Detection of a 100,000 M-circle dot 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 show a strong rotation (similar to 20 km s(-1)) and a velocity dispersion rise toward the center (37 km s(-1)). 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(-2.8)(+2.6) x 10(4) M-circle dot (1 sigma uncertainties). The inferred stellar mass of the cluster is 6.22(-0.05)(+)(0.03)( ) x 10(6) M-circle dot, 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(circle dot), 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 M-circle dot 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-circle dot) stripped nucleus, and one of the few dynamically detected intermediate-mass BHs.