poor than the field stars. 5) Dwarfs within ~40 kpc of the giant ellipticals M87 and M49 are red and have few or no GCs, suggesting that they have been tidally stripped and have contributed their GCs to the halos of their giant neighbors. The central dwarfs with high GC mass fractions are thus likely to be the survivors most similar to the protogalaxies that assembled the rich M87 globular cluster system.(Abridged)

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arXiv2007

The ACS Fornax Cluster Survey. II. The Central Brightness Profiles of Early-Type Galaxies: A Characteristic Radius on Nuclear Scales and the Transition from Central Luminosity Deficit to Excess

We analyse HST surface brightness profiles for 143 early-type galaxies in the Virgo and Fornax Clusters. Sersic models provide accurate descriptions of the global profiles with a notable exception: the observed profiles deviate systematically inside a characteristic "break" radius of R_b ~ 0.02R_e where R_e is the effective radius of the galaxy. The sense of the deviation is such that bright galaxies (M_B < -20) typically show central light deficits with respect to the inward extrapolation of the Sersic model, while the great majority of low- and intermediate-luminosity galaxies (-19.5 < M_B < -15) show central light excesses; galaxies occupying a narrow range of intermediate luminosities (-20 < M_B < -19.5) are usually well fitted by Sersic models over all radii. The slopes of the central surface brightness profiles, when measured at fixed fractions of R_e, vary smoothly as a function of galaxy luminosity in a manner that depends sensitively on the choice of measurement radius. We show that a recent claim of strong bimodality in slope is likely an artifact of the galaxy selection function used in that study. To provide a more robust characterization of the inner regions of galaxies, we introduce a parameter that describes the central luminosity deficit or excess relative to the inward extrapolation of the outer Sersic model. We find that this parameter varies smoothly over the range of ~ 720 in blue luminosity spanned by the Virgo and Fornax sample galaxies, with no evidence for a dichotomy. We argue that the central light excesses (nuclei) in M_B > -19 galaxies may be the analogs of the dense central cores that are predicted by some numerical simulations to form via gas inflows. (ABRIDGED)

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arXiv2007

The ACS Virgo Cluster Survey. XIII. SBF Distance Catalog and the Three-Dimensional Structure of the Virgo Cluster

The ACS Virgo Cluster Survey consists of HST ACS imaging for 100 early-type galaxies in the Virgo Cluster, observed in the F475W and F850LP filters. We derive distances for 84 of these galaxies using the method of surface brightness fluctuations (SBFs), present the SBF distance catalog, and use this database to examine the three-dimensional distribution of early-type galaxies in the Virgo Cluster. The SBF distance moduli have a mean (random) measurement error of 0.07 mag (0.5 Mpc), or roughly 3 times better than previous SBF measurements for Virgo Cluster galaxies. Five galaxies lie at a distance of ~23 Mpc and are members of the W' cloud. The remaining 79 galaxies have a narrow distribution around our adopted mean distance of 16.5+/-0.1 (random mean error) +/-1.1 Mpc (systematic). The rms distance scatter of this sample is 0.6+/-0.1 Mpc, with little dependence on morphological type or luminosity class (i.e., 0.7+/-0.1 and 0.5+/-0.1 Mpc for the giants and dwarfs, respectively). The back-to-front depth of the cluster measured from our sample of galaxies is 2.4+/-0.4 Mpc (i.e., +/-2sigma of the intrinsic distance distribution). The M87 (cluster A) and M49 (cluster B) subclusters are found to lie at distances of 16.7+/-0.2 and 16.4+/-0.2 Mpc, respectively. There may be a third subcluster associated with M86. A weak correlation between velocity and line-of-sight distance may be a faint echo of the cluster velocity distribution not having yet completely virialized. In three dimensions, Virgo's early-type galaxies appear to define a slightly triaxial distribution, with axis ratios of (1:0.7:0.5). The principal axis of the best-fit ellipsoid is inclined ~20-40 deg. from the line of sight, while the galaxies belonging to the W' cloud lie on an axis inclined by ~10-15 deg.

