Modeling projection effects in optically-selected cluster catalogues

TL;DR

This paper develops an empirical method to quantify and model projection effects in galaxy cluster catalogues, crucial for accurate mass calibration and cosmological parameter estimation.

Contribution

It introduces a robust empirical approach validated with simulations to account for projection effects in cluster abundance studies.

Findings

Modeling projection effects reduces bias in cosmological parameters.

Ignoring projection effects biases the S8 parameter by more than twice its uncertainty.

The method is applicable to current and future cluster surveys like DES.

Abstract

The cosmological utility of galaxy cluster catalogues is primarily limited by our ability to calibrate the relation between halo mass and observable mass proxies such as cluster richness, X-ray luminosity or the Sunyaev-Zeldovich signal. Projection effects are a particularly pernicious systematic effect that can impact observable mass proxies; structure along the line of sight can both bias and increase the scatter of the observable mass proxies used in cluster abundance studies. In this work, we develop an empirical method to characterize the impact of projection effects on redMaPPer cluster catalogues. We use numerical simulations to validate our method and illustrate its robustness. We demonstrate that modeling of projection effects is a necessary component for cluster abundance studies capable of reaching $\approx 5\%$ mass calibration uncertainties (e.g. the Dark Energy Survey Year 1 sample). Specifically, ignoring the impact of projection effects in the observable--mass relation --- i.e. marginalizing over a log-normal model only --- biases the posterior of the cluster normalization condition $S_8 \equiv \sigma_8 (\Omega_{\rm m}/0.3)^{1/2}$ by $\Delta S_8 =0.05$, more than twice the uncertainty in the posterior for such an analysis.