Secular Evolution and the Formation of Pseudobulges in Disk Galaxies

Abstract

▪ Abstract  The Universe is in transition. At early times, galactic evolution was dominated by hierarchical clustering and merging, processes that are violent and rapid. In the far future, evolution will mostly be secular—the slow rearrangement of energy and mass that results from interactions involving collective phenomena such as bars, oval disks, spiral structure, and triaxial dark halos. Both processes are important now. This review discusses internal secular evolution, concentrating on one important consequence, the buildup of dense central components in disk galaxies that look like classical, merger-built bulges but that were made slowly out of disk gas. We call these pseudobulges. We begin with an “existence proof”—a review of how bars rearrange disk gas into outer rings, inner rings, and stuff dumped onto the center. The results of numerical simulations correspond closely to the morphology of barred galaxies. In the simulations, gas is transported to small radii, where it reaches high densities and plausibly feeds star formation. In the observations, many barred and oval galaxies have dense central concentrations of gas and star formation. Optical colors and spectra often imply young stellar populations. So the formation of pseudobulges is well supported by theory and observations. It is embedded in a broader evolution picture that accounts for much of the richness observed in galaxy structure. If secular processes built dense central components that masquerade as bulges, how can we distinguish them from merger-built bulges? Observations show that pseudobulges retain a memory of their disky origin. That is, they have one or more characteristics of disks: (a) flatter shapes than those of classical bulges, (b) correspondingly large ratios of ordered to random velocities, (c) small velocity dispersions σ with respect to the Faber-Jackson correlation between σ and bulge lumi nosity, (d) spiral structure or nuclear bars in the “bulge” part of the light profile, (e) nearly exponential brightness profiles, and ( f ) starbursts. All these structures occur preferentially in barred and oval galaxies, where secular evolution should be most rapid. So the cleanest examples of pseudobulges are recognizable. Are their formation timescales plausible? We use measurements of central gas densities and star-formation rates to show that pseudobulges of the observed densities form on timescales of a few billion years. Thus a large variety of observational and theoretical results lead to a new picture of galaxy evolution that complements hierarchical clustering and merging. Secular evolution consists of more than the aging of stellar populations. Every galaxy is dynamically evolving.