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arXiv2007

The Low-Mass X-ray Binary and Globular Cluster Connection in Virgo Cluster Early-type Galaxies: Optical Properties

(Abridged) Chandra and Hubble ACS observations of 11 early-type galaxies probe the low-mass X-ray binary (LMXB) - globular cluster (GC) connection. We explore the optical properties of 270 GCs with LMXBs and 6,488 GCs without LMXBs. More massive, redder, and more compact GCs are more likely to contain LMXBs. Unlike Galactic GCs, a large number of GCs with LMXBs have half-mass relaxation times > 2.5 Gyr. We fit the dependence of the expected number of LMXBs per GC, λ_t, on the GC mass M, color (g-z), and half-mass radius r_{h,cor}, and find that λ_t \propto M^{1.24\pm0.08} 10^{0.9^{+0.2}_{-0.1} (g-z)} r_{h,cor}^{-2.2^{+0.3}_{-0.4}}. Our fit rules out that the number of LMXBs per GC is linearly proportional to GC mass and that most GCs with high X-ray luminosities contain a single LMXB. The detailed dependence of λ_t on GC properties appears essentially equivalent to a dependence on the encounter rate Γ_h and the metallicity Z, λ_t \propto Γ_h^{0.82\pm0.05} Z^{0.39\pm0.07}. Our analysis provides strong evidence that dynamical formation and metallicity play the primary roles in determining the presence of an LMXB in extragalactic GCs. The shallower than linear dependence for our sample requires an explanation by theories of dynamical binary formation. The abundance dependence is consistent with a metallicity-dependent variation in the number o neutron stars and black holes per unit mass GC, effects from irradiation induced winds or suppression of magnetic braking in metal-poor stars.

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arXiv2007

The ACS Virgo Cluster Survey. XII. The Luminosity Function of Globular Clusters in Early Type Galaxies

We analyze the luminosity function of the globular clusters (GCs) belonging to the early-type galaxies observed in the ACS Virgo Cluster Survey. We have obtained estimates for a Gaussian representation of the GC luminosity function (GCLF) for 89 galaxies. We have also fit the GCLFs with an "evolved Schechter function", which is meant to reflect the preferential depletion of low-mass GCs, primarily by evaporation due to two-body relaxation, from an initial Schechter mass function similar to that of young massive clusters. We find a significant trend of the GCLF dispersion with galaxy luminosity, in the sense that smaller galaxies have narrower GCLFs. We show that this narrowing of the GCLF in a Gaussian description is driven by a steepening of the GC mass function above the turnover mass, as one moves to smaller host galaxies. We argue that this behavior at the high-mass end of the GC mass function is most likely a consequence of systematic variations of the initial cluster mass function. The GCLF turnover mass M_TO is roughly constant, at ~ 2.2 x 10^5 M_sun in bright galaxies, but it decreases slightly in dwarfs with M_B >~ -18. We show that part of the variation could arise from the shorter dynamical friction timescales in smaller galaxies. We probe the variation of the GCLF to projected galactocentric radii of 20-35 kpc in the Virgo giants M49 and M87, finding that M_TO is essentially constant over these spatial scales. Our fits of evolved Schechter functions imply average dynamical mass losses (Delta) over a Hubble time that fall in the range 2 x 10^5 <~ (Delta/M_sun) < 10^6 per GC. We agree with previous suggestions that if the full GCLF is to be understood in more detail GCLF models will have to include self-consistent treatments of dynamical evolution inside time-dependent galaxy potentials. (Abridged)

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arXiv2007

The ACS Fornax Cluster Survey. I. Introduction to the Survey and Data Reduction Procedures

The Fornax Cluster is a conspicuous cluster of galaxies in the southern hemisphere and the second largest collection of early-type galaxies within <~ 20 Mpc after the Virgo Cluster. In this paper,we present a brief introduction to the ACS Fornax Cluster Survey - a program to image, in the F475W (g_475) and F850LP (z_850) bandpasses, 43 early-type galaxies in Fornax using the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. Combined with a companion survey of Virgo, the ACS Virgo Cluster Survey, this represents the most comprehensive imaging survey to date of early-type galaxies in cluster environments in terms of depth, spatial resolution, sample size and homogeneity. We describe the selection of the program galaxies, their basic properties, and the main science objectives of the survey which include the measurement of luminosities, colors and structural parameters for globular clusters associated with these galaxies, an analysis of their isophotal properties and surface brightness profiles, and an accurate calibration of the surface brightness fluctuation distance indicator. Finally, we discuss the data reduction procedures adopted for the survey.

