Mind the gap: tightening the mass–richness relation with magnitude gaps

We investigate the potential to improve optical tracers of cluster mass by exploiting measurements of the magnitude gap, m 12 , defined as the difference between the r-band absolute magnitudes of the two brightest cluster members. We find that in a mock sample of galaxy groups and clusters constructed from the Bolshoi simulation, the scatter about the massrichness relation decreases by 15-20 per cent when magnitude gap information is included. A similar trend is evident in a volume-limited, spectroscopic sample of galaxy groups observed in the Sloan Digital Sky Survey (SDSS). We find that SDSS groups with small magnitude gaps are richer than large-gap groups at fixed values of the one-dimensional velocity dispersion among group members v , which we use as a mass proxy. We demonstrate explicitly that m 12 contains information about cluster mass that supplements the information provided by group richness and the luminosity of the brightest cluster galaxy, L BCG . In so doing, we show that the luminosities of the members of a group with richness N are inconsistent with the distribution of luminosities that results from N random draws from the global galaxy luminosity function. As the cosmological constraining power of galaxy clusters is limited by the precision in cluster mass determination, our findings suggest a new way to improve the cosmological constraints derived from galaxy clusters.