Intergalactic stellar populations in intermediate redshift clusters

ABSTRACT A substantial fraction of the total stellar mass in rich clusters of galaxies resides in a diffuse intergalactic component usually referred to as the intracluster light (ICL). Theoretical models indicate that these intergalactic stars originate mostly from the tidal interaction of the cluster galaxies during the assembly history of the cluster, and that a significant fraction of these stars could have formed in situ from the late infall of cold metal‐poor gas clouds on to the cluster. However, these models also overpredict the fraction of stellar mass in the ICL by a substantial margin, something that is still not well understood. The models also make predictions about the age distribution of the ICL stars, which may provide additional observational constraints. Here we present population synthesis models for the ICL of an intermediate redshift (z = 0.29) X‐ray cluster that we have extensively studied in previous papers. The advantage of observing intermediate redshift clusters rather than nearby ones is that the former fit the field of view of multi‐object spectrographs in 8‐m telescopes and therefore permit us to encompass most of the ICL with only a few well‐placed slits. In this paper we show that by stacking spectra at different locations within the ICL it is possible to reach sufficiently high signal‐to‐noise ratios to fit population synthesis models and derive meaningful results. The models provide ages and metallicities for the dominant populations at several different locations within the ICL and the brightest cluster galaxies (BCG) halo, as well as measures of the kinematics of the stars as a function of distance from the BCG. We thus find that the ICL in our cluster is dominated by old metal‐rich stars, at odds with what has been found in nearby clusters where the stars that dominate the ICL are old and metal poor. While we see weak evidence of a young, metal‐poor component, if real, these young stars would amount to less than 1 per cent of the total ICL mass, much less than the up to 30 per cent predicted by the models. We propose that the very metal‐rich (i.e. 2.5× solar) stars in the ICL of our cluster, which comprise ∼40 per cent of the total mass, originate mostly from the central dumb‐bell galaxy, while the remaining solar and metal‐poor stars come from spiral, post‐starburst (E+A) and metal‐poor dwarf galaxies. About 16 per cent of the ICL stars are old and metal poor.