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

4.7. Velocity Dispersions and the Faber-Jackson Relation

Figure 18 correlates velocity dispersion and absolute magnitude for elliptical galaxies and for bulges of disk galaxies. Most early-type galaxy bulges are consistent with the well known correlation that bulge luminosity L_B ⁴ (Faber & Jackson 1976; see Minkowski 1962 for an early hint). But a few early-type bulges and a large majority of Sbc - Sm "bulges" fall well below the correlation. One possible interpretation is that the bulges of late-type galaxies are low in velocity dispersion. This is disky behavior. Alternatively, the late-type "bulges" may lie to the right of the early-type systems because they are actively forming stars and therefore have small mass-to-light ratios. Figure 18 then implies that most late-type bulges have young stellar population ages. This means that the star formation must be secular, not episodic. In either case, Figure 18 suggests that most late-type galaxies contain pseudobulges. In contrast, most early-type bulges are similar to elliptical galaxies.

Figure 18. Correlation between central velocity dispersion and bulge absolute magnitude for all galaxies of the indicated Hubble types that have velocity dispersions tabulated in Hypercat. The straight line is a least-squares fit to the ellipticals. Updated from Kormendy & Illingworth (1983), this figure is from Kormendy & Cornell (2004).