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

The ACS Virgo Cluster Survey is a Hubble Space Telescope program to obtain high-resolution imaging, in widely separated bandpasses (F475W ≈ g and F850LP ≈ z), for 100 early-type members of the Virgo Cluster, spanning a range of ≈ 460 in blue luminosity. We use this large, homogenous dataset to examine the innermost structure of these galaxies and to characterize the properties of their compact central nuclei. We present a sharp upward revision in the frequency of nucleation in early-type galaxies brighter than MB ≈ −15 (66 . fn . 82%) and show that ground-based surveys underestimated the number of nuclei due to surface brightness selection effects, limited sensitivity and poor spatial resolution. We speculate that previously reported claims that nucleated dwarfs are more concentrated to the center of Virgo than their non-nucleated counterparts may be an artifact of these selection effects. There is no clear evidence from the properties of the nuclei, or from the overall incidence of nucleation, for a change at MB ∼ −17.6, the traditional dividing point between dwarf and giant galaxies. There does, however, appear to be a fundamental transition at MB ∼ −20.5, in the sense that the brighter, “core-S´ersic” galaxies lack resolved (stellar) nuclei. A search for nuclei which may be offset from the photocenters of their host galaxies reveals only five candidates with displacements of more than 0.′′5, all of which are in dwarf galaxies. In each case, though, the evidence suggests that these “nuclei” are, in fact, globular clusters projected close to the galaxy photocenter. Working from a sample of 51 galaxies with prominent nuclei, we find a median half-light radius of hrhi = 4.2 pc, with the sizes of individual nuclei ranging from 62 pc down to ≤ 2 pc (i.e., unresolved in our images) in about a half dozen cases. Excluding these unresolved objects, the nuclei sizes are found to depend on nuclear luminosity according to the relation rh ∝ L0.50±0.03. Because the large majority of nuclei are resolved, we can rule out low-level AGN as an explanation for the central luminosity excess in almost all cases. On average, the nuclei are ≈ 3.5 mag brighter than a typical globular cluster. Based on their broadband colors, the nuclei appear to have old to intermediate-age stellar populations. The colors of the nuclei in galaxies fainter than MB ≈ −17.6 are tightly correlated with their luminosities, and less so with the luminosities of their host galaxies, suggesting that their chemical enrichment histories were governed by local or internal factors. 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. A formation mechanism that is rather insensitive to the detailed properties of the host galaxy is required to explain this ubiquity and homogeneity. The mean of the frequency function for the nucleus-togalaxy luminosity ratio in our nucleated galaxies, hlog10 ηi = −2.49 ± 0.09 dex (σ = 0.59 ± 0.10), is indistinguishable from that of the SBH-to-bulge mass ratio, hlog10 (M•/Mgal)i = −2.61 ± 0.07 dex (σ = 0.45±0.09), calculated in 23 early-type galaxies with detected supermassive black holes (SBHs). We argue that the compact stellar nuclei found in many of our program galaxies are the low-mass counterparts of the SBHs detected in the bright galaxies. If this interpretation is correct, then 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. In this view, SBHs would be the dominant formation mode above MB ≈ −20.5