Annu. Rev. Astron. Astrophys. 2000. 38: 289-335 Copyright © 2000 by Annual Reviews. All rights reserved

4.3. Galaxy Richness and Optical Luminosity

Most authors have found little or no correlation between X-ray luminosity and the number of luminous galaxies in a group or the total optical luminosity of the group (Ebeling et al 1994, Doe et al 1995, Mulchaey et al 1996a, Ponman et al 1996). The lack of correlation between X-ray luminosity and number of group members is not too surprising because galaxy-galaxy merging is likely prevalent in groups, and thus the number of galaxies in a group is likely not conserved in time (Ponman et al 1996). The fact that there is no relationship between optical and X-ray luminosity is important because it suggests that the X-ray emission is not associated with individual galaxies for most of the samples studied (Ponman et al 1996).

Mahdavi et al (1997) came to a very different conclusion with their RASS survey of optically-selected groups: They found a strong correlation between X-ray luminosity and optical luminosity. The differences between Mahdavi et al's (1997) results and those of other authors suggests that the groups in Mahdavi et al (1997) may be systems dominated by X-ray emission from individual galaxies and not intragroup gas.