Unveiling the dynamical state of massive clusters through the ICL fraction

TL;DR

This study investigates how the dynamical state of massive galaxy clusters affects the intra-cluster light (ICL) fraction, revealing higher ICL fractions in merging clusters and suggesting differences in stellar populations compared to relaxed clusters.

Contribution

It introduces a robust method to measure ICL fractions across multiple wavelengths and compares these fractions between relaxed and merging galaxy clusters.

Findings

Higher ICL fractions in merging clusters (~7-23%) compared to relaxed ones (~2-11%).

ICL colors are similar to cluster galaxies in relaxed clusters, indicating similar stellar populations.

Merging clusters show excess low-metallicity/younger stars in the ICL, likely from galaxy outskirts during mergers.

Abstract

We have selected a sample of eleven massive clusters of galaxies observed by the Hubble Space Telescope in order to study the impact of the dynamical state on the IntraCluster Light (ICL) fraction, the ratio of total integrated ICL to the total galaxy member light. With the exception of the Bullet cluster, the sample is drawn from the Cluster Lensing and Supernova Survey and the Frontier Fields program, containing five relaxed and six merging clusters. The ICL fraction is calculated in three optical filters using the CHEFs IntraCluster Light Estimator, a robust and accurate algorithm free of a priori assumptions. We find that the ICL fraction in the three bands is, on average, higher for the merging clusters, ranging between $\sim7-23\%$, compared with the $\sim 2-11\%$ found for the relaxed systems. We observe a nearly constant value (within the error bars) in the ICL fraction of the regular clusters at the three wavelengths considered, which would indicate that the colors of the ICL and the cluster galaxies are, on average, coincident and, thus, their stellar populations. However, we find a higher ICL fraction in the F606W filter for the merging clusters, consistent with an excess of lower-metallicity/younger stars in the ICL, which could have migrated violently from the outskirts of the infalling galaxies during the merger event.