2019MNRAS.483.4354S

We investigate the stellar populations of galaxies in clusters at different dynamical stages, aiming to identify possible effects of the relaxation state of the cluster or subcluster on the star formation histories of its galaxies. We have developed and applied a code for kinematic substructure detection to a sample of 412 galaxy clusters drawn from the Tempel et al. (2012) catalogue, finding a frequency of substructures of 45 per cent. We have extracted mean stellar ages with the STARLIGHT spectral synthesis code applied to Sloan Digital Sky Survey III spectra of the sample galaxies. We found lower mean stellar ages in unrelaxed clusters relative to relaxed clusters. For unrelaxed clusters, we separated primary and secondary subhaloes and found that, while relaxed clusters and primaries present similar masses and age distributions, secondaries present younger stellar populations, mainly due to low-mass galaxies (log M_*/M_☉ <= 11 dex). An age-clustercentric radius relation is seen for all subhaloes irrespective of the presence of substructures. We also observe relations between the mean stellar age and mass of relaxed and unrelaxed clusters, massive systems presenting higher mean ages. The locus of these relations is distinct between relaxed and unrelaxed clusters, but become indistinguishable when separating primaries and secondaries. Our results suggest that differences between relaxed and unrelaxed clusters are mainly driven by low-mass systems in the clusters outskirts, and that, while pre-processing can be seen in the subcomponents of dynamically young clusters, some evolution in the stellar populations must occur during the clusters relaxation.