ON THE GALAXY LUMINOSITY FUNCTION IN THE CENTRAL REGIONS OF THE COMA CLUSTER

We have obtained new redshifts for 265 objects in the central 48 x 25 arcmin(2) region of the Coma cluster. When supplemented with literatur e data, our redshift sample is 95 % complete up to a magnitude b(26.5) =18.0 (the magnitudes are taken from the photometric sample of Godwin et al. 1983). Using redshift-confirmed membership for 205 galaxies, an d the location in the colour-magnitude diagram for another 91 galaxies , we have built a sample of cluster members which is complete up to b( 26.5)=20.0. We show that the Coma cluster luminosity function cannot b e adequately fitted by a single Schechter (1976) function, because of a dip in the magnitude distribution at b(26.5)similar to 17. The super position of an Erlang (or a Gauss) and a Schechter function provides a significantly better fit. We compare the luminosity function of Coma to those of other clusters, and of the field. Luminosity functions for rich clusters look similar, with a maximum at M(b) similar or equal t o -19.5 + 5 x log h(50), while the Virgo and the field luminosity func tions show a nearly monotonic behaviour. These differences may be prod uced by physical processes related to the environment which affect the luminosities of a certain class of cluster galaxies.