The Edinburgh-Durham Southern Galaxy Catalogue -- VIII. The cluster galaxy luminosity function

Abstract

We have re-examined the nature of the cluster galaxy luminosity function using the data from the Edinburgh-Durham Southern Galaxy Catalogue and the Edinburgh-Milano Redshift Survey. We derive a best-fitting luminosity function (LF) over the range −18 to − 21 in M(bj), for a composite sample of 22 of the richer clusters that has M(bj)⋆= −20.16 ±0.02 and α= −1.22 ± 0.04. The dominant error in these values results from the choice of background subtraction method. From extensive simulations we can show that when the LF is fitted over this narrow range, it is difficult to discriminate against bright values of M ⋆ in the single cluster fits, but that faint values provide a strong test of the universality of the luminosity function. We find that all the individual cluster data are well-fitted by a Schechter function with α fixed at −1.25, and that ≤ 10 per cent of these have fitted values of M ⋆ that disagree from the average at the 99 per cent confidence level. We further show that fitting only a single parameter Schechter function to composite subsets of the data can give erroneous results for the derived M⋆, as might be expected from the known tight correlation between M⋆ and α. By considering two parameter fits, the results of Monte Carlo simulations and direct two-sample χ2 tests, we conclude that there is only weak evidence for differences between the data when broken down into subsets based on physical properties (Bautz-Morgan class, richness, velocity dispersion): from our simulations, only the evidence for a difference between subsets based on velocity dispersion may in fact be significant. However, we find no evidence at all that a Schechter function is not a good model for the intrinsic cluster luminosity function over this absolute magnitude range. Models that invoke strong evolution of galaxy luminosity of all galaxies within clusters are inconsistent with our results.