Characterizing diffused stellar light in simulated galaxy clusters

In this paper, we carry out a detailed analysis of the performance of two different methods to identify the diffuse stellar light in cosmological hydrodynamical simulations of galaxy clusters. One method is based on a dynamical analysis of the stellar component, which separates the brightest central galaxy (BCG) from the stellar component not gravitationally bound to any galaxy, what we call ’diffuse stellar component’ (DSC). The second method is closer to techniques commonly employed in observational studies. We generate mock images from simulations, and assume a standard surface brightness limit (SBL) to disentangle the BCG from the intra-cluster light (ICL). Both the dynamical method and the method based on the SBL criterion are applied to the same set of hydrodynamical simulations for a large sample of about 80 galaxy clusters. We analyse two sets of radiative simulations: a first set includes the effect of cooling, star formation, chemical enrichment and galactic outflows triggered by supernova feedback (CSF set); a second one also includes the effect of thermal feedback from active galactic nuclei triggered by gas accretion on to supermassive black holes (AGN set). We find significant differences between the ICL and DSC fractions computed with the two corresponding methods, which amounts to about a factor of 2 for the AGN simulations, and a factor of 4 for the CSF set. We also find that the inclusion of AGN feedback boosts the DSC and ICL fractions by a factor of 1.5–2, respectively, while leaving the BCG+ICL and BCG+DSC mass fraction almost unchanged. The sum of the BCG and DSC mass stellar mass fraction is found to decrease from ∼80 per cent in galaxy groups to ∼60 per cent in rich clusters, thus in excess of that found from observational analysis. We identify the average SBLs that yield the ICL fraction from the SBL method close to the DSC fraction from the dynamical method. These SBLs turn out to be brighter in the CSF than in the AGN simulations. This is consistent with the finding that AGN feedback makes BCGs to be less massive and with shallower density profiles than in the CSF simulations. The BCG stellar components, as identified by both methods, are slightly older and more metal-rich than the stars in the diffuse component. Relaxed clusters have somewhat higher stellar mass fractions in the diffuse component. The metallicity and age of both the BCG and diffuse components in relaxed clusters are also richer in metals and older.