Figure 5 shows fundamental plane projections (Djorgovski & Davis 1987; Faber et al. 1987; Djorgovski et al. 1988) and Sérsic index versus total magnitude for various kinds of stellar systems (adapted from Kormendy et al. 2008: KFCB).
Figure 5. Global parameter correlations for pseudobulges (blue), classical bulges (brown), ellipticals (red), and spheroidal galaxies (green). Pseudobulge and most bulge points are from Fisher & Drory (2007a). The ellipticals, five bulge points, and the green squares are from Kormendy et al. (2008: KFCB). Green triangles show all spheroidals from Ferrarese et al. (2006) that are not in KFCB. Crosses show all spheroidals from Gavazzi et al. (2005) that are not in KFCB or in Ferrarese et al. (2006). Open squares are Local Group spheroidals (Mateo 1998; McConnachie & Irwin 2006). The bottom panels show major-axis Sérsic index n and effective surface brightness µe versus total galaxy absolute magnitude. The top panel shows µe vs. effective radius re (the Kormendy 1977 relation, which shows the fundamental plane almost edge-on).
Critical to the interpretation of this figure is high-accuracy photometry of all known E and selected Sph galaxies in the Virgo cluster from KFCB. Composite HST and ground-based profiles over large radius ranges provide accurate Sérsic parameters. Then the intrinsically small scatter of the fundamental plane (Saglia et al. 1993; Jørgensen et al. 1996) is seen in the top panel, which shows the plane almost edge-on. Figure 5 confirms the results of Kormendy (1985, 1987), Binggeli & Cameron (1991), and Bender, Burstein, & Faber (1992) that E and Sph galaxies satisfy different parameter correlations. This result has been criticized by Jerjen & Binggeli (1997), Graham & Guzmán (2003), Gavazzi et al. (2005), and Ferrarese et al. (2006) in part because the n - MV correlation is continuous. We agree. But the observation that n is not sensitive to the difference between E and Sph galaxies does not mean that they are related. The fundamental plane correlations (top panels and Figure 6) show that lower-luminosity Es are monotonically higher in density, whereas lower-luminosity Sphs are monotonically lower in density. Spheroidals are not faint ellipticals. Instead, Kormendy (1985, 1987) showed that they have similar parameter correlations to dwarf spiral and irregular galaxies. Spheroidals and ellipticals almost certainly had very different formation processes. We believe that Es formed via major galaxy mergers. Evidence discussed in KFCB suggests that spheroidal galaxies are defunct late-type galaxies transformed by internal processes such as supernova-driven gas ejection (Dekel & Silk 1986) and environmental processes such as secular galaxy harassment (Moore et al. 1996, 1998) and ram-pressure gas stripping (e. g., Chung et al. 2008).