U-, B- and r-band luminosity functions of galaxies in the Coma cluster

We present a deep multicolour CCD mosaic of the Coma cluster (Abell 1656), covering 5.2 deg(2) in the B and r bands, and 1.3 deg(2) in the U band. This large, homogeneous data set provides a valuable low-redshift comparison sample for studies of galaxies in distant clusters. In this paper we present our survey, and study the dependence of the galaxy luminosity function (LF) on passband and radial distance from the cluster centre. The U-, B- and r-band LFs of the complete sample cannot be represented by single Schechter functions. For the central area, r < 245 h(100)(-1) kpc, we find best-fitting Schechter parameters of M-U* = -18.60(-0.18)(+0.13) and alpha(U) = -1.32(-0.028)(+0.018), M-B* = 19.79(-0.17)(+0.18) and alpha(B) = -1.37(-0.016)(+0.024) and M-r* = -20.87(-0.17)(+0.12) and alpha(r) = -1.16(-0.019)(+0.012). The LF becomes steeper at larger radial distances from the cluster centre. The effect is most pronounced in the U band. This result is consistent with the presence of a star-forming dwarf population at a large distance from the cluster centre, which may be in the process of being accreted by the cluster. The shapes of the Us of the NGC 4839 group support a scenario in which the group has already passed through the centre.