Secular Evolution in Disk Galaxies: Pseudobulge Growth and the Formation of Spheroidal Galaxies

Updating Kormendy & Kennicutt (2004, ARA&A, 42, 603), we review internal secular evolution of galaxy disks. One consequence is the growth of pseudobulges that often are mistaken for true (merger-built) bulges. Many pseudobulges are recognizable as cold, rapidly rotating, disky structures. Bulges have Sérsic function brightness profiles with index n > ∼ 2 while most pseudobulges have n < ∼ 2. Recognition of pseudobulges makes the biggest problem with cold dark matter galaxy formation more acute: How can hierarchical clustering make so many pure disk galaxies with no evidence for merger-built bulges? E. g., the giant Scd galaxies M101 and NGC 6946 have rotation velocities of Vcirc ∼ 200 km s but nuclear star clusters with velocity dispersions of 25 to 40 km s. Within 8 Mpc of us, 11 of 19 galaxies with Vcirc > 150 km s −1 show no evidence for a classical bulge, one may contain a classical plus a pseudo bulge, and 7 are ellipticals or have classical bulges. So it is hard to understand how bulgeless galaxies could form as the quiescent tail of a distribution of merger histories. Our second theme is environmental secular evolution. We confirm that spheroidal galaxies have fundamental plane correlations that are almost perpendicular to those for bulges and elliptical galaxies. Spheroidals are not dwarf ellipticals. Rather, their structural parameters are similar to those of late-type galaxies. We suggest that spheroidals are defunct late-type galaxies transformed by internal processes such as supernova-driven gas ejection and environmental processes such as secular harassment and ram-pressure stripping. Minus spheroidals, the fundamental plane correlations for ellipticals and bulges have small scatter. With respect to these, pseudobulges are larger and less dense. They fade out by becoming fluffy, not by becoming compact, like nuclei. Pseudobulges and nuclear star clusters appear to have different origins. 1. Internal and Environmental Secular Evolution Internal, slow (secular) evolution of galaxy disks occurs when nonaxisymmetries such as bars and spiral structure redistribute energy and angular momentum and rearrange disk structure. Environmentally driven evolution can also be secular (e. g., galaxy harassment), although better known processes are rapid (mergers). We concentrate on one consequence of environmental secular evolution. It is one of several processes that can transform late-type dwarfs into “spheroidals”, i. e., galaxies that are morphologically similar to ellipticals but that have different structural parameter correlations indicative of different formation physics. Figure 1 puts these galaxy formation processes into a more general context.