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.