The Age and Metallicity Range of Early-Type Galaxies in Clusters

We present an unbiased method for evaluating the ranges of ages and metallicities that are allowed by the photometric properties of the stellar populations that dominate the light of early-type galaxies in clusters. The method is based on the analysis of morphologically classified early-type galaxies in 17 clusters at redshifts 0.3 ≲ z ≲ 0.9 and in the nearby Coma Cluster using recent stellar population synthesis models that span a wide range of metallicities. We confirm that metallicity effects must play a role in the origin of the slope of the color-magnitude relation for cluster early-type galaxies. We show, however, that the small scatter of the color-magnitude relation out to redshifts z ~ 1 does not formally imply a common epoch of major star formation for all early-type galaxies. Instead, it requires that galaxies more recently assembled should be, on average, more metal-rich than older galaxies of similar luminosity. Regardless of the true ages and metallicities of early-type galaxies within the allowed range, their photometric properties and the implied strengths of several commonly used spectral indices are found to be consistent with the apparently passive evolution of the stellar populations. Also, the implied dependence of the mass-to-light ratio on galaxy luminosity is consistent with the observed trend. The results of our unbiased analysis define the boundaries in age and metallicity that must be satisfied by theoretical studies aimed at explaining the formation and evolution of early-type galaxies in clusters.