Selection effects

The disks of galaxies on the Hubble sequence Sa–Sb–Sc have central surface brightnesses that appear to fall in a rather narrow range (Freeman 1970). Zwicky (1957, p. 113) and Disney (1976) were among the first to emphasize the fact that this might be the result of a selection effect which is due to the difficulty in discovering galaxies of very low surface brightness. The fact that dwarf spheroidal galaxies, which are now known to be the most common type of extragalactic objects, were not discovered until the 1930s (Shapley 1939) supports this notion. It was originally thought (e.g. van den Bergh 1959) that all galaxies with a low surface brightness were early or late-type dwarfs. However, radial velocity observations by Fisher & Tully (1975) showed that some galaxies with low surface brightnesses are actually quite large and luminous. This effect is clearly shown in Figure 24 which compares the surface brightnesses of the disks of normal and of low-luminosity galaxies. An interesting feature of Figure 24 (see also Figure 1 of Bothun, Impey & McGaugh (1997)), which is presently not well understood, is that disk galaxies with surface brightnesses that are significantly higher than those of normal spirals do not appear to exist. This is shown most clearly in Courteau (1996b), who finds that there is a rather well-defined upper cut-off at a red central surface brightness of ∼17.5 mag arcsec−2.