Detection of a $\sim$100,000 M$_\odot$ black hole in M31's most massive
  globular cluster: A tidally stripped nucleus
  We investigate the presence of a central black hole (BH) in B023-G078, M31's
most massive globular cluster. We present high-resolution, adaptive-optics
assisted, integral-field spectroscopic kinematics from Gemini/NIFS that shows a
strong rotation ($\sim$20 km/s) and a velocity dispersion rise towards the
center (37 km/s). We combine the kinematic data with a mass model based on a
two-component fit to $HST$ ACS/HRC data of the cluster to estimate the mass of
a putative BH. Our dynamical modeling suggests a $>$3$\sigma$ detection of a BH
component of 9.1$^{+2.6}_{-2.8}\times$10$^4$ M$_\odot$ (1$\sigma$
uncertainties). The inferred stellar mass of the cluster is
6.22$^{+0.03}_{-0.05}\times$10$^6$ M$_\odot$, consistent with previous
estimates, thus the BH makes up 1.5% of its mass. We examine whether the
observed kinematics are caused by a collection of stellar mass BHs by modeling
an extended dark mass as a Plummer profile. The upper limit on the size scale
of the extended mass is 0.56 pc (95% confidence), which does not rule out an
extended mass. There is compelling evidence that B023-G078 is the tidally
stripped nucleus of a galaxy with a stellar mass $>$10$^9$ M$_{\odot}$,
including its high mass, two-component luminosity profile, color, metallicity
gradient, and spread in metallicity. Given the emerging evidence that the
central BH occupation fraction of $>$10$^9$ M$_{\odot}$ galaxies is high, the
most plausible interpretation of the kinematic data is that B023-G078 hosts a
central BH. This makes it the strongest BH detection in a lower mass ($<$10$^7$
M$_{\odot}$) stripped nucleus, and one of the few dynamically detected
intermediate-mass BHs.
