Photometric redshifts as a tool for studying the Coma cluster galaxy populations

Aims. We apply photometric redshift techniques to an investigation of the Coma cluster galaxy luminosity function (GLF) at faint magnitudes, in particular in the u* band where basically no studies are presently available at these magnitudes. Methods. Cluster members were selected based on probability distribution function from photometric redshift calculations applied to deep u*, B, V, R, Iimages covering a region of almost 1 deg² (completeness limit ∼ 24). In the area covered only by the u* image, the GLF was also derived after a statistical background subtraction. Results. Global and local GLFs in the B, V, R, and I bands obtained with photometric redshift selection are consistent with our previous results based on a statistical background subtraction. The GLF in the u* band shows an increase in the faint end slope towards the outer regions of the cluster. The analysis of the multicolor type spatial distribution reveals that late type galaxies are distributed in clumps in the cluster outskirts, where X-ray substructures are also detected and where the GLF in the u* band is steeper.Conclusions. We can reproduce the GLFs computed with classical statistical subtraction methods by applying a photometric redshift technique. The u* GLF slope is steeper in the cluster outskirts, varying from α ∼ -1 in the cluster center to α ∼ -2 in the cluster periphery. The concentrations of faint late type galaxies in the cluster outskirts could explain these very steep slopes, assuming a short burst of star formation in these galaxies when entering the cluster.