Mind the Gap: Tightening the Mass-Richness Relation with Magnitude Gaps

TL;DR

This paper demonstrates that incorporating the magnitude gap between the two brightest cluster members significantly reduces the scatter in the mass-richness relation, thereby improving galaxy cluster mass estimates for cosmological studies.

Contribution

It introduces the use of the magnitude gap as an additional mass proxy, showing its effectiveness in both simulated and observed galaxy cluster samples.

Findings

Including magnitude gap reduces scatter in mass-richness relation by 15-20%.

Small gap groups are richer at fixed velocity dispersion.

Magnitude gap provides supplementary information beyond traditional proxies.

Abstract

We investigate the potential to improve optical tracers of cluster mass by exploiting measurements of the magnitude gap, m12, defined as the difference between the r-band absolute magnitude 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% 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 sigma_v, which we use as a mass proxy. We demonstrate explicitly that m12 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.