An Intermediate-mass Black Hole in the Globular Cluster G1: Improved Significance from New Keck and Hubble Space Telescope Observations1

We present dynamical models for the massive globular cluster G1. The goal is to measure or place a significant upper limit on the mass of any central black hole. Whether or not globular clusters contain central massive black holes has important consequences for a variety of studies. We use new kinematic data obtained with Keck and new photometry from the Hubble Space Telescope. The Keck spectra allow us to obtain kinematics out to large radii that are required to pin down the mass-to-light ratio of the dynamical model and the orbital structure. The Hubble Space Telescope observations give us a factor of two better spatial resolution for the surface brightness profile. By fitting non-parametric, spherical, isotropic models we find a best-fit black hole mass of 1.7(±0.3)×104 M⊙. Fully general axisymmetric orbit-based models give similar results, with a black hole mass of 1.8(±0.5)×104 M⊙. The no-black hole model has ∆χ2 = 5 (marginalized over mass-to-light ratio), implying less than 3% significance. We have taken into account any change in the mass-to-light ratio in the center due to stellar remnants. These results are consistent with our previous estimate in Gebhardt, Rich & Ho (2002), and inconsistent with the analysis of Baumgardt et al. (2003) who claim that G1 does not show evidence for a black hole. These new results make G1 the best example of a cluster that contains an intermediate-mass black hole. Subject headings: galaxies: individual (M31) — galaxies: star clusters — globular clusters: general — globular clusters: individual (Mayall II = G1)