5 Conclusions

We computed the composite LF of 39 clusters of galaxies at 0.08<z<0.3 in three filters from the DPOSS plates, using the well known fact that clusters are galaxy overdensities with respect to the field. Our LF agrees with previous determinations of the cluster LF, obtained using specifically tailored observations, while we use sky survey plate data. The LFs are well described by a Schechter function, with a shallow slope $\alpha\sim-1.1$ with minor variations from blue to red filters and $M^*\sim-22.4,~-22.2,~-21.7$ (H₀=50 km s^-1 Mpc^-1) in g, r and ifilters, respectively. The LFs are computed without the assumption of an average background along the cluster line of sight, and use actual measurement of the background fluctuation instead of relying on the formalism and hypothesis presented in Huang et al. (1996) or, as in older works, assuming an "average" error. The existence of compact/misclassified galaxies have no impact on our LF determination: they are a minority population or a magnitude independent fraction of the number other galaxies.

The similarity of composite LFs by GMA99, measured from CCD photometry of the cluster central regions, suggests minor differences between the LF in the cluster outskirts and in the central one, or a minor contribution of galaxies in the cluster outskirts to the global LF.

When our cluster sample is grouped in classes of richness, dynamical and morphological type, we find no significant differences among the classes. However, our cluster sample may be not large enough for detecting the differences found in other studies, or the differences may be intrinsically too small to be detected in a sample, like ours, which is large but not huge (and the latter sample still does not exist).

Our results on the cluster LFs are not completely new: other authors found similar results, and for this reason we avoid repeating the cosmological implication of our results. However, we wish to stress that: we have a better control of the errors, due to the nearby control field and the direct measure of the field variance; we identify in the literature a few discrepant LFs in certain magnitude ranges; we show that the statistical subtraction of the background is sound, since we found the same LF shape found by Garilli et al., who removed interlopers by adopting an independent method; we obtain these results by using all-purpose photographic plates, instead of a multi-year CCD campaign.

We are currently increasing the present sample by an order of magnitude in order to explore with greater statistical significance the dependence of the cluster LF on the physical parameters.

Acknowledgements

Bianca Garilli, Dario Bottini and Dario Maccagni are warmly thanked for providing us with the electronic access to the data shown in Fig. 3 of Garilli et al. (1996). We also thank M. Fukugita for providing us k-correction data and N. Trentham for the information about the photometry in his articles.

The work on production and cataloguing of DPOSS at Caltech was supported by a generous grant from The Norris Foundation. R. Gal acknowledges a partial support from a NASA Graduate Fellowship. We are also thankful to the POSS-II and DPOSS teams for their efforts.