Detection of Enhanced Central Mass-to-Light Ratios in Low-Mass
  Early-Type Galaxies: Evidence for Black Holes?
  We present dynamical measurements of the central mass-to-light ratio ($M/L$)
of a sample of 27 low-mass early-type ATLAS$^{3D}$ galaxies. We consider all
ATLAS$^{3D}$ galaxies with 9.7$<$log(M$_\star/$M$_\odot)$$<$10.5 in our
analysis, selecting out galaxies with available high-resolution Hubble Space
Telescope (HST) data, and eliminating galaxies with significant central color
gradients or obvious dust features. We use the HST images to derive mass models
for these galaxies and combine these with the central velocity dispersion
values from ATLAS$^{3D}$ data to obtain a central dynamical $M/L$ estimate.
These central dynamical $M/L$s are higher than dynamical $M/L$s derived at
larger radii and stellar population estimates of the galaxy centers in
$\sim$80\% of galaxies, with a median enhancement of $\sim$14\% and a
statistical significance of 3.3$\sigma$. We show that the enhancement in the
central $M/L$ is best described either by the presence of black holes in these
galaxies or by radial IMF variations. Assuming a black hole model, we derive
black hole masses for the sample of galaxies. In two galaxies, NGC 4458 and NGC
4660, the data suggests significantly over-massive BHs, while in most others
only upper limits are obtained. We also show that the level of $M/L$
enhancements we see in these early-type galaxy nuclei are consistent with the
larger enhancements seen in ultracompact dwarf galaxies (UCDs), supporting the
scenario where massive UCDs are created by stripping galaxies of these masses.
