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

(Abridged) The ACS Virgo Cluster Survey is an HST program to obtainhigh-resolution, g and z-band images for 100 early-type members of the VirgoCluster, spanning a range of ~460 in blue luminosity. Based on this large,homogeneous dataset, we present a sharp upward revision in the frequency ofnucleation in early-type galaxies brighter than M_B ~ -15 (66 < f_n < 82%), andfind no evidence for nucleated dwarfs to be more concentrated to the center ofVirgo than their non-nucleated counterparts. Resolved stellar nuclei are notpresent in galaxies brighter than M_B ~ -20.5, however, there is no clearevidence from the properties of the nuclei, or from the overall incidence ofnucleation, for a change at M_B ~ -17.6, the traditional dividing point betweendwarf and giant galaxies. On average, nuclei are ~3.5 mag brighter than atypical globular cluster and have a median half-light radius ~4.2 pc. Nuclearluminosities correlate with nuclear sizes and, in galaxies fainter than M_B ~-17.6, nuclear colors. Comparing the nuclei to the "nuclear clusters" found inlate-type spiral galaxies reveals a close match in terms of size, luminosityand overall frequency, pointing to a formation mechanism that is ratherinsensitive to the detailed properties of the host galaxy. The meannuclear-to-galaxy luminosity ratio is indistinguishable from the meanSBH-to-bulge mass ratio, calculated in early-type galaxies with detectedsupermassive black holes (SBHs). We argue that compact stellar nuclei might bethe low-mass counterparts of the SBHs detected in the bright galaxies, and thatone should think in terms of "Central Massive Objects" -- either SBHs orcompact stellar nuclei -- that accompany the formation of almost all early-typegalaxies and contain a mean fraction ~0.3% of the total bulge mass.