The Velocity Dispersion Profile of NGC 6388 from Resolved-star Spectroscopy: No Evidence of a Central Cusp and New Constraints on the Black Hole Mass

Abstract

By combining high spatial resolution and wide-field spectroscopy performed, respectively, with SINFONI and FLAMES at the ESO/VLT we measured the radial velocities of more than 600 stars in the direction of NGC 6388, a Galactic globular cluster which is suspected to host an intermediate-mass black hole. Approximately 55% of the observed targets turned out to be cluster members. The cluster velocity dispersion has been derived from the radial velocity of individual stars: 52 measurements in the innermost 2'', and 276 stars located between 18'' and 600''. The velocity dispersion profile shows a central value of ~13 km s^-1, a flat behavior out to ~60'' and a decreasing trend outward. The comparison with spherical and isotropic models shows that the observed density and velocity dispersion profiles are inconsistent with the presence of a central black hole more massive than ~2000 M _⊙. These findings are at odds with recent results obtained from integrated light spectra, showing a velocity dispersion profile with a steep central cusp of 23-25 km s^-1 at r < 2'' and suggesting the presence of a black hole with a mass of ~1.7 × 10⁴ M _⊙. We also found some evidence of systemic rotation with amplitude A _rot ~ 8 km s^-1 in the innermost 2'' (0.13 pc), decreasing to A _rot = 3.2 km s^-1 at 18'' < r < 160''.

Based on observations collected at the European Southern Observatory, Cerro Paranal, Chile (under proposals 381.D-0329, 073.D-0211, and 073.D-0760). Also based on observations with the NASA/ESA HST (Prop. 19835), obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555.