Intra-cluster Light

The most revealing signature of galaxy cluster assembly is in a diffuse component that permeates clusters of galaxies. This component is the so-called intracluster light (ICL). This light is a by-product of interactions between galaxies in clusters. In the course of these interactions, individual stars are stripped from their galaxies and float freely, following the potential of the cluster. Characterising the ICL therefore provides unique insights into the processes that shaped the cluster. The ICL can also help us understand the assembly history of the Brightest Cluster Galaxy (BCG). For example, theoretical models have predicted a larger growth rate than the rate inferred by observations. This tension can be alleviated if we assume that a significant percentage of the accreted mass ends up in the cluster's ICL.

Despite its enormous importance for understanding galaxy clusters, the ICL is still mostly unexplored. This component is extremely challenging to probe due to its very low surface brightness and very few clusters have been studied so far. LSST will enable the community to perform the first comprehensive survey of ICL in galaxy clusters and groups within a uniform dataset and over a significant range in redshift.

The outstanding questions that LSST will answer are:

• When does the ICL first emerge i.e. at what redshift and does it depends on halo mass?
• How does the fraction of light in this component evolve with mass and redshift?
• Do the properties of the ICL correlate with the properties of the host cluster (e.g. mass or dynamical state)?
• Are these properties different at different redshifts?
• Where does the ICL begin and the large diffuse cD halo of the Brightest Cluster Galaxy (BCG) end, and do they have the same origin?