4.1. Supermassive Black Holes and their Host Galaxies
Measuring SBH masses in the nuclei of "normal" nearby galaxies has been a staple of the astronomical literature since the late 1970s. It all started in 1978, when Wallace Sargent and collaborators published an investigation of the nuclear dynamics of the Virgo cluster cD, M87 (Sargent et al. 1978), claiming the detection of a five billion solar mass black hole. Other famous detections followed: M32 in the mid '80s (Tonry 1984), M31 a few years later (Kormendy 1988). Each claim, however, seemed to have its detractors, beginning with Binney & Mamon (1982) who dismantled Sargent's M87 black hole detection and alerted the community to the perils of the now familiar "mass-to-light ratio - velocity anisotropy degeneracy".
Important ground-based work on SBHs continued through the '90s (Richstone, Bower & Dressler 1990; Kormendy et al. 1996a, 1996b; Magorrian et al. 1998), producing a series of tantalizing but frustratingly inconclusive results. Indeed, it was not until the launch of HST that dramatic progress was made. It was HST data that firmly established the existence of a SBH in M87 (Harms et al. 1994), thereby ending a two-decade controversy. Since then, SBH masses based on HST/FOS and STIS data have been published for ten additional galaxies (Ferrarese, Ford & Jaffe 1996; Bower et al. 1998; van der Marel & van den Bosch 1998; Ferrarese & Ford 1999; Emsellem et al. 1999; Cretton & van den Bosh 1999; Verdoes Kleijn et al. 2000; Gebhardt et al. 2000; Joseph et al. 2001; Barth et al. 2001; Sarzi et al. 2001).
The success of HST can be ascribed to the fact that its unprecedented spatial resolution (in the optical regime, at least!) makes it possible to resolve, in favorable cases, the region of space within which the SBH's gravitational potential dominates that of the surrounding stars, i.e., the "SBH sphere of influence". This is more crucial than might at first be realized. It has become obvious (Ferrarese & Merritt 2000) that resolving the sphere of influence does not simply aid the SBH detection: it is a necessary condition for a detection to be made. Ground-based observations generally lack the spatial resolution necessary to penetrate the SBH sphere of influence, and this condition inevitably leads to spurious detections and overestimated masses (see Merritt & Ferrarese 2001c for a more thorough discussion of this issue). To date, with a few notable exceptions (the Milky Way, Genzel et al. 2000, Ghez et al. 2000; NGC 4258, Miyoshi et al. 1995; NGC 5128, Marconi et al. 2001), all firm SBH detections - detections based on data which resolve the SBH sphere of influence - are based on HST data (see Table 1 of Merritt & Ferrarese 2001c).
Figure 2.(left) Correlation between central
velocity dispersion and black hole
mass for all secure SBH detections. Published data are shown as solid
symbols, data based on unpublished analyses as open symbols.
It was by isolating these secure detections that it became possible to unveil the existence of a fundamental, seemingly perfect correlation between black hole mass, M, and velocity dispersion, , of the host bulge (Fig. 2, Ferrarese & Merritt 2000; Gebhardt et al. 2000): the relation emerged from what had appeared almost as a scatter plot when the sample was restricted only to galaxies in which the SBH sphere of influence had been resolved (Ferrarese & Merritt 2000; Merritt & Ferrarese 2001b; Merritt & Ferrarese 2001c). A regression analysis, accounting for errors in both coordinates, of all published SBH detections (listed in Table 1 of Merritt & Ferrarese 2001c) gives
Including the few preliminary masses based on unpublished analyses produces an indistinguishable slope of 4.64 ± 0.47. The reduced 2 of the fit, 0.74, points to a relation with negligible intrinsic scatter, in agreement with the observations made by Ferrarese & Merritt (2000) based on the smaller sample available at the time. Because of its tightness, the M - relation has largely supplanted the well known correlation between M and bulge magnitude MB (Fig. 3; Kormendy & Richstone 1995; Magorrian et al. 1998), and has emerged as the tool of choice in the study of SBH demographics (Merritt & Ferrarese 2001b). Indeed, the tightness of the M - relation is its most puzzling feature, presenting formidable challenges to theoretical models for the formation and evolution of SBHs (e.g Haehnelt & Kauffmann 2000; Kauffmann & Haehnelt 2000; Adams et al. 2000; Ciotti & van Albada 2001; Burkert & Silk 2001). Even if one assumes that a tight relation was imprinted at an early stage of galaxy/SBH formation, it is difficult to understand how it could have survived unaltered in the face of mergers. It is especially remarkable that the relation should hold true for galaxies of disparate Hubble types (from SBs to compact ellipticals to cDs) belonging to wildly different environments (from rich clusters to the field), showing perfectly smooth (e.g. NGC 6251) or highly disturbed (e.g. NGC 5128) morphologies. For instance, it has recently been noted (McLure & Dunlop 2001) that the large scatter in the M - MB relation (a reduced 2 of 23, Ferrarese & Merritt 2000) can be significantly reduced, but only at the expense of excluding most lenticular and spiral galaxies (and the odd elliptical, cf. Fig. 3). Even the remarkably tight correlation discovered by Graham et al. (2001) between M and "concentration parameter" C(the fraction of total bulge light contained within a predetermined radius) is marred by the occasional outlier. However, every galaxy, even the ones which do not obey the M - MB or M - C relations, seems to conform magically to the M - relation.