The Transition Region between Brightest Cluster Galaxies and Intracluster Light in Galaxy Groups and Clusters

We take advantage of a state-of-art semianalytic model of galaxy formation, and the model presented by Contini & Gu, to investigate the mass distribution of brightest cluster galaxies (BCGs) and intracluster light (ICL) by addressing two points: (1) the region of transition between a BCG-dominated distribution and an ICL-dominated one, and (2) the relation between the total BCG+ICL mass and the ICL mass alone. We find the transition radius to be independent of both BCG+ICL and halo masses, with an average of 60 +/- 40 kpc, in good agreement with previous observational measurements, but given the large scatter it can be considered as a sort of physical separation between the two components only on a cluster scale. From the analysis of the M (ICL)-M (BCG+ICL) relation, we construct a method able to extract the ICL mass directly from knowledge of the BCG+ICL mass. Given the large scatter in low-mass systems, such a method under/overpredicts the true value of the ICL in a significant way, up to a factor of three in the worst cases. On the other hand, for log(MBCG+ICL) >12 log M-Halo > 14 (where masses are in units of M (circle dot)), the difference between the true value and the one extracted from the M (ICL)-M (BCG+ICL) relation ranges between +/- 30%. We therefore suggest this relation as a reliable test for observational works aiming to isolate the ICL from the BCG, for systems hosted by haloes on a cluster scale.