Luminosity distributions within rich clusters - III. A comparative study of seven Abell/ACO clusters

We recover the luminosity distributions over a wide range of absolute magnitude (-24.5 <M-R <-16.5) for a sample of seven rich southern galaxy clusters. We find a large variation in the ratio of dwarf to giant galaxies, DGR: 0.8 less than or equal to DGR less than or equal to 3.1. This variation is shown to be inconsistent with a ubiquitous cluster luminosity function. The DGR shows a smaller variation from cluster to cluster in the inner regions (r less than or similar to 0.56 Mpc). Outside these regions we find the DGR to be strongly anticorrelated with the mean local projected galaxy density, with the DGR increasing towards lower densities. In addition, the DGR in the outer regions shows some correlation with Bautz-Morgan type. Radial analysis of the clusters indicates that the dwarf galaxies are less centrally clustered than the giants, and they form a significant halo around clusters. We conclude that measurements of the total cluster luminosity distribution based on the inner core alone are likely to be severe underestimates of the dwarf component, the integrated cluster luminosity and the contribution of galaxy masses to the cluster's total mass. Further work is required to quantify this. The observational evidence that the unrelaxed, lower density outer regions of clusters are dwarf-rich adds credence to the recent evidence and conjecture that the field is a predominantly dwarf-rich environment, and that the dwarf galaxies are under-represented in measures of the local field luminosity function.