The dawn of intracluster light in protoclusters

Intracluster light (ICL) appears in observations as a faint, diffuse glow at the heart of galaxy clusters, composed primarily of stars that drift freely within the cluster's gravitational potential, untethered to any galaxy. To unravel the story of its early formation, we must journey back in time to when galaxy clusters were still assembling. In this work, we use observations and simulations to address ICL formation and assembly. We report on the detection of intracluster light within two proto-clusters at z=2 using deep HST images, suggesting that intracluster stars are already present within the core of proto-clusters. Using simulations, we analyze the origins of ICL in 100 massive, simulated proto-clusters from the Manhattan Suite simulations. Our goal is to pinpoint when and where ICL stars were born, the environments that shaped them, and the role of pre-processing in their assembly. We discover a surprising amount of ICL in the infall regions of proto-clusters, with a modest fraction in groups at 2<z<4. Most ICL stars originated within galaxies and were later ejected during galaxy interactions. Our findings also reveal that galaxies hosting ICL stars in the past are more massive than the average galaxy population, a trend especially pronounced in groups. This suggests that massive galaxies, through processes like mergers, play a key role in ejecting stars into the intracluster medium. Together, these results shed light on the intricate and dynamic processes that give rise to intracluster light in the earliest stages of proto-cluster evolution.