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

• Published in: Monthly notices of the Royal Astronomical Society 2013-04, Vol.430 (2), p.1238-1246
• Main Authors: Hearin, Andrew P., Zentner, Andrew R., Newman, Jeffrey A., Berlind, Andreas A.
• Format: Article
• Language: English
• Subjects: Astronomy; Cosmology; Luminosity; Simulation; Spectrum analysis; Star & galaxy formation
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/sts699

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 mass-richness 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 σv, 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.