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arXiv2006

The Inner Workings of Early-Type Galaxies: Cores, Nuclei and Supermassive Black Holes (Including a Critical Comparison of Nuker and core-Sersic/Sersic models)

Recent years have seen dramatic progress in the study of the core and nuclear properties of galaxies. The structure of the cores has been shown to vary methodically with global and nuclear properties, as cores respond to the mechanisms by which galaxies form/evolve. The dynamical centers of galaxies have been found capable of hosting two seemingly disparate objects: supermassive black holes (SBHs) and compact stellar nuclei. In a drastic departure from previous beliefs, it has been discovered that both structures are common: galaxies lacking SBHs and/or stellar nuclei are the exception, rather than the norm. This review explores the connection between cores, SBHs and stellar nuclei in early-type galaxies, as revealed by the ACS Virgo Cluster Survey. We find that, from parsec to kiloparsec scales, the surface brightness profiles of early-type galaxies are well described by a Sersic model -- a parametrization that is shown to be preferable to the so called `Nuker' model. Relative to the inward extrapolation of the best fitting Sersic model, the profiles within the inner few tens of parsecs transition from luminosity `deficits', in the brightest (M_B < ~-20) galaxies, to luminosity `excesses' in the faintest. The compact stellar nuclei responsible for such luminosity excesses comprise approximatively 0.2% of the total galactic mass. Remarkably, this is the same fraction believed to reside in the SBHs, which are predominantly found in the brightest galaxies showing luminosity deficits.

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arXiv2006

Trends in the Globular Cluster Luminosity Function of Early-Type Galaxies

We present results from a study of the globular cluster luminosity function (GCLF) in a sample of 89 early-type galaxies observed as part of the ACS Virgo Cluster Survey. Using a Gaussian parametrization of the GCLF, we find a highly significant correlation between the GCLF dispersion, sigma, and the galaxy luminosity, M_B, in the sense that the GC systems in fainter galaxies have narrower luminosity functions. The GCLF dispersions in the Milky Way and M31 are fully consistent with this trend, implying that the correlation between sigma and galaxy luminosity is more fundamental than older suggestions that GCLF shape is a function of galaxy Hubble type. We show that the sigma - M_B relation results from a bonafide narrowing of the distribution of (logarithmic) cluster masses in fainter galaxies. We further show that this behavior is mirrored by a steepening of the GC mass function for relatively high masses, M >~ 3 x 10^5 M_sun, a mass regime in which the shape of the GCLF is not strongly affected by dynamical evolution over a Hubble time. We argue that this trend arises from variations in initial conditions and requires explanation by theories of cluster formation. Finally, we confirm that in bright galaxies, the GCLF "turns over" at the canonical mass scale of M_TO ~ 2 x 10^5 M_sun. However, we find that M_TO scatters to lower values (~1-2 x 10^5 M_sun) in galaxies fainter than M_B >~ -18.5, an important consideration if the GCLF is to be used as a distance indicator for dwarf ellipticals.

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arXiv2006

The ACS Virgo Cluster Survey. XIV. Analysis of Color-Magnitude Relations in Globular Cluster Systems

We examine the correlation between globular cluster (GC) color and magnitude using HST/ACS imaging for a sample of 79 early-type galaxies (-21.7<M_B<-15.2 mag) with accurate SBF distances from the ACS Virgo Cluster Survey. Using the KMM mixture modeling algorithm, we find a highly significant correlation, d(g-z)/dz = -0.037 +- 0.004, between color and magnitude for the subpopulation of blue GCs in the co-added GC color-magnitude diagram of the three brightest Virgo galaxies (M49, M87 and M60): brighter GCs are redder than their fainter counterparts. For the single GC systems of M87 and M60, we find similar correlations; M49 does not appear to show a significant trend. There is no correlation between (g-z) and M_z for GCs of the red subpopulation. The correlation d(g-z)/dg for the blue subpopulation is much weaker than d(g-z)/dz. Using Monte Carlo simulations, we attribute this to the fact that the blue subpopulation in M_g extends to higher luminosities than the red subpopulation, which biases the KMM fits. The correlation between color and M_z thus is a real effect. This conclusion is supported by biweight fits to the same color distributions. We identify two environmental dependencies of the color-magnitude relation: (1) the slope decreases in significance with decreasing galaxy luminosity; and (2) the slope is stronger for GCs at smaller galactocentric distances. We examine several mechanisms that might give rise to the observed color-magnitude relation: (1) presence of contaminators; (2) accretion of GCs from low-mass galaxies; (3) stochastic effects; (4) capture of field stars by individual GCs; and (5) GC self-enrichment. We conclude that self-enrichment and field-star capture, or a combination of these processes, offer the most promising means of explaining our observations.

