The relative importance of different processes of galactic evolution (Fig. 1) is changing as the universe expands. Rapid processes that happen in discrete events are giving way to slow, ongoing processes. Hierarchical clustering that builds classical bulges is giving way to the secular growth of pseudobulges.
Figure 1. Morphological box (Zwicky 1957) of processes of galactic evolution. Updated from Kormendy (1982a), this figure is from Kormendy & Kennicutt (2004). Processes are divided vertically into fast (top) and slow (bottom). Fast evolution happens on a free-fall timescale, tff ~ (G )-1/2; is the density of the object produced and G is the gravitational constant. Slow means many galaxy rotation periods. Processes are divided horizontally into ones that happen internally in one galaxy (left) and ones that are driven by environmental effects such as galaxy interactions (right). The processes at center are aspects of all types of galaxy evolution. This paper reviews the internal and slow processes at lower-left.
Galactic evolution studies over the past 25 years show convincingly that hierarchical clustering (see White 1997 and Steinmetz 2001 for reviews) and mergers (Toomre 1977a, see Schweizer 1990 for a review) built and continue to build elliptical galaxies and elliptical-like classical bulges of disk galaxies. As the universe expands and as galaxy clusters virialize and acquire large velocity dispersions, mergers get less common (Toomre 1977a; Le Fevre et al. 2000; Conselice et al. 2003). Very flat disks in pure disk galaxies show that at least some galaxies have suffered no major merger violence since disk star formation began (see Freeman 2000 for a review). Therefore there has been time to reshape galaxies via the interactions of individual stars or gas clouds with collective phenomena such as bars, oval distortions, spiral structure, and triaxial dark matter halos. These secular processes are reviewed in Kormendy (1993) and in Kormendy & Kennicutt (2004). This paper provides a summary.