6. THE M - M_B,bulge CORRELATION. II.
BULGES VERSUS PSEUDOBULGES

So far, we have discussed elliptical galaxies and the bulges of disk galaxies as if they were equivalent. In terms of BH content, they are indistinguishable: they are consistent with the same M - M_{B, bulge} and M - _E correlations. But a variety of observational and theoretical results show that there are two different kinds of high-density central components in disk galaxies. Both have steep surface brightness profiles. But, while classical bulges in (mostly) early-type galaxies are like little ellipticals living in the middle of a disk, the "pseudobulges" of (mostly) late-type galaxies are physically unrelated to ellipticals.

Pseudobulges are reviewed in Kormendy (1993b). Observational evidence for disklike dynamics includes (i) velocity dispersions that are smaller than those predicted by the Faber-Jackson (1976) - M_B correlation, (ii) rapid rotation V that implies V / values above the "oblate line" describing rotationally flattened, isotropic spheroids in the V / - ellipticity diagram, and (iii) spiral structure that dominates the pseudobulge part of the galaxy. These observations and n-body simulations imply that high-density central disks can form out of disk gas that is transported toward the center by bars and oval distortions. They heat themselves, e.g. by scattering of stars off of bars (Pfenniger & Norman 1990). The observations imply that most early-type galaxies contain bulges, that later-type galaxies tend to contain pseudobulges, and that only pseudobulges are seen in Sc - Sm galaxies.

Andredakis & Sanders (1994), Andredakis, Peletier, & Balcells (1995), and Courteau, de Jong, & Broeils (1996) show that the "bulges" of many late-type galaxies have nearly exponential surface brightness profiles. It is likely that these profiles are a signature of pseudobulges, especially since blue colors imply that they are younger than classical bulges (Balcells & Peletier 1994).

HST observations strengthen the evidence for pseudobulges. Carollo et al. (1997, 1998a, b) find that many bulges have disk-like properties, including young stars, spiral structure, central bars, and exponential brightness profiles. It seems safe to say that no-one who saw these would suggest that they are mini-ellipticals living in the middle of a disk. They look more like late-type or irregular galaxies. To be sure, Peletier et al. (2000) find that bulges of early-type galaxies generally have red colors: they are old. True bulges that are similar to elliptical galaxies do exist; M31 and NGC 4594 contain examples. But the lesson from the Carollo papers is that pseudobulges are more important than we expected. Like Kormendy (1993b) and Courteau et al. (1996), Carollo and collaborators argue that these are not real bulges but instead are formed via gas inflow in disks.

So there is growing evidence that the "bulges" in Fig. 2 are two different kinds of objects. Classical bulges are thought to form like ellipticals, in a dissipative collapse triggered by a merger. Pseudobulges are thought to form by secular evolution in disks. In both cases, gas flows inward and may feed BHs. One way to explore this is to ask whether bulges and pseudobulges have the same BH content.

The answer is shown in Figure 5. Pseudobulges have relatively low luminosities; this is plausible, since they are made from disks. But for their low luminosities, they have normal BH masses. The indentification of pseudobulges is still somewhat uncertain, and only a few have been observed. It will be important to check our result with a larger sample. However, it is consistent with the hypothesis that (pseudo)bulge formation and BH feeding are closely connected. Present data do not show any dependence of M on the details of whether BH feeding happens rapidly during a collapse or slowly via secular evolution of the disk.

Figure 5. The M - MB, bulge (left) and M - E (right) correlations for elliptical galaxies (filled circles), bulges (open circles) and pseudobulges (stars).

The smallest BHs provide an argument that most BH growth did not happen after bulge formation. Some pseudobulges are still forming now; there is little time after bulge formation. Also, these objects do not contain fuel in the form of x-ray gas. And galaxies like M32 contain little gas of any sort for late accretion.