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Luminosity Distributions within Rich Clusters - I: A Ubiquitous Dwarf-Rich Luminosity Function ?

From deep CCD observations of the cluster Abell 2554 we have recovered the cluster's luminosity distribution over a wide range of magnitude (-24 < M(R) < -16). We compare the derived A2554 cluster luminosity function (at redshift 0.1) with that of the local Coma Cluster (A1656) and the more distant (z = 0.2) cluster A963. The distribution is remarkably similar for these three clusters of comparable richness and morphology. All show a flat (α= -1.0) luminosity function for the giant galaxies (-24 < M(R) < -19.5) which exhibits a sharp upturn (α= -1.7) at some intermediate magnitude (M(R) = -19) and continues to rise to the limits of existing data. We suggest that such a luminosity function may be ubiquitous among rich clusters and that a similar form may apply for poorer clusters and possibly the field as well. The three cluster dwarf LFs are seen over a range of lookback times covering a quarter of the age of the universe. Therefore the similarity between the three measured LFs seems to rule out strong evolution of the dwarf populations in rich cluster environments, at least out to z = 0.2, unless richness effects conspire to conceal evolutionary changes.