Copyright © 2004 by Annual Reviews. All rights reserved
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| Annu. Rev. Astron. Astrophys. 2004. 42:
603-683 Copyright © 2004 by Annual Reviews. All rights reserved |
The suggestion that some "bulges" were built by the secular processes discussed in Sections 2 and 3 was first made by Kormendy (1982a, b). A decade later, both the evidence for evolution and the case that it can construct what we now call pseudobulges had grown substantially (Kormendy 1993). Now, after another decade, it is a struggle to review the wealth of new evidence in a single ARA&A article.
Other early papers that focused on the building of "bulges" by bars include Combes & Sanders (1981) and Pfenniger & Norman (1990). Two processes were discussed. One is the inward transport of gas by bars and ovals. The other involves dissipationless processes that can produce vertically thickened central components when bars suffer buckling instabilities and when disk stars scatter off of bars and are heated in the axial direction. Both processes can happen in the same galaxy and both make bulge-like components out of disk material. Therefore we refer to the products of both processes as pseudobulges. In this section, we discuss the observed properties of pseudobulges. As discussed in Section 1.1, we need the context of the above formation mechanisms to make sense of the wealth (or plague) of detail in galactic centers.
How can we tell whether a "bulge" is like an elliptical or whether it formed secularly? The answer - and the theme of this section - is that pseudobulges retain enough memory of their disky origin so that the best examples are easily recognizable. In the pre-HST era reviewed by Kormendy (1993), the cleanest evidence was dynamical. Pseudobulges are more dominated by rotation and less dominated by random motions than are classical bulges and ellipticals. This evidence remains compelling (Sections 4.6 and 4.7). However, as a result of spectacular progress from HST imaging surveys, morphology and surface photometry now provide the best evidence for disk-like "bulges". We begin with these surveys.