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
  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, a flat behavior out to ~60" and a decreasing trend outwards. 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 Msol. 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 at r<2" and
suggesting the presence of a black hole with a mass of 17,000 Msol (Lutzgendorf
et al. 2011). We also found some evidence of systemic rotation with amplitude
Arot ~8 km/s in the innermost 2" (0.13 pc), decreasing to Arot= 3.2 km/s at
18"<r<160".
