5. MOVING FORWARD: OPEN ISSUES

5.1. What More Can Be Learned about the M - Relation?

With so much progress in the past few years, it is only natural to be optimistic about what the near future might bring. Indeed, a considerable amount of effort will be devoted to the study of supermassive black holes in nearby galaxies, with HST remaining the instrument of choice for the investigation. Roughly 130 galaxies have, or will be, observed with HST/STIS within the next year. While only a fraction of these observations are likely to lead to secure SBH detections (Merritt & Ferrarese 2001c), these results are highly anticipated, and will help to better define the slope and scatter of the M - relation.

Nevertheless, one important section of parameter space will remain unexplored. Now that the existence of SBHs is as well established as that of the galaxies in which they reside, the most pressing need has become, in my opinion, an exploration of the low mass end of the M - relation. However, the vast majority of the galaxies in the HST pipeline are expected to host SBHs with M ~ 10⁸ M, a range already well-sampled by the current data. None of the ongoing programs is likely to measure a SBH of M < 10⁷ M (Merritt & Ferrarese 2001c).

This is unfortunate since determining how far the M - relation extends is key for discriminating between different scenarios for the formation of SBHs. The smallest nuclear SBHs whose masses have been established dynamically are in the Milky Way (Genzel et al. 2000) and M32 (Joseph et al. 2001), both with M 3 × 10⁶ M (Fig. 2). Evidence for black holes with 10³ < M < 10⁶ M (dubbed "intermediate" mass black holes, or IBHs) is so far circumstantial, the most likely candidates being the super-luminous off-nuclear X-ray sources (ULXs) detected by Chandra in a number of starburst galaxies (Fabbiano et al. 2001; Matsumoto et al. 2001).

The link between IBHs and SBHs is unclear. If Chandra's off-nuclear ULXs are indeed IBHs, they could sink slowly to the galaxy center through dynamical friction and provide the seeds for nuclear SBHs (Ebisuzaki et al. 2001). Or the latter might be born in situ, through collapse of a protogalactic cloud, possibly before the onset of star formation in the bulge (Loeb 1993; Silk & Rees 1998; Haehnelt, Natarajan & Rees 1997). Deciding between these and competing formation scenarios will undoubtedly keep theorists busy for many years. However, different theories would almost certainly make different predictions about the form of the M - relation, and this is the most promising route for distinguishing between them. For instance, in situ formation in nuclei is unlikely to result in black holes less massive than ~ 10⁶ M (e.g Haehnelt, Natarajan & Rees 1998), while accumulation of IBHs would probably not result in as tight a correlation between M and unless some additional feedback mechanism were invoked (e.g. Burkert & Silk 2001). But little progress is likely to be made until we know whether IBHs are present in galaxy nuclei and if so, where they lie relative to the M - relation defined by SBHs. Therefore, exploring the M - relation in the M < 10⁶ M range will be an important challenge in the years to come.

A first step in this direction has been taken recently with the derivation of an upper limit, of a few thousand solar masses, for the putative black hole inhabiting the nucleus of the nearby spiral M33 (Merritt, Ferrarese & Joseph 2001; Gebhardt et al. 2001; Valluri et al. 2002). As small as this upper limit might seem, it is still consistent with the M - relation as characterized in this paper, when extrapolated (by three orders of magnitude!) to the thousand solar mass range. Unfortunately, until the next technological leap, there is little hope of significantly tightening this upper limit: at the distance of M33, the black hole's sphere of influence is well below (by at least an order of magnitude) the resolution capabilities of HST. Indeed, with one notable exception, there are no galaxies expected to contain a black hole below the 10⁶ M mark that are close enough, and have high enough central surface brightness, to allow HST to measure M. The one exception, the Local Group spheroidal galaxy NGC 205, is scheduled to be observed by HST as part of program 9448 (P.I. L. Ferrarese). NGC 205 is expected to host a ~ 7.5 × 10⁵ M black hole; at a distance of 740 kpc, a black hole as small as 6 × 10⁵ M can be detected. Even so, it seems inevitable that, to fully characterize the low mass range of the M - relation, we must look beyond HST.

In my opinion, the answer is reverberation mapping. Although the obvious drawback is that it is only applicable to the 1% of galaxies with Type 1 AGNs, reverberation mapping is intrinsically unbiased with respect to black hole mass, provided the galaxies can be monitored with the appropriate time resolution. Furthermore, reverberation mapping can probe galaxies at high redshifts and with a wide range of nuclear activity, opening an avenue for the exploration of possible dependences of the M - relation on cosmic time and activity level.

The stage is being set to embark upon this new endeavor. In the past few years, the reliability of reverberation-mapping masses has been called into question on both observational (e.g. Ho 1999; Richstone et al. 1998) and theoretical (Krolik 2001) grounds. However, on the observational side, the doubts appear to be dissipating. The observation that SBHs in AGNs appeared to be undermassive, by a factor ~ 50, compared to SBHs in quiescent galaxies (Wandel 1999), was apparently the result of two erroneous assumptions: the overestimate (by a factor ~ six) of SBH masses in quiescent galaxies derived from the M - M_B relation of Magorrian et al. (1998); and an overestimate of the AGN host bulge magnitudes (by up to ~ 3.5 mag) adopted by Wandel (McLure & Dunlop 2000; Merritt & Ferrarese 2001c; Wandel 2002). Indeed, Merritt & Ferrarese (2001c) conclude that the ratio of SBH to bulge mass in Seyfert, QSO and quiescent galaxy samples are all consistent: <M / M_bulge > = 0.09% (QSOs) and 0.12% (Seyferts), < M / M_bulge > = 0.13% for quiescent galaxies.

On the theoretical side, Krolik (2001) argues that the unknown BLR geometry, radial emissivity distribution, and angular radiation pattern of the line emission, coupled with the often less than optimal temporal sampling of the data, can lead to systematic errors in the reverberation masses of a factor ~ three or more. While there is little doubt that Krolik's objections are all well-justified, my collaborators and I have taken an observational approach to this issue. Since there are no independent measurements of M for any of the reverberation-mapped AGNs, we have opted for an indirect comparison by placing these galaxies onto the M - plane. Initial results (Ferrarese et al. 2001) suggest that the AGN sample follows the same M - relation as the quiescent galaxies on which the relation is defined. More secure conclusions should be reached within the next year, once the AGN sample is doubled (Pogge et al. 2002). At the moment, the evidence suggests that reverberation mapping works, in spite of the theoretically motivated concerns.