Which Globular Clusters Contain Intermediate-Mass Black Holes?

• Published in: The Astrophysical journal 2005-02, Vol.620 (1), p.238-243
• Main Authors: Baumgardt, Holger, Makino, Junichiro, Hut, Piet
• Format: Article
• Language: English
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/426893

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 , M sub(BH) , 1000 M sub( )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 the "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 sub(C) containing an IMBH of mass M sub(BH), the influence of the IMBH becomes significant only at a fraction 2.5 M sub(BH)/M sub(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.