5.3. Intermediate-mass black holes?
An interesting question is related to the range of black hole masses over which the observed relation (16) applies. Most recently, high-spatial-resolution spectroscopy with HST, (van der Marel et al. 2002, Gerssen et al. 2002, Gerssen et al. 2003), of the central part of the globular cluster M15, revealed evidence for the existence of a dense, central concentration of dark mass (see also Dull et al. 2002, Baumgardt et al. 2002). The interpretation of the nature of this dark mass has been somewhat controversial. Dull et al. (2002) and Baumgardt et al. (2002) have shown that the sharp rise in M/L that was observed toward the center of M15 could be explained by a central concentration of neutron stars and massive white dwarfs. On the other hand, Gerssen et al. (2003) argued that if one were to allow for the fact that a large fraction of the neutron stars born within the cluster cannot be retained (due to "kick" velocities of a few hundred km s-1 at birth), the observed M/L might require the existence of a central black hole of mass MBH = 1.7+2.7-1.7 × 103 M. Interestingly, the derived mass fits quite well onto the MBH - relationship (Tremaine et al. 2002, Gerssen et al. 2002). Similarly, evidence for a central black hole with a mass of ~ 2 × 104 M has been found in the stellar cluster G1 in the Andromeda galaxy M31 (Gebhardt, Rich, and Ho 2002). The latter black hole also fits the MBH - relation. In an entirely independent work (D'Amico et al. 2002), pulse timing observations determined positions for five millisecond pulsars in the cluster NGC 6752 with a 20 mas accuracy. Three were found to have line-of-sight accelerations larger than the maximum value predicted on the basis of the central mass density derived from optical observations. The measured accelerations thus provided dynamical evidence for a central mass-to-light ratio of M/L 10. All of these findings, if confirmed, are extremely exciting, since the implied masses make these black holes intermediate between the stellar black holes (with MBH ~ 10 M) and the suppermassive ones (MBH ~ 106-109 M).
The potential existence of a class of intermediate-mass black holes has also been suggested following the discovery of ultraluminous x-ray sources (e.g., Colbert and Mushotzky 1999). The latter are x-ray sources outside the nuclei of external galaxies, with luminosities in excess of 1039 erg s-1. These sources were originally discovered by the Einstein satellite (Fabbiano 1989), but have been found in large numbers by the ROSAT and Chandra observatories (e.g., Colbert and Mushotzky 1999, Lira, Johnson, and Lawrence 2002, Jeltema et al. 2002). More recently, however, it has been pointed out that although the ultraluminous x-ray sources may form a heterogeneous class of systems, the most likely explanation for the majority of them is that they constitute the high-luminosity tail of the stellar-mass black-hole binary distribution (e.g., King et al. 2001, Roberts et al. 2001, Podsiadlowski et al. 2002). In particular, Podsiadlowski et al. (2002) have shown that in binary systems in which the donor star becomes a giant and the evolution is driven by the nuclear evolution of the hydrogen-burning shell, luminosities that are potentially as high as ~ 1041 erg s-1 can be obtained. The existence of ultraluminous x-ray sources by itself, therefore, may not imply the existence of intermediate-mass black holes.
The potential existence of intermediate-mass black holes also led to a new scenario for the formation of the supermassive ones (Ebisuzaki et al. 2001). The idea is that first, intermediate-mass black holes form in young clusters, due to runaway mergers of massive stars. The massive stars are assumed to sink to the cluster center due to dynamical friction, on a timescale of (e.g., Binney and Tremaine 1987)
where log is the Coulomb logarithm, is the velocity dispersion, r is the distance to the cluster center, rh is the half-mass radius and M and m are the masses of the cluster and the star, respectively. During the same time that the intermediate-mass black holes are forming, the host clusters themselves sink to the galactic center (again due to dynamical friction), evaporate, and deposit their black holes. The latter can then form black hole binaries which merge due to gravitational radiation, eventually leading to the formation of supermassive black holes. I should note that this is only one path out of the many that have been suggested over the years for the formation of supermassive black holes (Rees 1984).
The installation of HST's Advanced Camera for Surveys (ACS) in March 2002, with its superior sensitivity (by a factor of about 3-5 over the previous Wide Field Planetary Camera 2) and increased resolution (in the high-resolution channel), combined with X-ray observations by the Chandra and XMM-Newton Observatories, and infrared observations of dust-obscured active galactic nuclei by the Space Infrared Telescope Facility (SIRTF) promise that the study of black holes in clusters (if they exist) and in galactic centers is only beginning. In particular, future observations will help clarify the relation between galaxy evolution in general, and the formation and evolution of the cental black holes.
Observations with HST have provided important insights not only in the study of supermassive black holes, but also in researches related to the probable formation of stellar-mass black holes - the study of gamma-ray bursters.