Flaming, bright galaxies along the filaments of A 2744

Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany e-mail: [fbraglia;dpierini;hxb]@mpe.mpg.de

Received: 27 February 2007
Accepted: 30 April 2007

Abstract

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 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 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 for the main body of the cluster and with angular sizes of 13–22  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.