Detection of a 100,000 M _⊙ black hole in M31's Most Massive Globular Cluster: A Tidally Stripped Nucleus

Abstract

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 (~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 >3σ detection of a BH component of ${9.1}_{-2.8}^{+2.6}\times {10}^{4}\,{M}_{\odot }$ (1σ uncertainties). The inferred stellar mass of the cluster is ${6.22}_{-0.05}^{+0.03}\times {10}^{6}\,{M}_{\odot }$ , 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 >10⁹ M _⊙, 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 >10⁹ M _⊙ 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 (<10⁷ M _⊙) stripped nucleus, and one of the few dynamically detected intermediate-mass BHs.