Internal Secular Evolution in Disk Galaxies: The Growth of Pseudobulges

TL;DR

This paper reviews how internal, secular processes in disk galaxies lead to the formation of pseudobulges, challenging traditional views on galaxy bulge formation and highlighting the importance of non-merger evolution.

Contribution

It emphasizes the fundamental role of secular evolution in galaxy disks and reviews new findings on pseudobulge properties and their implications for galaxy formation theories.

Findings

Pseudobulges form from disk stars and gas, not mergers.

Distinct structural types of pseudobulges identified: boxy and disky.

Secular evolution complicates hierarchical galaxy formation models.

Abstract

Observational and theoretical evidence that internal, slow ("secular") evolution reshapes galaxy disks is reviewed in Kormendy & Kennicutt (2004, ARAA, 42, 603). This update has three aims. First, I emphasize that this evolution is very general -- it is as fundamental to the evolution of galaxy disks as (e.g.) core collapse is to globular clusters, as the production of hot Jupiters is to the evolution of protoplanetary disks, and as evolution to red giants containing proto-white-dwarfs is to stellar evolution. One consequence for disk galaxies is the buildup of dense central components that get mistaken for classical (i.e., merger-built) bulges but that were grown out of disk stars and gas. We call these pseudobulges. Second, I review new results on pseudobulge star formation and structure and on the distinction between boxy and disky pseudobulges. Finally, I highlight how these results make a galaxy formation problem more acute. How can hierarchical clustering produce so many pure disk galaxies with no evidence for merger-built bulges?