Intermediate-mass black holes in globular clusters

This work is focused on the search for intermediate-mass black
holes (IMBHs) in the centers of globular clusters. It has been
demonstrated that supermassive black holes (SMBHs) at the centers
of galaxies show a tight correlation between their mass (M•) and
the velocity dispersion (σ) of the galaxy. Investigating this M• −
σ and similar correlations is crucial to constrain scenarios of
galaxy formation and evolution. If they formed by runaway
collisions of massive stars in young and dense stellar clusters,
IMBHs could still be present in the centers of globular clusters,
today. We measured the inner kinematic profiles for a sample of 9
galactic globular clusters using integral-field spectroscopy and
combined them with existing outer kinematics and photom- etry
obtained form HST archive images. In order to constrain the mass of
a possible black hole we applied analytical Jeans models in
combinations with varying M/LV profiles to each of the clusters.
The results of these fits range from strong hints towards an IMBH
(e.g. NGC 6388) to globular clusters which do not show any
indications of a rising velocity dis- persion profile in their
center (e.g. NGC 2808). Furthermore, the discovery of two high
velocity stars in NGC 2808 opened another opportunity to study the
internal kinematics of this particular cluster and indicates a high
number of stellar-mass black holes in NGC 2808. We finally combined
our results with measurements from the literature and investigated
known scaling relations for SMBHs in galaxies (e.g. M• − σ) at the
low-mass end by plac- ing the results and upper limits of IMBH
measurements on these correlations. We found that IMBHs follow
similar, but more shallow correlations of their mass and the
properties of their host systems. This might be caused with the
severe mass-loss the cluster suffers during its life time. In
addition we ran numerical N-body simulations and compared globular
clusters with dif- ferent black-hole retention fractions, IMBH
masses and binary fractions. We found that IMBHs lead to a higher
ejection rate of massive stars so that clusters with less depleted
mass functions might therefore be good candidates to host IMBHs at
their centers. In the future more N-body simulations will be
performed in order to reproduce our observations in a more
sophisticated way and perform crucial tests to our observing and
analysis methods. The search for IMBHs requires both high spatial
and spectral resolution and will remain at the edge of feasibility.
However, it is crucial to continue the investigations in order to
shed light on black-hole formation and growth.