Searching for filaments and large-scale structure around DAFT/FADA clusters

Context. Clusters of galaxies are located at the intersection of cosmic filaments and are still accreting galaxies and groups along these preferential directions. However, because of their relatively low contrast on the sky, filaments are difficult to detect (unless a large amount of spectroscopic data are available), and unambiguous detections have been limited until now to relatively low redshifts (z< ~ 0.3). Aims. This project is aimed at searching for extensions and filaments around clusters, traced by galaxies selected to be at the cluster redshift based on the red sequence. In the 0.4 <z< 0.9 redshift range of our sample, clusters are believed to be already well formed, but still to be accreting material along filaments. Methods. We have searched for extensions and filaments around the thirty clusters of the DAFT/FADA survey for which we had deep wide field photometric data. For each cluster, based on a colour–magnitude diagram, we selected galaxies that were likely to belong to the red sequence, and hence to be at the cluster redshift, and built density maps. By computing the background for each of these maps and drawing 3σ contours, we estimated the elongations of the structures detected in this way. Whenever possible, we identified the other structures detected on the density maps with clusters listed in NED. Results. We find clear elongations in twelve clusters out of thirty, with sizes that can reach up to 7.6 Mpc. Eleven other clusters have neighbouring structures, but the zones linking them are not detected in the density maps at a 3σ level. Three clusters show no extended structure and no neighbours, and four clusters are of too low contrast to be clearly visible on our density maps. Conclusions. The simple method we have applied appears to work well to show the existence of filaments and/or extensions around a number of clusters in the redshift range 0.4 <z< 0.9. We plan to apply it to other large cluster samples such as the clusters detected in the CFHTLS and SDSS-Stripe 82 surveys in the near future