Large and small elliptical galaxies

Figure 1 shows the local optical luminosity function for elliptical and S0 galaxies (Sadler 1982). Although galaxies with (B-band) absolute magnitudes fainter than M = -18 (for H = 100 km s Mpc) make up only about 10% of most optically-selected samples of elliptical galaxies, their true space density is higher than that of the giant ellipticals.

Several properties of elliptical galaxies appear to change at an absolute magnitude around M Large elliptical galaxies (defined here as those with M brighter than -19) are usually dominated by an old stellar population (e.g. Bressan et al. 1996), and have most of their gas in a hot X-ray corona (Forman et al. 1985; Canizares et al. 1987). Small elliptical galaxies (M fainter than -18) are less well-studied, partly because they are severely under-represented in magnitude-limited galaxy catalogues, but there are strong hints that many of them contain significant amounts of HI (Lake & Schommer 1984) and are still forming stars (Phillips et al. 1986). There are other differences too -- active nuclei are common in the large ellipticals and rare or non-existent in the small ones (e.g. Sadler & Slee 1994); and most large ellipticals have little or no stellar rotation, while many small ones are rotationally flattened (Davies et al. 1983). Since there are many qualitative differences between large and small ellipticals, it is intriguing that their photometric properties define a continuous family. It is usually impossible to tell, based on the light profile alone, whether an individual elliptical galaxy is distant and luminous or nearby and small.

 
Figure 1: The local optical luminosity function for a combined sample of elliptical and S0 galaxies (points), and for elliptical galaxies alone (solid line). Since elliptical galaxies with dust lanes or other `peculiar' features are often classified as S0s, the true space density of ellipticals probably lies somewhere between the two sets of points. The faint end of the luminosity function is not well defined for field galaxies, but studies of nearby clusters (Binggeli et al. 1988) suggest that it turns over near M.

© Copyright Astronomical Society of Australia 1997