The future is promising: (1) the census of BHs is expected to grow rapidly as HST reaches its full potential and as new techniques allow us to measure M in more distant galaxies; (2) the ongoing unification of the BH and galaxy formation paradigms is fundamental progress, and (3) x-ray satellites and gravitational wave detectors are expected to probe the immediate vicinity of the Schwarzschild radius.
8.1. Black Holes in Distant Galaxies
Measuring M by making dynamical models of observations that spatially resolve the central kinematics (Tables 1 and 2) are well tested techniques. Confidence is growing that the resulting BH masses are accurate to within ~ 30% in the best cases. This has allowed us to begin demographic studies of BHs in nearby galaxies. But the above techniques have a fundamental limitation. They cannot be applied unless the galaxies are close enough so that we can spatially resolve the region that is dynamically affected by the BH. Within a few more years, the most interesting galaxies that are accessible with HST resolution will have been observed, and new detections will slow down. Expected advances in spatial resolution will enable important but only incremental progress. The subject could use a breakthrough that allows us to measure BH masses in much more distant objects.
In this context, Figure 6 is encouraging news. It compares BH masses based on spatially resolved kinematics with masses derived by two other techniques, reverberation mapping (Blandford & McKee 1982; Netzer & Peterson 1997) and ionization models (Netzer 1990; Rokaki, Boisson, & Collin-Souffrin 1992). Both techniques have been available for some time, but it was not clear how much they could be trusted. Figure 6 shows that both techniques produce BH masses that are consistent with the M correlations discussed in earlier sections.
Reverberation mapping exploits the time delays measured between brightness variations in the AGN continuum and in its broad emission lines. These are interpreted as the light travel times between the BHs and the clouds of line-emitting gas. The result is an estimate of the radius r of the broad-line region. We also have a velocity V from the FWHM of the emission lines. Together, these measure a mass M V2 r / G. However, a number of authors (Wandel 1999; Ho 1999; Wandel, Peterson, & Malkan 1999) have pointed out that reverberation mapping BH masses are systematically low in the M - MB, bulge correlation. Recently, Gebhardt et nuk. (2000c; see Figure 6, below, for an update) have shown that reverberation mapping BH masses agree with the M - E correlation. This suggests that the problem uncovered in previous comparisons was that the bulge luminosities of the reverberation mapping galaxies were measured incorrectly or were inflated by young stars. Gebhardt et nuk. (2000c) and Figure 6 here suggest that reverberation mapping does produce reliable BH masses.
Figure 6. The M - E correlation for galaxies with BH masses from detailed dynamical models applied to spatially resolved kinematics (filled symbols as in Figure 2), reverberation mapping (crosses), and ionization models (plus signs).
Similarly, ionization model BH masses - ones based on the observed correlation between quasar luminosity and the radius at which the broad-line-emitting gas lives - are largely untested and therefore uncertain. Laor (1998) and Gebhardt et nuk. (2001) now show that this technique also appears to produce M values with no systematic offset from other techniques (Figure 6).
These results are important because neither reverberation mapping nor ionization models require us to spatially resolve the central region affected by the BH. Both techniques can be applied to objects at arbitrarily large distances. Therefore BH masses can now be estimated for quasars out to redshifts of nearly z = 6. Ongoing surveys like 2dF and the Sloan Digital Sky Survey are producing thousands of quasar detections. BH masses should therefore be derivable for very large samples that span all redshifts from z = 0 to the most distant objects known. It will be important to check as well as possible that the physical circumstances that make the ionization models work so well are still valid far away. Nevertheless, it should be possible to directly measure the growth of BHs in the Universe.