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...

Imaging surveys with the Hubble Space Telescope (HST) have shown that ≈ 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 NGC 205 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 M gal ∼ a few 10 10 M ⊙ , SBHs appear to be the dominant mode of CMO formation.

We present imaging of the recently discovered Hercules Milky Way satellite and its surrounding regions to study its structure, star formation history and to thoroughly search for signs of disruption. We robustly determine the distance, luminosity, size and morphology of Hercules utilizing a bootstrap approach to characterize our uncertainties. We derive a distance to Hercules of 133 ± 6 kpc via a comparison to empirical and theoretical isochrones. As previous studies have found, Hercules is very elongated, with ǫ = 0.67 ± 0.03 and a half light radius of r h ≃ 230 pc. Using the color magnitude fitting package StarFISH, we determine that Hercules is old (> 12 Gyr) and metal poor ([F e/H] ∼ −2.0), 1 Based on data acquired using the Large Binocular Telescope (LBT). The LBT is an international collaboration among institutions in the US

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Core radii of globular clusters in the Large and Small Magellanic Clouds show an increasing trend with age. We propose that this trend is a dynamical effect resulting from the accumulation of massive stars and stellar-mass black holes at the cluster centers. The black holes are remnants of stars with initial masses exceeding 20-25 solar masses; as their orbits decay by dynamical friction, they heat the stellar background and create a core. Using analytical estimates and N-body experiments, we show that the sizes of the cores so produced and their growth rates are consistent with what is observed. We propose that this mechanism is responsible for the formation of cores in all globular clusters and possibly in other systems as well.Comment: 5 page