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

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 (MB = −15.1 mag) to giants (MB = −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 BT = 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 . MB . −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 S´ersic 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 S´ersic model, and are better described by a single power-law function. Core galaxies are clearly distinct in having fainter central surface brightness, μ0, 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 μ0 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 multiple morphological components (disks or bars) is unimodal across the entire magnitude range (a factor 460 in B−band luminosity) spanned by the ACSVCS galaxies. Although core galaxies have shallow inner profiles, the shallowest profiles (lowest γ values) are found in faint dwarf systems. 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 γ values, is therefore untenable based on the present study. Once core galaxies are removed, dwarf and bright ellipticals display a continuum in their morphological parameters, contradicting some previous beliefs that the two belong to structurally distinct classes. However, dwarfs span a wider range in morphological characteristics than brighter systems: their surface brightness profiles vary from exponential to almost r1/4 laws, they comprise both nucleated and non-nucleated varieties, and several systems display evidence of disks, spiral structures and recent star formation. This is taken as evidence that dwarf galaxies, as currently classified, form a heterogeneous class