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arXiv2006

A Fundamental Relation Between Compact Stellar Nuclei, Supermassive Black Holes, and Their Host Galaxies

Imaging surveys with the Hubble Space Telescope (HST) have shown that roughly 50-80% of low- and intermediate-luminosity galaxies contain a compact stellar nucleus at their center, regardless of host galaxy morphological type. We combine HST imaging for early-type galaxies from the ACS Virgo Cluster Survey with ground-based long-slit spectra from KPNO to show that the masses of compact stellar nuclei in Virgo Cluster galaxies obey a tight correlation with the masses of the host galaxies. The same correlation is obeyed by the supermassive black holes (SBHs) found in predominantly massive galaxies. The compact stellar nuclei in the Local Group galaxies M33 and NGC205 are also found to fall along this same scaling relation. These results indicate that a generic by-product of galaxy formation is the creation of a Central Massive Object (CMO) -- either a SBH or a compact stellar nucleus -- that contains a mean fraction, ~ 0.2%, of the total galactic mass. In galaxies with masses greater than a few times 10^10 solar masses, SBHs appear to be the dominant mode of CMO formation.

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arXiv2006

Evolution of the Color-Magnitude relation in High-Redshift Clusters: Early-type Galaxies in the Lynx Supercluster at z~1.26

Color-magnitude relations have been derived in the clusters RX J0849+4452 and RX J0848+4453 at z~1.26. The color-magnitude relation was determined from Advanced Camera for Surveys imaging in the WFC F775W (i_775) and F850LP (z_850) filters combined with ground-based spectroscopy. Early-type cluster candidates have been identified according to the Postman et al. morphological classification. In both clusters the bright red early-type population defines a tight color-magnitude relation very similar in color, although the two clusters present different X-ray luminosities and shapes, with RX J0849+4452 being three times more X-ray luminous and more compact, and having a temperature two times higher. The elliptical galaxy color-magnitude relations (CMR) in RX J0849+4452 and RX J0848+4453 show an intrinsic (i_775-z_850) color scatter of 0.026 +/- 0.012 mag and 0.024 +/- 0.015 mag, respectively, within 2 arcminutes (~1Mpc at z=1.26) from the cluster X-ray emission centers. Simple modeling of the scatters using stellar population models from Bruzual and Charlot, gives a mean luminosity-weighted age t > 2.5 Gyr (z_f > 2.75). S0 galaxies follow the elliptical CMR; they show larger scatters about the CMR. The intrinsic scatter decreases and the CMR slopes are steeper at smaller radii, within both clusters. We conclude that old stellar populations in cluster elliptical galaxies are already in place at z=1.26, both in the more evolved cluster RX J0849+4452, and in its less evolved companion RX J0848+4453. Even at a lookback time of 9 Gyr, in the early merging and buildup of massive clusters, the bulk of the stellar content of the bright elliptical galaxy population was in place - apparently formed some 2.5~Gyr earlier at z~3

