Abstract

Intracluster light is thought to originate from stars that were ripped away from their parent galaxies by gravitational tides and galaxy interactions during the build up of the cluster. The stars from such interactions will accumulate over time, so semi-analytic models suggest that the abundance of intracluster stars is negligible in young proto-clusters at z~2 and grows to around a quarter of the stellar mass in the oldest, most mature clusters. In contrast to these theoretical expectations, I will show you detections of intracluster light within two proto-clusters using deep HST images, with >50% of the stellar mass located more than 10 kpc away from the brightest cluster galaxies (BCGs). I will compare these quantitative observations to theoretical predictions from the latest semi-analytic models and the Hydrangea hydrodynamical galaxy cluster simulations to show you that we still do not have a galaxy formation theory that can adequately describe the growth of BCGs and intracluster light.