Detection of Supermassive Black Holes in Two Virgo Ultracompact Dwarf
  Galaxies
  We present the detection of supermassive black holes (BHs) in two Virgo
ultracompact dwarf galaxies (UCDs), VUCD3 and M59cO. We use adaptive optics
assisted data from the Gemini/NIFS instrument to derive radial velocity
dispersion profiles for both objects. Mass models for the two UCDs are created
using multi-band Hubble Space Telescope (HST) imaging, including the modeling
of mild color gradients seen in both objects. We then find a best-fit stellar
mass-to-light ratio ($M/L$) and BH mass by combining the kinematic data and the
deprojected stellar mass profile using Jeans Anisotropic Models (JAM). Assuming
axisymmetric isotropic Jeans models, we detect BHs in both objects with masses
of $4.4^{+2.5}_{-3.0} \times 10^6$ $M_{\odot}$ in VUCD3 and $5.8^{+2.5}_{-2.8}
\times 10^6$ $M_{\odot}$ in M59cO (3$\sigma$ uncertainties). The BH mass is
degenerate with the anisotropy parameter, $\beta_z$; for the data to be
consistent with no BH requires $\beta_z = 0.4$ and $\beta_z = 0.6$ for VUCD3
and M59cO, respectively. Comparing these values with nuclear star clusters
shows that while it is possible that these UCDs are highly radially
anisotropic, it seems unlikely. These detections constitute the second and
third UCDs known to host supermassive BHs. They both have a high fraction of
their total mass in their BH; $\sim$13% for VUCD3 and $\sim$18% for M59cO. They
also have low best-fit stellar $M/L$s, supporting the proposed scenario that
most massive UCDs host high mass fraction BHs. The properties of the BHs and
UCDs are consistent with both objects being the tidally stripped remnants of
$\sim$10$^9$ M$_\odot$ galaxies.
