RELICS: ICL Analysis of the $z=0.566$ merging cluster WHL J013719.8-08284

TL;DR

This study analyzes the intracluster light in the merging galaxy cluster WHL J013719.8-08284 at z=0.566 using HST imaging, revealing a significant ICL fraction, signs of recent star stripping, and potential classification as a fossil system progenitor.

Contribution

First detailed ICL analysis of the z=0.566 cluster WHL J013719.8-08284 using multi-band HST imaging and the CICLE algorithm, highlighting merger signatures and ICL properties.

Findings

ICL fractions range between 6% and 19% in optical bands.

Detected an ICL excess between 3800 Å and 4800 Å indicating young/low-metallicity stars.

Infrared ICL fractions are about 50% higher than optical, suggesting older or higher-metallicity populations.

Abstract

We present a pilot study of the intracluster light (ICL) in massive clusters using imaging of the $z=0.566$ cluster of galaxies WHL J013719.8-08284 observed by the RELICS project with the HST. We measure the ICL fraction in four optical ACS/WFC filters (F435W, F475W, F606W, and F814W) and five infrared WFC3/IR bands (F105W, F110W, F125W, F140W, and F160W). The ICL maps are calculated using the free of a priori assumptions algorithm CICLE, and the cluster membership is estimated from photometric properties. We find optical ICL fractions that range between $\sim$6\% and 19\% in nice agreement with the values found in previous works for merging clusters. We also observe an ICL fraction excess between 3800 \AA and 4800 \AA, previously identified as a signature of merging clusters at $0.18<z<0.55$. This excess suggests the presence of an enhanced population of young/low-metallicity stars in the ICL. All indicators thus point to WHL J013719.8-08284 as a disturbed cluster with a significant amount of recently injected stars, bluer than the average stars hosted by the cluster members and likely stripped out from infalling galaxies during the current merging event. Infrared ICL fractions are $\sim$50\% higher than the optical ones, which could be signature of an older and/or higher-metallicity ICL population that can be associated with the build-up of the BCG, the passive evolution of young stars, previously injected, or preprocessing in infalling groups. Finally, investigating the photometry of the cluster members, we tentatively conclude that WHL J013719.8-08284 fulfills the expected conditions for a fossil system progenitor.