Galaxy clusters as intrinsic alignment tracers: present and future

TL;DR

This paper compares galaxy and cluster intrinsic alignments as cosmological probes, analyzing their detectability and potential advantages in current and future surveys like SDSS and LSST.

Contribution

It provides forecasts of cluster and galaxy alignment signals, highlighting the potential of clusters in weak lensing studies despite their lower detectability at higher redshifts.

Findings

Clusters have higher alignment signal-to-noise than galaxies in SDSS.

Cluster alignment signals diminish at higher redshifts in LSST.

Clusters can be advantageous in weak lensing due to easier isolation of intrinsic alignments.

Abstract

Galaxies and clusters embedded in the large-scale structure of the Universe are observed to align in preferential directions. Galaxy alignment has been established as a potential probe for cosmological information, but the application of cluster alignments for these purposes remains unexplored. Clusters are observed to have a higher alignment amplitude than galaxies, but because galaxies are much more numerous, the trade-off in detectability between the two signals remains unclear. We present forecasts comparing cluster and galaxy alignments for two extragalactic survey set-ups: a currently-available low redshift survey (SDSS) and an upcoming higher redshift survey (LSST). For SDSS, we rely on the publicly available redMaPPer catalogue to describe the cluster sample. For LSST, we perform estimations of the expected number counts while we extrapolate the alignment measurements from SDSS. Clusters in SDSS have typically higher alignment signal-to-noise than galaxies. For LSST, the cluster alignment signals quickly wash out with redshift due to a relatively low number count and a decreasing alignment amplitude. Nevertheless, a potential strong-suit of clusters is in their interplay with weak lensing: intrinsic alignments can be more easily isolated for clusters than for galaxies. The signal-to-noise of cluster alignment can in general be improved by isolating close pairs along the line of sight.