The ACS Virgo Cluster Survey is a Hubble Space Telescope program to obtain high-resolution imaging , in widely separated bandpasses ( F475W \approx g and F850LP \approx z ) , for 100 early-type members of the Virgo Cluster , spanning a range of \approx 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 M _ { B } \approx - 15 ( 66 \lesssim f _ { n } \lesssim 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 M _ { B } \sim - 17.6 , the traditional dividing point between dwarf and giant galaxies .
There does , however , appear to be a fundamental transition at M _ { B } \sim - 20.5 , in the sense that the brighter , “ core-Sérsic ” 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 \farcs 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 \langle r _ { h } \rangle = 4.2 pc , with the sizes of individual nuclei ranging from 62 pc down to \leq 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 r _ { h } \propto { \cal L } ^ { 0.50 \pm 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 \approx 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 M _ { B } \approx - 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-to-galaxy luminosity ratio in our nucleated galaxies , \langle \log _ { 10 } \eta \rangle = -2.49 \pm 0.09 dex ( \sigma = 0.59 \pm 0.10 ) , is indistinguishable from that of the SBH-to-bulge mass ratio , \langle \log _ { 10 } { ( { \cal M } _ { \bullet } / { \cal M } _ { gal } ) } \rangle = -2.61 \pm 0.07 dex ( \sigma = 0.45 \pm 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 \approx 0.3 % of the total bulge mass .
In this view , SBHs would be the dominant formation mode above M _ { B } \approx - 20.5 .