Constraining ultra-compact dwarf galaxy formation with galaxy clusters
  in the local universe
  We compare the predictions of a semi-analytic model for ultra-compact dwarf
galaxy (UCD) formation by tidal stripping to the observed properties of
globular clusters (GCs) and UCDs in the Fornax and Virgo clusters. For Fornax
we find the predicted number of stripped nuclei agrees very well with the
excess number of GCs$+$UCDs above the GC luminosity function. GCs$+$UCDs with
masses $>10^{7.3}$ M$_\odot$ are consistent with being entirely formed by tidal
stripping. Stripped nuclei can also account for Virgo UCDs with masses
$>10^{7.3}$ M$_\odot$ where numbers are complete by mass. For both Fornax and
Virgo, the predicted velocity dispersions and radial distributions of stripped
nuclei are consistent with that of UCDs within $\sim$50-100 kpc but disagree at
larger distances where dispersions are too high and radial distributions too
extended. Stripped nuclei are predicted to have radially biased anisotropies at
all radii, agreeing with Virgo UCDs at clustercentric distances larger than 50
kpc. However, ongoing disruption is not included in our model which would cause
orbits to become tangentially biased at small radii. We find the predicted
metallicities and central black hole masses of stripped nuclei agree well with
the metallicities and implied black hole masses of UCDs for masses $>10^{6.5}$
M$_\odot$. The predicted black hole masses also agree well with that of
M60-UCD1, the first UCD with a confirmed central black hole. These results
suggest that observed GC$+$UCD populations are a combination of genuine GCs and
stripped nuclei, with the contribution of stripped nuclei increasing toward the
high-mass end.
