The Mass-Richness Relation of MaxBCG Clusters from Quasar Lensing Magnification using Variability

TL;DR

This study measures the mass-richness relation of MaxBCG galaxy clusters using quasar lensing magnification via variability, confirming high mass normalization consistent with shear measurements and addressing discrepancies with other methods.

Contribution

It introduces a magnification-based method using quasar variability to independently measure cluster masses, validating previous shear-based results.

Findings

Magnification measurements agree with shear-based mass normalization.

Line-of-sight orientation affects individual cluster mass estimates.

Minority of clusters with specific orientations do not bias overall relation.

Abstract

Accurate measurement of galaxy cluster masses is an essential component not only in studies of cluster physics, but also for probes of cosmology. However, different mass measurement techniques frequently yield discrepant results. The SDSS MaxBCG catalog's mass-richness relation has previously been constrained using weak lensing shear, Sunyaev-Zeldovich (SZ), and X-ray measurements. The mass normalization of the clusters as measured by weak lensing shear is >~25% higher than that measured using SZ and X-ray methods, a difference much larger than the stated measurement errors in the analyses. We constrain the mass-richness relation of the MaxBCG galaxy cluster catalog by measuring the gravitational lensing magnification of type I quasars in the background of the clusters. The magnification is determined using the quasars' variability and the correlation between quasars' variability amplitude and intrinsic luminosity. The mass-richness relation determined through magnification is in agreement with that measured using shear, confirming that the lensing strength of the clusters implies a high mass normalization, and that the discrepancy with other methods is not due to a shear-related systematic measurement error. We study the dependence of the measured mass normalization on the cluster halo orientation. As expected, line-of-sight clusters yield a higher normalization; however, this minority of haloes does not significantly bias the average mass-richness relation of the catalog.