Diffuse intracluster light (ICL) has now been observed in nearby and in intermediate redshift clusters. Individual intracluster stars have been detected in the Virgo and Coma clusters and the first color-magnitude diagram and velocity measurements have been obtained. Recent studies show that the ICL contains of the order of 10% and perhaps up to 30% of the mass in stars of the cluster, but in the cores of some dense and rich clusters like Coma, the local ICL fraction can be high as 40%-50%.
What can we learn from the ICL about the formation of galaxy clusters and the evolution of cluster galaxies? How and when did the ICL form? What is the connection to the central brightest cluster galaxy? Cosmological N-body and hydrodynamical simulations are now beginning to make predictions for the kinematics and origin of the ICL. The evolution of baryonic substructures in dense environments is traced by the ICL which can thus be used to constraint those aspects of cosmological simulations that are most uncertain, such as the modeling of star formation and the mass distribution of the baryonic component in galaxies.
This is the first time that a Joint Discussion dedicated to this subject of research will confront observational evidence and theoretical predictions. It aims at identifying future directions for understanding the origin and implications of this new component of galaxy clusters.