Isophotal signatures of the Brightest Cluster Galaxy-Intracluster Light transition

The distinction between brightest cluster galaxies (BCGs) and intracluster light (ICL) remains ambiguous, with inferred cluster properties depending strongly on the adopted definition and measurement methodology. We report the presence of a common inflection feature in the radial profiles of surface brightness, ellipticity, and position angle of BCG+ICL systems, indicative of a structural transition in the underlying stellar distribution. Using a combination of low-redshift clusters (z < 0.1) observed with the Wendelstein Wide Field Imager, deep Hyper Suprime-Cam observations of clusters at 0.2 < z < 0.4, and cosmological hydrodynamical simulations from Horizon-AGN and Hydrangea, we consistently identify a common inflection at radii of 60 − 85 kpc. We interpret this feature as a characteristic transition in the combined BCG+ICL light distribution, that may reflect the hierarchical assembly of the cluster. The persistence of this feature across multiple photometric bands, redshift ranges, and simulations supports its robustness. To identify these transitions, we employ an automated second-derivative-based approach that does not rely on parametric decompositions or arbitrary surface brightness thresholds. This framework provides a physically motivated and scalable means to characterise the BCG-ICL transition, and it will be of paramount importance for forthcoming large-scale surveys such as the Legacy Survey of Space and Time and the European Space Agency’s Euclid mission.