The impact of projection effects on cluster observables: stacked lensing and projected clustering

Abstract

An optical cluster finder inevitably suffers from projection effects, where it misidentifies a superposition of galaxies in multiple haloes along the line of sight as a single cluster. Using mock cluster catalogues built from cosmological N-body simulations, we quantify the impact of these projection effects with a particular focus on the observables of interest for cluster cosmology, namely the cluster-lensing and the cluster-clustering signals. We find that ‘observed’ clusters, i.e. clusters identified by our cluster finder algorithm, exhibit lensing and clustering signals that deviate from expectations based on a statistically isotropic halo model – while both signals agree with halo model expectations on small scales, they show unexpected boosts on large scales by up to a factor of 1.2 or 1.4, respectively. We identify the origin of these boosts as the inherent selection bias of optical cluster finders for clusters embedded within filaments aligned with the line of sight and show that a minority ($\sim 30{{\ \rm per\ cent}}$) of such clusters within the entire sample is responsible for this observed boost. We discuss the implications of our results on previous studies of optical cluster, as well as prospects for identifying and mitigating projection effects in future cluster cosmology analyses.

Dark Energy Survey Year 1 Results: Cosmological constraints from cluster abundances and weak lensing

“…During the finalization of this analysis similar findings have been presented in the work of [97] To account for this effect in our mass estimates we refit the stacked weak lensing data following the same methodology detailed in [15], but multiplying the projected mass density profile model [ΣðRÞ, Eq. 28 of [15] ] by the selection effect bias correction relevant for the bin considered: B Sel:Eff: ðRÞ ¼ΣðRÞ λ−Sel =ΣðRÞ RND−Sel prior to computing the predicted ΔΣðRÞ.…”

Dark Energy Survey Year 1 Results: Cosmological Constraints from Cluster Abundances, Weak Lensing, and Galaxy Correlations

“…Systematics of redMaPPer clusters. Photometrically selected galaxy clusters are subject to two important systematics: projection effects [17,51,52] and orientation biases [17,53]. These two systematics bias the observed galaxy and matter overdensities of the selected galaxy clusters relative to randomly selected halos of the same mass.…”

The effect of selection -- a tale of cluster mass measurement bias induced by correlation and projection

“…Specifically, the analysis in Abbott et al (2020) shows that the weak-lensing measured masses of galaxy clusters appear to have deviated from their model values significantly, which, in the end, affect cosmological parameters derived from modeling the cluster mass distribution and abundance. Abbott et al (2020) and further Sunayama et al (2020) find that the bias of cluster weak-lensing measurements likely originate from cluster selection. Their analyses based on simulations have demonstrated that galaxy clusters selected by their richness observable, defined as a weighted number count of ★ E-mail: ynzhang@tamu.edu red sequence cluster galaxies, is a biased population when compared to a unbiased population selected by their unbiased truth quantities.…”