The M_BH - sigma relation for intermediate-mass black holes in globular
  clusters
  For galaxies hosting supermassive black holes (SMBHs), it has been observed
that the mass of the central black hole (M_BH) tightly correlates with the
effective or central velocity dispersion (sigma) of the host galaxy. The origin
of this M_BH - sigma scaling relation is assumed to lie in the merging history
of the galaxies but many open questions about its origin and the behavior in
different mass ranges still need to be addressed. The goal of this work is to
study the black-hole scaling relations for low black-hole masses, where the
regime of intermediate-mass black holes (IMBHs) in globular clusters (GCs) is
entered. We collect all existing reports of dynamical black-hole measurements
in globular clusters, providing black-hole masses or upper limits for 14
candidates. We plot the black-hole masses versus different cluster parameters
including total mass, velocity dispersion, concentration and half-mass radius.
We search for trends and test the correlations in order to quantify their
significance using a set of different statistical approaches. For correlations
showing a large significance we perform a linear fit, accounting for
uncertainties and upper limits. We find a clear correlation between the mass of
the IMBH and the velocity dispersion of the globular cluster. As expected, the
total mass of the globular cluster then also correlates with the mass of the
IMBH. While the slope of the M_BH - sigma correlation differs strongly from the
one observed for SMBHs, the other scaling relations M_BH - M_TOT, and M_BH - L
are similar to the correlations in galaxies. Significant correlations of
black-hole mass with other cluster properties were not found in the present
sample.
