A ~50,000 solar mass black hole in the nucleus of RGG 118
  Scaling relations between black hole (BH) masses and their host galaxy
properties have been studied extensively over the last two decades, and point
towards co-evolution of central massive BHs and their hosts. However, these
relations remain poorly constrained for BH masses below $\sim10^{6}$ M_sun.
Here we present optical and X-ray observations of the dwarf galaxy RGG 118
taken with the Magellan Echellette Spectrograph on the 6.5m Clay Telescope and
Chandra X-ray Observatory. Based on Sloan Digital Sky Survey spectroscopy, RGG
118 was identified as possessing narrow emission line ratios indicative of
photoionization partly due to an active galactic nucleus. Our higher resolution
spectroscopy clearly reveals broad H$\alpha$ emission in the spectrum of RGG
118. Using virial BH mass estimate techniques, we calculate a BH mass of
$\sim50,000$ \msun. We detect a nuclear X-ray point source in RGG 118,
suggesting a total accretion powered luminosity of $L=4\times10^{40}~{\rm
erg~s^{-1}}$, and an Eddington fraction of $\sim1$ per cent. The BH in RGG 118
is the smallest ever reported in a galaxy nucleus and we find that it lies on
the extrapolation of the $M_{\rm BH}-\sigma_{\ast}$ relation to the lowest
masses yet.
