Annu. Rev. Astron. Astrophys. 1994. 32: 115-52 Copyright © 1994 by Annual Reviews. All rights reserved

Optical Luminosity

The optical luminosity L_B is a parameter of scale. Like the linear size, the range of median L_B values characteristic of classical galaxies varies only slightly, until the latest types, where the distinctiveness of the dwarfs becomes evident (Figure 2b). The ellipticals here are slightly brighter than spirals.

LUMINOSITY FUNCTION Binggeli et al. (1988) have carefully reviewed what is currently known about the luminosity function (L). In their study of the Virgo cluster (Binggeli et al. 1985), they derive the luminosity function (L, T) for each morphological type separately. The range of luminosities representative of the classical galaxies is seen to be similar, as in Figure 2b, with the later spirals and dwarfs showing a characteristic decrease and being clearly separate. Binggeli et al. find that while the brightest galaxies are ellipticals, the most common galaxies are the dE's. Because of morphological segregation, they conclude that (L, T) cannot be universal. We discuss the effects of morphological segregation in Section 4.

Optical Surface Brightness

Holmberg's survey (1958) included photometric parameters for both magnitude and diameter with which he investigated the surface brightnesses of galaxies and studied the effects of internal extinction. Because internal extinction causes a sytematic change in the observed surface brightness (and color) as a function of inclination, we adopt a definition of surface magnitude that attempts to account for extinction. As mentioned above, the issues of internal extinction are still under significant debate (Disney et al. 1989).

Figure 3a shows the global surface brightness within D₂₅, _B. The distribution of _B is nearly constant for the E, S0 and classical spiral galaxies, but distinguishes clearly the late dwarf categories Sm and Im. We emphasize again that, as discussed by Disney and Phillipps (1983), current catalogs are biased against systems of low surface brightness, and some of the relative constancy across most of the classical galaxy sequence may be due to catalog selection.

Figure 3. Same as Figure 2, for (a) optical (blue) surface brightness B, (b) log FIR surface density FIR, (c) log total mass surface density T, (d) log HI surface density HI. Dashed lines delineate the types with significantly fewer data.

DISK SURFACE BRIGHTNESS AND SCALE LENGTH By examining the available surface photometry, Freeman (1970) found that the face-on central surface brightnesses of most spiral disks are nearly constant, with small scatter: 21.67 ± 0.30 B-mag arcsec^-2. Deviations occur at the ends of the spiral sequence among the S0's on the one side and dwarfs on the other. Surprisingly, most E's also seem also to have constant central surface brightness. With the caveat that current catalogs are indeed biased against the low surface brightness systems, most E, S0 and classical spiral galaxies have the same scale length as a function of luminosity regardless of morphology. A thorough discussion of the details of this issue, including the effects of selection bias, is presented in Gilmore et al. (1990).