A Comprehensive Archival Chandra Search for X-ray Emission from
  Ultracompact Dwarf Galaxies
  We present the first comprehensive archival study of the X-ray properties of
ultracompact dwarf (UCD) galaxies, with the goal of identifying
weakly-accreting central black holes in UCDs. Our study spans 578 UCDs
distributed across thirteen different host systems, including clusters, groups,
fossil groups, and isolated galaxies. Of the 336 spectroscopically-confirmed
UCDs with usable archival Chandra imaging observations, 21 are X-ray-detected.
Imposing a completeness limit of $L_X>2\times10^{38}$ erg s$^{-1}$, the global
X-ray detection fraction for the UCD population is $\sim3\%$. Of the 21
X-ray-detected UCDs, seven show evidence of long-term X-ray time variability on
the order of months to years. X-ray-detected UCDs tend to be more compact than
non-X-ray-detected UCDs, and we find tentative evidence that the X-ray
detection fraction increases with surface luminosity density and global stellar
velocity dispersion. The X-ray emission of UCDs is fully consistent with
arising from a population of low-mass X-ray binaries (LMXBs). In fact, there
are fewer X-ray sources than expected using a naive extrapolation from globular
clusters. Invoking the fundamental plane of black hole activity for SUCD1 near
the Sombrero galaxy, for which archival Jansky Very Large Array imaging at 5
GHz is publicly available, we set an upper limit on the mass of a hypothetical
central black hole in that UCD to be $\lesssim10^5M_{\odot}$. While the
majority of our sources are likely LMXBs, we cannot rule out central black
holes in some UCDs based on X-rays alone, and so we address the utility of
follow-up radio observations to find weakly-accreting central black holes.
