Stellar splashback: the edge of the intracluster light

TL;DR

This study uses simulations to identify the physical boundary of galaxy clusters through the stellar and dark matter distributions, revealing that the intracluster light edge correlates with the splashback radius and can be observed with future telescopes.

Contribution

It demonstrates that the stellar distribution's edge, or ICL, aligns with the dark matter splashback radius, offering a new way to measure cluster boundaries.

Findings

The dark matter splashback radius is at approximately r_200m.

The stellar distribution edge correlates with the dark matter splashback radius.

Upcoming telescopes can observe the faint stellar splashback feature.

Abstract

We examine the outskirts of galaxy clusters in the C-EAGLE simulations to quantify the `edges' of the stellar and dark matter distribution. The radius of the steepest slope in the dark matter, commonly used as a proxy for the splashback radius, is located at ~r_200m; the strength and location of this feature depends on the recent mass accretion rate, in good agreement with previous work. Interestingly, the stellar distribution (or intracluster light, ICL) also has a well-defined edge, which is directly related to the splashback radius of the halo. Thus, detecting the edge of the ICL can provide an independent measure of the physical boundary of the halo, and the recent mass accretion rate. We show that these caustics can also be seen in the projected density profiles, but care must be taken to account for the influence of substructures and other non-diffuse material, which can bias and/or weaken the signal of the steepest slope. This is particularly important for the stellar material, which has a higher fraction bound in subhaloes than the dark matter. Finally, we show that the `stellar splashback' feature is located beyond current observational constraints on the ICL, but these large projected distances (>> 1 Mpc) and low surface brightnesses (mu >> 32 mag/arcsec^2) can be reached with upcoming observational facilities such as the Vera C. Rubin Observatory, the Nancy Grace Roman Space Telescope, and Euclid.