Line Strengths in Early-Type Cluster Galaxies at z = 0.33: Implications for α/Fe, Nitrogen, and the Histories of E/S0s

In this paper we analyze previously published spectra with high signal-to-noise ratios of E/S0s in the rich cluster CL 1358+62 at z = 0.33. Introducing techniques for fitting stellar population models to the data, we focus on the 19 Es and S0s with a homogeneous set of eight blue Lick indices. We explore the galaxy properties using six-parameter stellar population models from the literature and describe an approach for fitting the models differentially, such that the largest systematic errors are avoided. The results of the model fitting are accurate relative measures of the stellar population parameters. We find (1) no difference between the best-fit stellar population parameters of Es and S0s at fixed σ; (2) the stars in Es and S0s are uniformly old, consistent with previously published results using the fundamental plane; (3) a correlation of [Z/H] with σ, in a manner consistent with the observed B - V colors of the galaxies; (4) no strong correlation of [α/Fe] with σ, discrepant with the correlation inferred from data on nearby galaxies at the <3 σ level; and (5) a significant anticorrelation of [α/N] with σ, which we interpret as a correlation of the abundance of secondary nitrogen with mean metallicity. While the differences between our conclusions and the current view of stellar populations may point to serious deficiencies, our deduced correlation of mean metallicity with velocity dispersion does reproduce the observed colors of the galaxies and the slope of the local Mg-σ relation. More specifically, our data conclusively show that cluster S0s did not form their stars at significantly later epochs than cluster elliptical galaxies of the same mass, and the presence of secondary nitrogen indicates that both Es and S0s formed from self-enriching progenitors, presumably with extended star formation histories.