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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. Dust is detected in as many as 18, preferentially bright, galaxies. The incidence rate in the 26 galaxies brighter than B T = 12.15 mag, which form a magnitude limited sample, is 42%. The amount and distribution of dust show no obvious correlations with galaxy morphology; dust features range from faint wisps and patches on tens of parsec scales, to regular, highly organized kpc-scale dust disks. Blue star clusters are interspersed within the larger, clumpier dust disks, while thin, dynamically cold stellar disks are seen in association with the smaller, uniform nuclear dust disks. Kiloparsec-scale stellar disks, bars, and nuclear stellar disks are seen in 60% of galaxies with intermediate luminosity (−20 M B −17). In at least one case (VCC 1938 = NGC 4638), the large-scale stellar disk has a sharp inner edge, possibly produced when disk instabilities led to the formation of a (now dissolved) bar. This process might indeed be seen unfolding in one galaxy, VCC 1537 (=NGC 4528). A spiral structure might be present in VCC 1199, an elliptical companion of M49. In dwarf galaxies, spiral structures are confirmed in VCC 856 and detected for the first time in VCC 1695. Surface brightness profiles, ellipticities, major axis position angles, and isophotal shapes 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. For all but 10 of the galaxies, the surface brightness profiles are well described by a Sérsic model with index n which increases steadily from the fainter to the brightest galaxies. In agreement with previous claims, the inner profiles (typically within 100 pc of the center) of eight of the 10 brightest galaxies, to which we will refer as "core" galaxies, are lower than expected based on an extrapolation of the outer Sérsic model, and are better described by a single power-law function. Core galaxies are clearly distinct in having fainter central surface brightness, µ 0 , and shallower logarithmic slope of the inner surface brightness profile, γ, than expected based on the extrapolation of the trend followed by the rest of the sample, for which both µ 0 and γ increase steadily with galaxy magnitude. Large-scale, global properties also set core galaxies apart: the effective radius in particular is found to be almost one order of magnitude larger than for only slightly less luminous non-core galaxies. Contrary to previous claims, we find no evidence in support of a strong bimodal behavior of the inner profile slope, γ; in particular the γ distribution for galaxies which do not show evidence of m...

The ACS Virgo Cluster Survey consists of HST ACS imaging for 100 early-type galaxies in the Virgo Cluster, observed in the F475W (%SDSS g) and F850LP (%SDSS z) 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 d % 23 Mpc and are members of the W 0 cloud. The remaining 79 galaxies have a narrow distribution around our adopted distance of hdi ¼ 16:5 AE 0:1 (random mean error) AE1.1 Mpc (systematic). The rms distance scatter of this sample is (d ) ¼ 0:6 AE 0:1 Mpc, with little or no dependence on morphological type or luminosity class (i.e., 0:7 AE 0:1 and 0:5 AE 0:1 Mpc for the giants and dwarfs, respectively). The back-to-front depth of the cluster measured from our sample of early-type galaxies is 2:4 AE 0:4 Mpc (i.e., AE 2 of the intrinsic distance distribution). The M87 (cluster A) and M49 (cluster B) subclusters are found to lie at distances of 16:7 AE 0:2 and 16:4 AE 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 Y 40 from the line of sight, while the galaxies belonging to the W 0 cloud lie on an axis inclined by $10 Y15 .

We present the color distributions of globular cluster (GC) systems for 100 Virgo cluster earlytype galaxies observed in the ACS Virgo Cluster Survey, the deepest and most homogeneous survey of this kind to date. While the color distributions of individual GC systems can show significant variations from one another, their general properties are consistent with continuous trends across galaxy luminosity, color, and stellar mass. On average, galaxies at all luminosities in our study (−22 < M B < −15) appear to have bimodal or asymmetric GC color distributions. 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. The steeper correlation seen in the mean color of the entire GC system is driven by the increasing fraction of metal-rich GCs for more luminous galaxies.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±0.04 ∝ M 0.17±0.04 ⋆ and [Fe/H] MR ∝ L 0.26±0.03 ∝ M 0.22±0.03 ⋆, 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 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. The similarity in the M ⋆ -[Fe/H] relations for the two populations, implies that the conditions of GC formation for metal-poor and metal-rich GCs could not have been too different. Like the color-magnitude relation, these relations derived from globular clusters present stringent constraints on the formation and evolution of early-type galaxies.

