Coevolution of intracluster light and brightest cluster galaxies

Departamento de Astronomía, Universidad de La Serena, Avda. Raúl Bitrán No 1305 La Serena, Chile

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Received: 18 June 2025
Accepted: 6 February 2026

Abstract

Context. Intracluster light (ICL) is a faint stellar component of galaxy groups and clusters bound to the cluster potential, making up a significant fraction of the cluster mass. The ICL formation and evolution is strongly linked to the brightest cluster galaxies (BCGs) of clusters.

Aims. We aim to compare the properties and progenitor galaxies of the ICL and BCGs of clusters and groups at redshift z = 0 and to determine their processes of coevolution.

Methods. We selected 127 clusters and groups in the hydrodynamic IllustrisTNG-100 simulation above a mass of 10¹³ M_⊙. We divided the ICL from the BCG by applying a surface brightness cut at the Holmberg radius of 26.5 mag/arcsec⁻², where star particles within this radius are defined as being attached to the BCG. Those beyond outside are defined as being part of the ICL. We studied the properties and formation history of the ICL and BCG.

Results. We find the ICL is generally composed of material from stripped or merged intermediate-mass galaxies, with a smaller in situ component, whereas the BCG is composed of more massive merged galaxies and has a higher in situ fraction. The ICL mass fraction increases weakly with cluster mass, declines with concentration, and increases with time since the time of the BCG’s most recent major merger. The ICL is bluer and more metal-poor than the BCG, but there is no significant difference in the age of the material. Universally, BCG+ICL systems have negative colour and metallicity gradients. The ICL and BCG share a high fraction of progenitor galaxies, but the most significant progenitor is typically unknown.

Conclusions. The ICL properties and formation are tied to the formation histories of the host cluster and BCG and, thus, their properties are individual to each system. Although the ICL and BCG are coevolved, they have distinct formation histories and properties.