The impact of galaxy selection on the splashback boundaries of galaxy clusters

TL;DR

This study investigates how galaxy selection criteria influence the measurement of the splashback radius in galaxy clusters using simulations, revealing that galaxy properties affect the observed $R_{sp}$ and reconciling different observational results.

Contribution

It demonstrates how various galaxy selection methods impact splashback radius measurements and shows quenched galaxies reliably trace the dark matter boundary.

Findings

Massive and luminous galaxies show smaller $R_{sp}$ than dark matter.

Quenched galaxies can accurately trace the dark matter splashback radius.

Selection functions explain differences in observational $R_{sp}$ measurements.

Abstract

We explore how the splashback radius ($R_{\rm sp}$) of galaxy clusters, measured using the number density of the subhalo population, changes based on various selection criteria using the IllustrisTNG cosmological galaxy formation simulation. We identify $R_{\rm sp}$ by extracting the steepest radial gradient in a stacked set of clusters in 0.5 dex wide mass bins, with our clusters having halo masses $10^{13} \leq M_{\rm 200, mean} / {\rm M}_\odot \leq 10^{15}$. We apply cuts in subhalo mass, galaxy stellar mass, $i$-band absolute magnitude and specific star formation rate. We find that, generally, galaxies of increasing mass and luminosity trace smaller measured splashback radii relative to the intrinsic dark matter radius. We also show that quenched galaxies may be used to reliably reconstruct the dark matter splashback radius. This trend is likely due to changes in the galaxy population. Additionally, we are able to reconcile different observational predictions that $R_{\rm sp}$ based upon galaxy number counts and dark matter may either align or show significant offset (e.g. those using optically- or SZ-selected clusters) through the selection functions that these studies employ. Finally, we demonstrate that changes in $R_{\rm sp}$ measured through number counts are not due to a simple change in galaxy abundance inside and outside of the cluster.