The Surface Density Profile of NGC 6388: A Good Candidate for Harboring an Intermediate-Mass Black Hole

We have used a combination of HST high-resolution and ground-based wide-field observations of the Galactic globular cluster NGC 6388 to derive its center of gravity, projected density profile, and central surface brightness profile. While the overall projected profiles are well fit by a King model with an intermediate concentration (c=1.8) and a sizable core radius (r_c = 7.2"), a significant power-law (with slope α=-0.2) deviation from flat-core behavior has been detected within the inner 1". These properties suggest the presence of a central intermediate-mass black hole. The observed profiles are well reproduced by a multimass isotropic, spherical model, including a black hole with a mass of ~5.7 × 10³ M_solar.

Based on observations with the NASA/ESA Hubble Space Telescope (HST), obtained at the Space Telescope Science Institute (STScI), which is operated by AURA, Inc., under NASA contract NAS5-26555. Also based on WFI observations collected at the European Southern Observatory (ESO), La Silla, Chile.