A unifying hypothesis for the origin of bulgelike disks emerges from discussions by a number of authors (e.g., van den Bergh 1976a; Kormendy 1982a, b, 1988b; Gallagher, Goad, and Mould 1982; O'Connell 1983; Kormendy and Illingworth 1983; Kormendy and Djorgovski 1989; Kormendy and McClure 1993). The suggestion is that the central concentration of galaxy disks can be increased dramatically by inward gas transport and subsequent star formation. Bars and oval disks are particularly efficient engines for this process (see Kormendy 1982a; Prendergast 1983; Combes 1991 for reviews).
This idea also is a natural consequence of the hypothesis that nuclear activity in galaxies results when black holes are fueled by infalling gas. If gas can reach the black hole, it may form stars along the way when the density gets high enough in the gravitational funnel. This may even be a necessary step in the formation of nuclear black holes, since core collapse times in giant ellipticals are long, while nuclei can evolve more rapidly (Kormendy 1988c).
Shlosman and Begelman (1987, 1989) and Shlosman, Frank, and Begelman (1989) take this idea one step further and suggest that hierarchical bar formation through inward gas transport also solves the problem of how to get fuel to small radii where the main bar no longer affects the gravitational potential. They suggest that a bar transports gas inward to radii much smaller than its length; at this point the central concentration has increased enough to make the gas self-gravitating; it becomes unstable to the formation of a new and shorter bar, and the process repeats itself with residual gas. It is not clear that all of this actually happens, nor is it clear that this is the dominant transport process (contrast Gunn 1979). But it is interesting to note that the nuclear bars discussed in the previous section are predicted by the Shlosman and Begelman mechanism. Space Telescope observations should tell us whether there exist additional levels of bar-within-bar hierarchy.
There has recently been a resurgence of theoretical work on the building of nuclear disks and bulges by secular processes. For example, Hasan and Norman (1990), Norman and Hasan (1990), and Hasan, Pfenniger, and Norman (1993) point out that bars may be destroyed when the central mass concentration is increased sufficiently. Related papers include Duschl (1988a, b) and Pfenniger and Norman (1990). Many papers at this meeting also discuss aspects of this picture (Gerhard 1993; Hasan and Norman 1993; Sellwood 1993; Wada and Habe 1993).