34 2 v 1 1 9 Ja n 20 00 Structure of the globular cluster M 15 and constraints on a massive central black hole ⋆

Globular clusters could harbor massive central black holes (BHs), just as galaxies do. So far, no unambiguous detection of a massive BH has been reported for any globular cluster. However, the dense core-collapsed cluster M15 seems to be a good candidate. I review the available photometric and kinematic data for this cluster. Both are consistent with a BH of mass M• ≈ 2000M⊙, although such a BH is not unambiguously required by the data. I discuss some ongoing studies with Keck and HST which should shed more light on this issue in the coming years. 1 Massive Central Black Holes in Globular Clusters Massive BHs have been convincingly detected in the centers of some nearby galaxies (e.g., Miyoshi et al. 1995), including our own (Genzel et al. 1997). In certain galaxies the BH directly reveals itself through its associated accretion and activity. Such activity is never observed in globular clusters, but nonetheless, it may well be that (some) globular clusters also contain BHs. There are many ways in which globular cluster evolution at high densities (Meylan & Heggie 1997) can lead to the formation of a massive BH in the center (Rees 1984). For example, core collapse induced by two-body relaxation may lead to sufficiently high densities for individual stars or stellar-mass black holes to interact or collide, with a single massive BH as the likely end product (Quinlan & Shapiro 1987; Lee 1993, 1995). The black hole mass M• in galaxies correlates with galaxy (bulge) mass M such that M•/M ≈ 10 −3±1 (Kormendy & Richstone 1995; Magorrian et al. 1998; van der Marel 1999). One may use this correlation to obtain a crude estimate for the possible BH masses in globular clusters (although it should be kept in mind that the BHs in galaxies are often hypothesized to have formed through gas collapse and accretion, instead of through collapse of a stellar cluster; e.g., Haehnelt et al. 1997). This yields M• ≈ 10 3M⊙. The presence or growth of a BH in a globular cluster affects both the stellar density profile and the stellar dynamics. Observational constraints on BHs in globular clusters can therefore be found from photometric and ⋆ To appear in ‘Black Holes in Binaries and Galactic Nuclei’, Proceedings of an ESO Workshop in September 1999, Kaper L., van den Heuvel E. P. J., Woudt P. A., eds., Springer-Verlag, 2000. 2 Roeland P. van der Marel kinematic studies. So far, no detection of a massive BH has been obtained for any globular cluster, but only few, if any, studies have had sufficient sensitivity to unambiguously detect BHs with masses as low as M• ≈ 10 3M⊙. On the other hand, advances in observational capabilities and techniques are now making it possible to study BH masses down to this limit, so this is becoming a more active area of interest. The globular cluster M15 (NGC 7078) at a distance of 10 kpc is one of the densest globular clusters in our Galaxy. The presence of a bright Xray source (Hut et al. 1992) and several millisecond pulsars (Phinney 1993) are manifestations of its extreme density, which makes M15 one of the best a priori candidates to search for evidence of a central BH. For this reason, M15 has been intensively observed in the past decade using a variety of techniques, and it is also the subject of several ongoing studies.