A stellar-mass black hole population in the globular cluster NGC 6101?
  Dalessandro et al. observed a similar distribution for blue straggler stars
and main-sequence turn-off stars in the Galactic globular cluster NGC 6101, and
interpreted this feature as an indication that this cluster is not
mass-segregated. Using direct N-body simulations, we find that a significant
amount of mass segregation is expected for a cluster with the mass, radius and
age of NGC 6101. Therefore, the absence of mass segregation cannot be explained
by the argument that the cluster is not yet dynamically evolved. By varying the
retention fraction of stellar-mass black holes, we show that segregation is not
observable in clusters with a high black hole retention fraction (>50% after
supernova kicks and >50% after dynamical evolution). Yet all model clusters
have the same amount of mass segregation in terms of the decline of the mean
mass of stars and remnants with distance to the centre. We also discuss how
kinematics can be used to further constrain the presence of a stellar-mass
black hole population and distinguish it from the effect of an
intermediate-mass black hole. Our results imply that the kick velocities of
black holes are lower than those of neutron stars. The large retention fraction
during its dynamical evolution can be explained if NGC 6101 formed with a large
initial radius in a Milky Way satellite.
