Annu. Rev. Astron. Astrophys. 1995. 33: 581-624 Copyright © 1995 by Annual Reviews. All rights reserved

6. BLACK HOLE DEMOGRAPHICS

Kormendy's CFHT BH search is being extended to 14 E-Sb galaxies with M_B -17.5 and distance 10 Mpc. Evidence for MDOs is seen in ~ 20% of these galaxies (M31, NGC 3115, and NGC 3377). Failures often occur where detection is unlikely because of nuclear starbursts or because the galaxy is too face-on. An example (not from the statistical sample) is NGC 1316 (Fornax A). The central dispersion is only 222 ±4 km s^-1 despite an unusually small and dense core (Schweizer 1980; Kormendy 1987b; Fabbiano et al. 1994). The core has M / L_V 1.5, presumably due to an ongoing starburst (Schweizer 1980). In such a case it is impossible to conclude anything about whether there is a BH.

It is no accident that MDOs have been found only in galaxies with favorable conditions; i.e., in edge-on bulges with rapidly rotating nuclear disks and in rotating ellipticals. Despite this, we have already found 10⁸ M of dark matter per L* of galaxy luminosity [where L* = characteristic luminosity of the Schechter 1976 luminosity function]. This is approximately the mass in dead quasars that is predicted by energy arguments (Soltan 1982; Rees 1984; Chokshi & Turner 1992).

With eight detections, we can start to look at demographics. If MDOs are BHs, then Figure 14 shows that BH mass correlates with bulge luminosity. The mean BH mass fraction is <M_BH / M_bulge> = 0.0022^+0.0016_-0.0009 (we averaged log M_BH / M_bulge from Table 1). If we omit the Galaxy, which has the smallest M_BH / M_bulge = 0.00017, then <M_BH / M_bulge> = 0.0032^+0.0018_-0.0012. M33 is consistent with this correlation, but also with the hypothesis that supermassive BHs are found only in bulges.

Figure 14. BH mass as a function of bulge absolute magnitude. The symbols are the same as in Figure 3. As in Figure 3, the correlation may be only the upper envelope of a distribution that extends to smaller MBH.

Figure 15. Correlations with distance D of detected BH mass (left) and BH mass divided by bulge mass (right).

Figure 15 shows the correlations of M_BH and M_BH / M_bulge with distance. It illustrates the selection effects in the search. Not surprisingly, M_BH correlates well with distance. Giant galaxies like M87 and NGC 4594 are rare; if M_BH M_bulge, then we are unlikely to find 10⁹-M BHs nearby. This is why the upper-left part of the diagram is empty. On the other hand, 10⁷-M BHs cannot be detected much outside the Local Group. This ensures that the lower-right part of the diagram will remain empty until we get better spatial resolution.

There is little correlation of M_BH / M_bulge with D. Most BH detections were approximately equally difficult, a point made also by Rix (1993). NGC 3115 has the largest fractional BH mass and is one of the most definitive detections. M31 and the Galaxy have small fractional BH masses; a detection was possible in M31 because the nucleus rotates rapidly and in the Galaxy because the center is so nearby.