Secular Evolution and the Growth of Pseudobulges in Disk Galaxies

Galactic evolution is in transition from an early universe dominated by hierarchical clustering to a future dominated by secular processes. These result from interactions involving collective phenomena such as bars, oval disks, spiral structure, and triaxial dark halos. A detailed review is in Kormendy & Kennicutt (2004). This paper provides a summary illustrated in part with different galaxies. Figure 2 summarizes how bars rearrange disk gas into outer rings, inner rings, and galactic centers, where high gas densities feed starbursts. Consistent with this picture, many barred and oval galaxies are observed to have dense central concentrations of gas and star formation. Measurements of star formation rates show that bulge-like stellar densities are constructed on timescales of a few billion years. We conclude that secular evolution builds dense central components in disk galaxies that look like classical – that is, merger-built – bulges but that were made slowly out of disk gas. We call these pseudobulges. Many pseudobulges can be recognized because they have characteristics of disks – (1) flatter shapes than those of classical bulges, (2) correspondingly large ratios of ordered to random velocities, (3) small velocity dispersions σ with respect to the Faber-Jackson correlation between σ and bulge luminosity, (4) spiral structure or nuclear bars, (5) nearly exponential brightness profiles, and (6) starbursts. These structures occur preferentially in barred and oval galaxies in which secular evolution should be most rapid. Thus a variety of observational and theoretical results contribute to a new paradigm of secular evolution that complements hierarchical clustering.