A 20,000 M⊙ black hole in the stellar cluster G1

We present the detection of a 2.0(-0.8)(+1.4) x 10(4) M. black hole (BH) in the stellar cluster G1 (Mayall II), based on data taken with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. G1 is one of the most massive stellar clusters in M31. The central velocity dispersion (25 km s(-1)) and the measured BH mass of G1 place it on a linear extrapolation of the correlation between BH mass and bulge velocity dispersion established for nearby galaxies. The detection of a BH in this low-mass stellar system suggests that (1) the most likely candidates for seed massive BHs come from stellar clusters, (2) there is a direct link between massive stellar clusters and normal galaxies, and (3) the formation process of both bulges and massive clusters is similar because of their concordance in the M-.-sigma relation. Globular clusters in our Galaxy should be searched for central BHs.