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Galaxy Populations and Evolution in Clusters. II. Defining Cluster Populations

This paper presents quantitative techniques for studying, in an unbiased manner, the photometric and structural properties of galaxies in clusters, including a means to identify likely background objects in the absence of redshift information. We develop self-consistent and reproducible measurements of fundamental properties of galaxies such as radius, surface brightness, concentration of light, and structural asymmetry. We illustrate our techniques through an application to deep UBR images, taken with the WIYN 3.5 m telescope, of the central ∼173 arcmin2 (or 0.3 Mpc × 0.3 Mpc) of the cluster Abell 426 (Perseus). Our techniques allow us to study the properties of the galaxy population in the center of Perseus down to MB = -11. Using these methods, we describe and characterize a well-defined relation between absolute magnitude and surface brightness for galaxy cluster members across the entire wide range of galaxy luminosity from MB = -20 to -11 independent of galaxy type. The galaxies that are assigned by our techniques to the background show no such tight relationship between apparent magnitude and surface brightness, with the exception of those we identify as being members of a background cluster of galaxies at z ∼ 0.55. We, however, find that at the fainter magnitudes, MB > -16, there is a large scatter about the underlying color-magnitude relation defined by the brighter galaxies. Our analysis of galaxies at the center of the Perseus Cluster further indicates that the vast majority are "normal," with little evidence for structural or photometric properties associated with active evolution; we, however, discuss the detailed properties of a handful of unusual galaxies. Finally, the galaxy luminosity function of the Perseus Cluster center is computed, with a derived faint-end slope of α = -1.44 ± 0.04, a value similar to those previously obtained for other nearby rich galaxy clusters.