A Search for an Intermediate-mass Black Hole in the Core of the Globular Cluster NGC 6266

It has long been thought that intermediate-mass black holes (IMBHs) might be located in the cores of globular clusters. However, studies attempting to confirm this possibility have been inconclusive. To refine the search for these objects, Baumgardt et al. completed a series of N-body simulations to determine the observational properties that a host globular cluster should possess. Keys to revealing the presence of an IMBH were found to be the shape of the cluster's core proper motion dispersion profile and its surface density profile. Among the possible host clusters identified by Baumgardt et al., NGC 6266 was found to be the most suitable object to search. Hubble Space Telescope Wide Field Planetary Camera 2 images with an epoch difference of eight years were, therefore, used to measure this cluster's internal proper motion dispersion profile from 0.8 arcsec to 17 arcsec from the cluster center. This profile and the surface density profiles obtained by Noyola & Gebhardt and Trager et al. were then compared to those produced by N-body simulations of NGC 6266 with and without an IMBH. We find that a centrally located IMBH is not required to match these profiles, but that an IMBH with a 1σ upper limit mass of less than a few thousand M _⊙ cannot be excluded. To establish the existence of this object, the exact location of the density center and more precise velocity measurements within the inner 1 arcsec of this center are required. Our best-fitting model of NGC 6266 without an IMBH yields a cluster mass of M = 8.22 ± 0.17 × 10⁵ M _⊙, leading to a mass-to-light ratio of M/L_V = 2.05 ± 0.04.