Back to Volume
Paper: Secular Evolution in Disk Galaxies: Pseudobulge Growth and the Formation of Spheroidal Galaxies
Volume: 396, Formation and Evolution of Galaxy Disks
Page: 297
Authors: Kormendy, J.; Fisher, D.B.
Abstract: Updating Kormendy & Kennicutt (2004), 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 M 101 and NGC 6946 have rotation velocities of Vcirc ∼ 200 km s−1 but nuclear star clusters with velocity dispersions of 25 to 40 km s−1. 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.

Back to Volume