MUSE crowded field 3D spectroscopy of over 12,000 stars in the globular
  cluster NGC 6397 - II. Probing the internal dynamics and the presence of a
  central black hole
  We present a detailed analysis of the kinematics of the Galactic globular
cluster NGC 6397 based on more than ~18,000 spectra obtained with the novel
integral field spectrograph MUSE. While NGC 6397 is often considered a core
collapse cluster, our analysis suggests a flattening of the surface brightness
profile at the smallest radii. Although it is among the nearest globular
clusters, the low velocity dispersion of NGC 6397 of <5km/s imposes heavy
demands on the quality of the kinematical data. We show that despite its
limited spectral resolution, MUSE reaches an accuracy of 1km/s in the analysis
of stellar spectra. We find slight evidence for a rotational component in the
cluster and the velocity dispersion profile that we obtain shows a mild central
cusp. To investigate the nature of this feature, we calculate spherical Jeans
models and compare these models to our kinematical data. This comparison shows
that if a constant mass-to-light ratio is assumed, the addition of an
intermediate-mass black hole with a mass of 600M_sun brings the model
predictions into agreement with our data, and therefore could be at the origin
of the velocity dispersion profile. We further investigate cases with varying
mass-to-light ratios and find that a compact dark stellar component can also
explain our observations. However, such a component would closely resemble the
black hole from the constant mass-to-light ratio models as this component must
be confined to the central ~5arcsec of the cluster and must have a similar
mass. Independent constraints on the distribution of stellar remnants in the
cluster or kinematic measurements at the highest possible spatial resolution
should be able to distinguish the two alternatives.