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arXiv2006

The ACS Virgo Cluster Survey. VIII. The Nuclei of Early-Type Galaxies

(Abridged) The ACS Virgo Cluster Survey is an HST program to obtain high-resolution, g and z-band images for 100 early-type members of the Virgo Cluster, spanning a range of ~460 in blue luminosity. Based on this large, homogeneous dataset, we present a sharp upward revision in the frequency of nucleation in early-type galaxies brighter than M_B ~ -15 (66 < f_n < 82%), and find no evidence for nucleated dwarfs to be more concentrated to the center of Virgo than their non-nucleated counterparts. Resolved stellar nuclei are not present in galaxies brighter than M_B ~ -20.5, however, there is no clear evidence from the properties of the nuclei, or from the overall incidence of nucleation, for a change at M_B ~ -17.6, the traditional dividing point between dwarf and giant galaxies. On average, nuclei are ~3.5 mag brighter than a typical globular cluster and have a median half-light radius ~4.2 pc. Nuclear luminosities correlate with nuclear sizes and, in galaxies fainter than M_B ~ -17.6, nuclear colors. Comparing the nuclei to the "nuclear clusters" found in late-type spiral galaxies reveals a close match in terms of size, luminosity and overall frequency, pointing to a formation mechanism that is rather insensitive to the detailed properties of the host galaxy. The mean nuclear-to-galaxy luminosity ratio is indistinguishable from the mean SBH-to-bulge mass ratio, calculated in early-type galaxies with detected supermassive black holes (SBHs). We argue that compact stellar nuclei might be the low-mass counterparts of the SBHs detected in the bright galaxies, and that one should think in terms of "Central Massive Objects" -- either SBHs or compact stellar nuclei -- that accompany the formation of almost all early-type galaxies and contain a mean fraction ~0.3% of the total bulge mass.

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arXiv2006

The ACS Virgo Cluster Survey. VI. Isophotal Analysis and the Structure of Early-Type Galaxies

(Abridged) We present a detailed analysis of the morphology, isophotal parameters and surface brightness profiles for 100 early-type members of the Virgo Cluster, from dwarfs (M_B = -15.1 mag) to giants (M_B = -21.8 mag). Each galaxy has been imaged in two filters, closely resembling the Sloan g and z passbands, using the Advanced Camera for Surveys on board the Hubble Space Telescope. Dust and complex morphological structures are common, with kiloparsec-scale stellar disks, bars, and nuclear stellar disks seen in 60% of galaxies with intermediate luminosity (-20 < M_B < -17), and dust seen in 42% of galaxies brighter than M_B = -18.9 mag. Dust morphologies range from faint wisps and patches on tens of parsec scales, to regular, highly organized kpc-scale dust disks, often showing evidence of recent star formation. Surface brightness profiles and isophotal parameters are derived typically within 8 kpc from the center for the brightest galaxies, and 1.5 kpc for the faintest systems, with a resolution (FWHM) of 7 pc. Based on a parametrization of the surface brightness profiles in terms of a Sersic or core-Sersic model, we find that 1) there is no evidence of a bimodal behavior of the slope, gamma, of the profile in the innermost regions; 2) although the brightest galaxies have shallow inner profiles, the shallowest profiles (lowest gamma values) are found in faint dwarf systems; 3) the widely adopted separation of early-type galaxies between "core" and "power-law" types, which had originally been prompted by the claim of a clear bimodal distribution of gamma values, is untenable; and 4) there is no evidence of a structural dichothomy between dwarf and regular ellipticals.

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arXiv2005

The ACS Virgo Cluster Survey XI. The Nature of Diffuse Star Clusters in Early-Type Galaxies

We use HST/ACS imaging of 100 early-type galaxies in the ACS Virgo Cluster Survey to investigate the nature of diffuse star clusters (DSCs). Compared to globular clusters (GCs), these star clusters have moderately low luminosities (M_V > -8) and a broad distribution of sizes (3 < r_h < 30 pc), but they are principally characterized by their low mean surface brightnesses which can be more than three magnitudes fainter than a typical GC (mu_g > 20 mag arcsec^-2). The median colors of diffuse star cluster systems are red, 1.1 < g-z < 1.6, which is redder than metal-rich GCs and often as red as the galaxy itself. Most DSC systems thus have mean ages older than 5 Gyr or else have super-solar metallicities. We find that 12 galaxies in our sample contain a significant excess of diffuse star cluster candidates -- nine are lenticulars (S0s), and five visibly contain dust. We also find a substantial population of DSCs in the halo of the giant elliptical M49, associated with the companion galaxy VCC 1199. Most DSC systems appear to be both aligned with the galaxy light and associated with galactic disks, but at the same time many lenticulars do not host substantial DSC populations. Diffuse star clusters in our sample share similar characteristics to those identified in other nearby lenticular, spiral, and dwarf galaxies, and we suggest that DSCs preferentially form, survive, and coevolve with galactic disks. Their properties are broadly consistent with those of merged star cluster complexes, and we note that despite being 3 - 5 magnitudes brighter than DSCs, ultra-compact dwarfs have similar surface brightnesses. The closest Galactic analogs to the DSCs are the old open clusters. We suggest that if a diffuse star cluster population did exist in the disk of the Milky Way, it would be very difficult to find. (Abridged)

