Which Globular Clusters contain Intermediate-Mass Black Holes

Baumgardt, H., Makino, J., Hut, P.

Abstract:

It has been assumed that intermediate-mass black holes (IMBHs) in globular clusters can only reside in the most centrally concentrated clusters, with a so-called `core-collapsed' density profile. While this would be a natural guess, it is in fact wrong. We have followed the evolution of star clusters containing IMBHs with masses between $125 \le M_{BH} \le 1000$ $M_{\odot}$ through detailed $N$-body simulations, and we find that a cluster with an IMBH, in projection, appears to have a relatively large `core' with surface brightness only slightly rising toward the center. This makes it highly unlikely that any of `core-collapsed' clusters will harbor an IMBH. On the contrary, the places to look for an IMBH are those clusters that can be fitted well by medium-concentration King models. The velocity dispersion of the visible stars in a globular cluster with an IMBH is nearly constant well inside the apparent core radius. For a cluster of mass $M_C$ containing an IMBH of mass $M_{BH}$, the influence of the IMBH becomes significant only at a fraction $2.5 M_{BH}/M_C$ of the half-mass radius, deep within the core, where it will affect only a small number of stars. In conclusion, observational detection of an IMBH may be possible, but will be challenging.


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