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

8.2. Can Minor Accretion Events Mimic Pseudobulge Growth?

The answer is surely "yes". Kannappan, Jansen, & Barton (2004) find a correlation between blue-centered, star-forming bulges and evidence of tidal encounters with neighboring galaxies. Well known examples are M 82 and NGC 3077, which are connected to M 81 by H I tidal bridges (Yun, Ho, & Lo 1994). Counterrotating gas and even stellar components in some galaxies also imply accretion. An example is NGC 4826 (Braun et al. 1994; Rubin 1994; Walterbos et al. 1994; Rix et al. 1995; Garcia-Burillo et al. 2003). Note, however, that NGC 4826's pseudobulge signature - a very low stellar velocity dispersion (Kormendy 1993) - is a property of corotating stars and therefore predates the accretion of counterrotating material and has not yet been affected by it. Examples of galaxies with pseudobulge characteristics that may instead be caused by gas accretions include NGC 5102 (Section 8.1) and NGC 7457 (Kormendy & Illingworth 1983; Kormendy 1993; Peletier et al. 1999).

There are three reasons why we suggest that secular evolution accounts for more pseudobulges than do accretion events. (1) Many of the most recognizable pseudobulges occur in strongly barred and oval galaxies, especially in ones in which radial dust lanes imply that gas infall is ongoing now. (2) If galaxies approach each other closely enough to transfer gas, then their dark matter halos are likely already to overlap and they are likely to merge after a few more orbits. A configuration like the M 81 - M 82 - NGC 3077 encounter can last for a billion years but not for a significant fraction of a Hubble time and not at all without being recognizable. Most pseudobulge galaxies show no signs of tidal interactions in progress. (3) Inhaling a tiny, gas-rich dwarf does no damage to an existing disk, but a major merger heats a thin disk too much to be consistent with flat, edge-on galaxies.

Nevertheless, the relative importance of internal and externally-driven secular evolution is not known and needs further study. It is likely that accretions create more than an occasional quasipseudobulge.