The Color-magnitude Effect in Early-type Cluster Galaxies

We present the analysis of the color-magnitude relation (CMR) for a sample of 57 X-ray detected Abell clusters within the redshift interval 0.02 ≤ z ≤ 0.18. We use the B − R vs R color-magnitude plane to establish that the CMR is present in all our low-redshift clusters and can be parameterized by a single straight line. We find that the CMRs for this large cluster sample of different richness and cluster types are consistent with having universal properties. The k-corrected color of the individual CMRs in the sample at a fixed absolute magnitude have a small intrinsic dispersion of ∼ 0.05 mag. The slope of the CMR is consistent with being the same for all clusters, with the variations entirely accountable by filter band shifting effects. We determine the mean of the dispersion of the 57 CMRs to be 0.074 mag, with a small rms scatter of 0.026 mag. However, a modest amount of the dispersion arises from photometric measurement errors and possible background cluster superpositions; and the derived mean dispersion is an upper limit. Models which explain the CMR in terms of metallicity and passive evolution can naturally reproduce the observed behavior of the CMR in this paper. The observed properties of the CMR are consistent with models in which the last episode of significant star formation in cluster early-type galaxies occurred significantly more than ∼ 3 Gyr ago, and that the core set of early-type galaxies in clusters were formed more than 7 Gyr ago. The universality of the CMR provides us with an important tool for cluster detection and redshift estimation. A very accurate photometric cluster redshift estimator can be devised based on the apparent color-shift of the CMR due to redshift. This calibrator has the additional advantage of being very efficient since only two bands are needed. An empirically calibrated redshift estimator based on the B−R color of the CMR for clusters with z < 0.20 produces an accuracy of ∆z ∼ 0.010. Background clusters, typically at z ∼ 0.25 and previously unknown, are found in this survey in the color-magnitude diagrams as secondary CMRs to the red of the target cluster CMRs. We also find clear cases of apparent X-ray substructure which are due to these cluster superpositions. This suggests that X-ray observations of clusters are also subject to a significant amount of projection contaminations. Subject headings: Galaxies: clusters: general — galaxies: elliptical and lenticular, cDs — galaxy: formation — galaxies: photometry