Assembly of the Intracluster Light in the Horizon-AGN Simulation

TL;DR

This study uses the Horizon-AGN simulation to trace the origins of intracluster light (ICL) in galaxy clusters, revealing that most ICL stars originate from massive infalling galaxies rather than in-situ formation.

Contribution

It provides a detailed analysis of ICL assembly, linking it mainly to galaxy infall and interactions, and highlights the dominant role of massive galaxies in this process.

Findings

Approximately 90% of ICL originates from galaxies with stellar masses above 10^9 M_sun.

About half of the ICL comes from galaxies near 10^11 M_sun.

In-situ star formation in the ICL is negligible.

Abstract

The diffuse stellar component of galaxy clusters made up of intergalactic stars is termed the intracluster light (ICL). Though there is a developing understanding of the mechanisms by which the ICL is formed, no strong consensus has yet been reached on which objects the stars of the ICL are primarily sourced from. We investigate the assembly of the ICL starting approximately $10$ Gyr before $z=0$ in 11 galaxy clusters (halo masses between $\sim1\times 10^{14}$ M$_{\odot}$ and $\sim7\times 10^{14}$ M$_{\odot}$ at $z\approx0$) in the Horizon-AGN simulation. By tracking the stars of galaxies that fall into these clusters past cluster infall, we are able to link almost all of the $z\approx0$ ICL back to progenitor objects. Satellite stripping, mergers, and pre-processing are all found to make significant contributions to the ICL, but any contribution from in-situ star-formation directly into the ICL appears negligible. Even after compensating for resolution effects, we find that approximately $90$ per cent of the stacked ICL of the 11 clusters that is not pre-processed should come from galaxies infalling with stellar masses above $10^{9}$ M$_{\odot}$, with roughly half coming from infalling galaxies with stellar masses within half a dex of $10^{11}$ M$_{\odot}$. The fact that the ICL appears largely sourced from such massive objects suggests that the ICL assembly of any individual cluster may be principally stochastic.