Luminosity function of clusters of galaxies

The composite galaxy luminosity function (hereafter LF) of 39 Abell cluster s of galaxies is derived by computing the statistical excess of galaxy coun ts in the cluster direction with respect to control fields. Due to the wide field coverage of the digitised POSS-II plates, we can measure field count s around each cluster in a fully homogeneous way. Furthermore, the availabi lity of virtually unlimited sky coverage allows us to directly compute the LF errors without having to rely on the estimated variance of the backgroun d. The wide field coverage also allows us to derive the LF of the whole clu ster, including galaxies located in the cluster outskirts. The global compo site LF has a slope alpha similar to -1.1 +/- 0.2 with minor variations fro m blue to red filters, and M-* similar to -21.7, -22.2, -22.4 mag (H-0 = 50 km s(-1) Mpc(-1)) in g, r and i filters, respectively (errors are detailed in the text). These results are in quite good agreement with several previ ous determinations and in particular with the LF determined for the inner r egion of a largely overlapping set of clusters, but derived making use of a completely different method for background subtraction. The similarity of the two LFs suggests the existence of minor differences between the LF in t he cluster outskirts and in the central region, or a negligible contributio n of galaxies in the cluster outskirts to the global LF.