We present (g 475 −z 850 ) color and z 850 -band surface brightness fluctuations (SBF) measurements for 43 earlytype galaxies in the Fornax cluster imaged with the Hubble Space Telescope Advanced Camera for Surveys. These are combined with our earlier measurements for Virgo cluster galaxies to derive a revised, nonlinear calibration of the z 850 -band SBF absolute magnitude M z as a function of (g 475 −z 850 ) color, valid for the AB color range 0.8 < (g 475 −z 850 ) < 1.6. In all, we tabulate recalibrated SBF distances for 134 galaxies in Virgo, Fornax, the Virgo W ′ group, and NGC 4697 in the Virgo Southern Extension. The calibration procedure yields a highly precise relative distance modulus for Fornax with respect to Virgo of ∆(m− M) FV = 0.42 ± 0.03 mag, or a distance ratio d F /d V = 1.214±0.017. The resulting Fornax distance modulus is (m−M) For = 31.51±0.03±0.15 mag, corresponding to d F = 20.0 ± 0.3 ± 1.4 Mpc, where the second set of error bars reflects the total systematic uncertainty from our assumed Virgo distance of 16.5 Mpc. The rms distance scatter for the early-type Fornax cluster galaxies is σ d = 0.49 +0.11 −0.15 Mpc, or a total line-of-sight depth of 2.0 +0.4 −0.6 Mpc, consistent with its compact appearance on the sky. This translates to a depth scatter smaller than the intrinsic, or "cosmic," scatter σ cos in the SBF calibration, unlike the case for the larger Virgo cluster. As a result, we are able to place the first tight constraints on the value of σ cos . We find σ cos = 0.06 ± 0.01 mag, with a firm upper limit of σ cos < 0.08 mag, for the subsample of galaxies with (g 475 −z 850 ) > 1.02, but it is about twice as large for bluer galaxies. We also present an alternative SBF calibration based on the 'fluctuation count' parameter N = m − m tot , a proxy for galaxy mass. This gives a consistent relative distance but with larger intrinsic scatter, and we adopt the result from the calibration on (g 475 −z 850 ) because of its basis in stellar population properties alone. 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.

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. This catalog represents the largest homogeneous catalog of GC number and mass fractions across a wide range of galaxy luminosity (−22 < M B < −15). We find that 1) GC mass fractions can be high in both giants and dwarfs, but are universally low in galaxies with intermediate luminosities (−20 < M B < −17).2) The fraction of red GCs increases with galaxy luminosity, but stays constant or decreases for galaxies brighter than M z = −22. As a result, although specific frequencies for blue and red GCs are both higher in massive galaxies, 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, where dwarf galaxies closer to the cluster center have higher GC fractions. Nearly all dwarfs with high GC fractions are within 1 Mpc of the cD galaxy M87, presenting the first strong evidence that GC formation in dwarf galaxies is biased toward dense environments. 4) GC formation in central dwarfs is biased because their stars form earliest and most intensely. Comparisons to early-type dwarf galaxies in the Millennium Simulation show that central dwarfs are likely to have older stellar populations and form more of their stars at higher star formation rates (SFRs) and star formation rate surface densities. In addition, 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 for their luminosities 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.

1-3 -(6) Five of the 13 DGTOs in Virgo are associated with a single galaxy: M87.This finding suggests that proximity to the Virgo center may be of critical importance for the formation of these objects, although we find M87 to be more abundant in DGTOs than would be expected on the basis of its luminosity, the size of its globular cluster system, or the local galaxy density.These results show that distinguishing bonafide UCDs from high-luminosity globular clusters requires a careful analysis of their detailed structural and dynamical properties, particularly their mass-to-light ratios. In general, the properties of the UCDs in our sample are consistent with models in which these objects form through tidal stripping of nucleated dwarf galaxies.

In hierarchical cosmological models, galaxies grow in mass through the continual accretion of smaller ones. The tidal disruption of these systems is expected to result in loosely bound stars surrounding the galaxy, at distances that reach 10-100 times the radius of the central disk. The number, luminosity and morphology of the relics of this process provide significant clues to galaxy formation history, but obtaining a comprehensive survey of these components is difficult because of their intrinsic faintness and vast extent. Here we report a panoramic survey of the Andromeda galaxy (M31). We detect stars and coherent structures that are almost certainly remnants of dwarf galaxies destroyed by the tidal field of M31. An improved census of their surviving counterparts implies that three-quarters of M31's satellites brighter than M(v) = -6 await discovery. The brightest companion, Triangulum (M33), is surrounded by a stellar structure that provides persuasive evidence for a recent encounter with M31. This panorama of galaxy structure directly confirms the basic tenets of the hierarchical galaxy formation model and reveals the shared history of M31 and M33 in the unceasing build-up of galaxies.