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arXiv2005

The ACS Virgo Cluster Survey IX: The Color Distributions of Globular Cluster Systems in Early-Type Galaxies

We present the color distributions of globular cluster (GC) systems for 100 Virgo cluster early-type galaxies observed in the ACS Virgo Cluster Survey. The color distributions of individual GC systems are consistent with continuous trends across galaxy luminosity, color, and stellar mass. On average, almost all galaxies possess a component of metal-poor GCs, with the average fraction of metal-rich GCs ranging from 15 to 60%. The colors of both subpopulations correlate with host galaxy luminosity and color, with the red GCs having a steeper slope. To convert color to metallicity, we also introduce a preliminary (g-z)-[Fe/H] relation calibrated to Galactic, M49 and M87 GCs. This relation is nonlinear with a steeper slope for [Fe/H] < -0.8. As a result, the metallicities of the metal-poor and metal-rich GCs vary similarly with respect to galaxy luminosity and stellar mass, with relations of [Fe/H]_MP ~ L^0.16 ~ M_star^0.17 and [Fe/H]_MR ~ L^0.26 ~ M_star^0.22, respectively. Although these relations are shallower than the mass-metallicity relation predicted by wind models and observed for dwarf galaxies, they are very similar to the mass-metallicity relation for star forming galaxies in the same mass range. The offset between the two GC populations varies slowly (~ M_star^0.05) and is approximately 1 dex across three orders of magnitude in mass, suggesting a nearly universal amount of enrichment between the formation of the two populations of GCs. We also find that although the metal-rich GCs show a larger dispersion in color, it is the *metal-poor GCs* that have an equal or larger dispersion in metallicity. Like the color-magnitude relation, these relations derived from globular clusters present stringent constraints on the formation and evolution of early-type galaxies. (Abridged)

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arXiv2005

The ACS Virgo Cluster Survey X. Half-light Radii of Globular Clusters in Early-Type Galaxies: Environmental Dependencies and a Standard Ruler for Distance Estimation

We have measured half-light radii, r_h, for globular clusters (GCs) belonging to the 100 early-type galaxies observed in the ACS Virgo Cluster Survey and the elliptical galaxy NGC 4697. An analysis of the dependencies of the measured r_h on both the properties of the GCs themselves and their host galaxies reveals that the average r_h increases with increasing galactocentric distance or, alternatively, with decreasing galaxy surface brightness. For the first time, we find that the average r_h decreases with the host galaxy color. We also show that there is no evidence for a variation of r_h with the luminosity of the GCs. Finally, we find in agreement with previous observations that the average r_h depends on the color of GCs, with red GCs being ~17% smaller than their blue counterparts. We show that this difference is probably a consequence of an intrinsic mechanism, rather than projection effects, and that it is in good agreement with the mechanism proposed in Jordan (2004). We discuss these findings in light of two simple pictures for the origin of the r_h of GCs and show that both lead to a behavior in rough agreement with the observations. After accounting for the dependencies found we show that the average GC half-light radii can be successfully used as a standard ruler for distance estimation. We outline the methodology, and provide a calibration for its use. We find = 2.7 +- 0.35 pc for GCs with (g-z)=1.2 mag in a galaxy with color (g-z)_{gal}=1.5 mag and at an underlying surface z-band brightness of mu_z = 21 mag arcsec^{-2}. Using this technique, we place an upper limit of 3.4 Mpc on the 1-sigma line-of-sight depth of the Virgo Cluster. Finally, we examine the form of the r_h distribution for our sample galaxies and provide an analytic expression which successfully describes this distribution. (Abridged)

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