The galaxy luminosity function of the Abell 496 cluster and its spatial variations

Context. The faint end slopes of galaxy luminosity functions (LFs) in clusters of galaxies have been observed in some cases to vary with clustercentric distance and should be influenced by the physical processes (mergers, tides) affecting cluster galaxies. However, there is a wide disagreement on the values of the faint end LF slopes, ranging from-1 to -2.3 in the magnitude range -18 < Mr < -14. Aims. We investigate the LF in the very relaxed cluster Abell 496. Methods. Our analysis is based on deep images obtained at CFHT with MegaPrime/MegaCam in four bands (u*g'r'i') covering a 1 x 1 deg(2) region, which is centred on the cluster Abell 496 and extends to near its virial radius. The LFs are estimated by statistically subtracting a reference field taken as the mean of the 4 Deep fields of the CFHTLS survey. Background contamination is minimised by cutting out galaxies redder than the observed Red Sequence in the g' - i' versus i' colour-magnitude diagram. Results. In Abell 496, the global LFs show a faint end slope of -1.55 +/- 0.06 and vary little with observing band. Without colour cuts, the LFs are much noisier but not significantly steeper. The faint end slopes show a statistically significant steepening from alpha = -1.4 +/- 0.1 in the central region (extending to half a virial radius) to -1.8 +/- 0.1 in the Southern envelope of the cluster. Cosmic variance and uncertain star-galaxy separation are our main limiting factors in measuring the faint end of the LFs. The large-scale environment of Abell 496, probed with the fairly complete 6dFGS catalogue, shows a statistically significant 36 Mpc long filament at PA = 137 degrees. Conclusions. Our LFs do not display the large number of dwarf galaxies (a approximate to -2) inferred by several authors, whose analyses may suffer from field contamination caused by non-existent or inadequate colour cuts. Alternatively, different clusters may have different faint end slopes, but this is hard to reconcile with the wide range of slopes found for given clusters and for wide sets of clusters.