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

4.8. Pseudobulges and the Fundamental Plane Correlations

If pseudobulges have Sérsic indices n ~ 1 that are smaller than those of elliptical galaxies (n ~ 4), then this signals a breakdown in the homology that, together with the Virial theorem, is the reason why classical bulges and ellipticals satisfy the Fundamental Plane (FP) correlations (e.g., Djorgovski & Davis 1987; Dressler et al. 1987; Faber et al. 1987; Lauer 1987; Djorgovski, de Carvalho, & Han 1988; Kormendy & Djorgovski 1989; Bender, Burstein, & Faber 1992, 1993; Burstein et al. 1997). That is, bulges and ellipticals lie in an inclined plane, R ^{1.4 ± 0.15} I^{-0.9 ± 0.1} in the space of observed radius R, surface brightess I, and velocity dispersion . The scatter of ellipticals around the FP is small (see the above papers and Saglia, Bender, & Dressler 1993). Therefore deviations from the FP are a sensitive test of whether the structure of a bulge-like component is or is not similar to that of an elliptical. Carollo (1999) finds that pseudobulges deviate from the FP of classical bulges and elliptical galaxies in the direction of having lower densities (see also Kent 1985; Andredakis, Peletier, & Balcells 1995). They lie closer to the locus of disks than to that of hot stellar systems. Similar studies of larger samples may provide an additional, quantitative way to recognize pseudobulges.