2008A&A...489...11B

We present an optical investigation of the Abell 85 cluster filament (z=0.055) previously interpreted in X-rays as groups falling on to the main cluster. We compare the distribution of galaxies with the X-ray filament, and investigate the galaxy luminosity functions in several bands and in several regions. We search for galaxies where star formation may have been triggered by interactions with intracluster gas or tidal pressure due to the cluster potential when entering the cluster. Our analysis is based on images covering the South tip of Abell 85 and its infalling filament, obtained with CFHT MegaPrime/MegaCam (1x1 deg² field) in four bands (u^*, g', r', i') and ESO 2.2m WFI (38x36arcmin² field) in a narrow band filter corresponding to the redshifted Hα line and in an R_C broad band filter. The LFs are estimated by statistically subtracting a reference field. Background contamination is minimized by cutting out galaxies redder than the observed red sequence in the g'-i' versus i' colour-magnitude diagram. The galaxy distribution shows a significantly flattened cluster, whose principal axis is slightly offset from the X-ray filament. The analysis of the broad band galaxy luminosity functions shows that the filament region is well populated. The filament is also independently detected as a gravitationally bound structure by the Serna & Gerbal (1996A&A...309...65S) hierarchical method. 101 galaxies are detected in the Hα filter, among which 23 have spectroscopic redshifts in the cluster, 2 have spectroscopic redshifts higher than the cluster and 58 have photometric redshifts that tend to indicate that they are background objects. One galaxy that is not detected in the Hα filter probably because of the filter low wavelength cut but shows Hα emission in its SDSS spectrum in the cluster redshift range has been added to our sample. The 24 galaxies with spectroscopic redshifts in the cluster are mostly concentrated in the South part of the cluster and along the filament. We find a number of galaxies showing evidence for star formation in the filament, and all our results are consistent with the previous hypothesis that the X-ray filament in Abell 85 is a gravitationally bound structure made of groups falling on to the main cluster.