The Intrinsic Alignment of Galaxy Clusters and Impact of Projection Effects

TL;DR

This study investigates how projection effects influence the measurement of galaxy cluster intrinsic alignments, revealing that such effects can weaken the observed signals and introduce features like a bump at certain scales, affecting cosmological analyses.

Contribution

It provides a detailed analysis of projection effects on cluster alignment measurements using mock catalogs, highlighting their impact on observed signals and implications for cosmological studies.

Findings

Projection effects decrease the intrinsic alignment signal at large scales.

A bump at ~1h^{-1}/Mpc is caused by interlopers and missed members.

Clusters with fewer missing members or more interlopers show higher alignment signals.

Abstract

Galaxy clusters, being the most massive objects in the Universe, exhibit the strongest alignment with the large-scale structure. However, mis-identification of members due to projection effects from the large scale structure can occur. We studied the impact of projection effects on the measurement of the intrinsic alignment of galaxy clusters, using galaxy cluster mock catalogs. Our findings showed that projection effects result in a decrease of the large scale intrinsic alignment signal of the cluster and produce a bump at $r_p\sim 1h^{-1}/Mpc$, most likely due to interlopers and missed member galaxies. This decrease in signal explains the observed similar alignment strength between bright central galaxies and clusters in the SDSS redMaPPer cluster catalog. The projection effect and cluster intrinsic alignment signal are coupled, with clusters having lower fractions of missing members or having higher fraction of interlopers exhibiting higher alignment signals in their projected shapes. We aim to use these findings to determine the impact of projection effects on galaxy cluster cosmology in future studies.