|Annu. Rev. Astron. Astrophys. 2004. 42:
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This section discusses issues that complicate the identification of pseudobulges. They do not threaten the conclusion that pseudobulges form by secular evolution of disks; on the contrary, they make it likely that such evolution has operated even in situations where this is not obvious. They highlight areas that need further work.
7.1. Pseudobulges Do Not Have To Be Flat
Several dynamical heating processes are expected to puff pseudobulges up in the axial direction. We are fortunate that some pseudobulges are disky enough so that we can detect the "smoking gun" that points to a secular origin. It is plausible that others are so similar to classical bulges that they cannot easily be recognized.
One heating mechanism that we have already discussed is bar buckling (Section 4.5). If the density profile along the ridge line of the bar is not too steep, then buckled bars produces box-shaped structures that can easily be recognized when they are seen edge-on.
Resonant vertical heating by the bar may also be important (Pfenniger 1984, 1985; Pfenniger & Norman 1990; Friedli 1999). It is not limited to the relatively few stars that are in resonance at any one time, because the radii of all resonances change as the central concentration increases.
Another heating mechanism involves in-the-plane instabilities that result if the nuclear disk gets too dense for its velocity dispersion. Secular gas inflow is slow compared to the disk's rotation velocity and even the disk's radial velocity dispersion r. If, as a result, star formation builds up the disk surface density µ without much changing r, then the disk gets less stable. Toomre (1964) showed that violent instability sets in when Q r / crit 1, where crit = 3.36 Gµ / . Gas inflow increases both the density and the epicyclic frequency, but density wins and crit increases as the evolution proceeds. As Q drops toward 1, instabilities should form and heat the growing pseudobulge in the disk plane. Toomre (1966) showed further that buckling instabilities heat the disk vertically if r gets bigger than about 3.3 times the vertical velocity dispersion even if there is no bar. So heating in the plane results in heating perpendicular to the plane. Because the density increases rapidly toward the center, it is unlikely that the result will look box-shaped. Rather, the thickness of the pseudobulge is likely to be larger at smaller radii, much like in a classical bulge.
Finally, at radii that are smaller than the disk thickness, it is no longer relevant that the incoming gas comes from a disk. There is no reason why the innermost parts of a pseudobulge should be flattened at all.