Annu. Rev. Astron. Astrophys. 2004. 42: 603-683 Copyright © 2004 by Annual Reviews. All rights reserved

7.2. Pseudobulges Do Not Have To Be Young

Most of this paper emphasizes evolution in progress, because this is the easiest way to see that evolution is happening at all. However, we do not mean to create the mistaken impression that pseudobulges must be young or that they must be made of young stars.

Secular evolution is by definition slower than nonequilibrium processes such as mergers, but it can have timescales that are much shorter than a Hubble time. Rates are uncertain, but evolution is thought to be possible on timescales as short as ~ 5 galactic rotations. Bars started to be abundant at least 5 Gyr ago (Abraham et al. 1999; van den Bergh et al. 2002; van den Bergh 2002). Therefore it is possible that secular evolution built some pseudobulges quickly 5 Gyr ago and then stopped.

Also, heating by bars can elevate disk stars to scale heights characteristic of pseudobulges. These stars can be as old as the oldest disks.

So we expect that pseudobulges have a range of stellar population ages from nearly zero to at least 5 Gyr (e.g., Bouwens, Cayón, & Silk (1999). Much older stellar populations are not out of the question; how much older is plausible is not known. The signs from population studies are mixed. Some support a continuity from classical bulges in early-type galaxies to pseudobulges in late-type galaxies. Others point to a possible collision with the secular evolution picture. We defer a review to Section 8.1.