Flaming, bright galaxies along the filaments of A2744

Context. The existence of a lumpy, large-scale filamentary structure is at the basis of the current paradigm of cosmic formation and evolution of clusters. The star-formation history of galaxies falling into a cluster is altered as a result of the environmental stresses. Aims. We investigate the relation between substructure and properties of the galaxy population in a 30' x 30' region centred on the massive merging cluster A 2744 at z - 0.3. Methods. Multi-object spectroscopy at low resolution and BVR photometry are used to detect the presence of substructure through a Dressler-Schectman analysis and the photometric redshift technique, respectively. Galaxies at the same photometric redshift of the cluster are divided into red and blue according to their distribution in the B - R vs. R colour - magnitude diagram. Results. We identify two large-scale filaments associated with A 2744. Along these filaments, the blue-to-red galaxy number ratio increases together with the cluster-centric distance but peaks slightly beyond the cluster virial radius. The filaments host a population of bright, large ( i.e. more luminous than R* for the main body of the cluster and with angular sizes of 13 - 22 h(-1) (70) kpc) blue galaxies that is hardly found among galaxies lying in a neighbouring low-density environment at the same redshift of the cluster. Conclusions. These results can be interpreted as a manifestation of galaxy harassment.