Annu. Rev. Astron. Astrophys. 1988. 26: 509-560 Copyright © 1988 by Annual Reviews. All rights reserved

5.1. E and S0 Galaxies

The LFs of both E and S0 galaxies have a maximum and decrease toward zero at a magnitude brighter than M_BT ~ -15 (see Figure 1). The LF of E galaxies is skewed toward faint magnitudes, being fairly shallow at the bright end but with a sharp cutoff at the faint end. Just the opposite holds for the LF of S0 galaxies, which is skewed toward bright magnitudes, steeply rising at the bright end and shallow at the faint end. This characteristic behavior at the bright end (S0s being steeper than Es) has not only been observed in the Virgo cluster but also in the Coma cluster (Thompson & Gregory 1980) and in the field (Sandage et al. 1979, their Figure 1). As a consequence, E galaxies can reach brighter luminosities than S0s, a fact already noted by van den Bergh & McClure (1979). The brightest E galaxies in the RSA sample reach M_BT ~ -23, while the brightest S0s reach M_BT ~ -22 (TYS, their Figure 4; Kraan-Korteweg et al. 1984). The very brightest Es with extended envelopes are separated, following Matthews et al. (1964), into a morphologically distinct class called cD type.

The faint end of (M) for S0 galaxies in the region that goes to zero has so far only been observed in the Virgo cluster (SBT). For E galaxies it has additionally been observed in the Fornax cluster (Caldwell 1987, following Wirth & Gallagher 1984). The surprisingly sharp cutoff of the LF of Es at M_BT ~ -16 in the Virgo cluster has been confirmed by Caldwell (1987) in the Fornax cluster. However, the existence of a faint-end cutoff of the LF of Es is not without difficulties, because the Es must be separated from the dEs in the magnitude range where their LFs overlap. This range is -18 M_BT -16 (see Figure 1). The problem is discussed in the next section.