Multi-wavelength study of X-ray luminous clusters at z ∼ 0.3 I. Star-formation activity of cluster galaxies

Context. The current paradigm of cosmic formation and evolution of galaxy clusters foresees growth mostly through merging. Galaxies in the infall region or in the core of a cluster undergo transformations owing to different environmental stresses. Aims. For two X-ray luminous clusters at redshift z similar to 0.3 with opposite X-ray morphologies (i.e., dynamical states), RXCJ0014.3-3022 and RXCJ 2308.3-0211, we assess differences in galaxy populations as a function of cluster topography. This is a pilot study for the joint X-ray and optical analysis of the REFLEX-DXL cluster sample. Methods. Cluster large-scale structure and substructure are determined from the combined photometry in the B, V, and R bands, and from multi-object optical spectroscopy at low resolution. Photometric redshifts and broad-band optical colours are determined. A spectral index analysis is performed, based on the [OII](lambda lambda 3726, 3728 angstrom) and H-delta(lambda 4102 angstrom) features, and the D4000 break, which are available for more than 100 member galaxies per cluster. Additional far-ultraviolet (FUV) photometry is retrieved from the GALEX archive. Combination of spectral indices and FUV-optical colours provides a picture of the star-formation history in galaxies. Results. In spite of the potential presence of a small fraction of galaxies with obscured star-formation activity, the average star-formation history of cluster members is found to depend on clustercentric distance and, more interestingly, on cluster substructure. The core regions of both clusters mainly host galaxies dominated by old, passively evolving stellar populations, which define the same red sequence in a (B-R) colour-R magnitude diagram. However, a sharp increase in star-formation activity is found along two clearly evident filamentary structures of the merging cluster RXCJ 0014.3-3022, out to its virial radius and beyond. It is produced by luminous (i.e., L-R >= L-R(star)) and sub-L-star galaxies. In contrast, the regular cool-core cluster RXCJ 2308.3-0211 mostly hosts galaxies that either populate the red sequence or are becoming more passive. This finding holds out to the cluster virial radius, and also for its immediate large-scale environment. Conclusions. These results suggest the existence of a correspondence between assembly state and overall age of the stellar populations of galaxies inside the virialized region and in the surrounding large-scale structure of massive clusters at z similar to 0.3.