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 and S0 galaxies in the rich cluster CL1358+62 at z = 0.33, and introduce techniques for fitting stellar population models to the data. These data and methods will be used further in a larger study of the evolution of absorption line strengths in intermediate redshift clusters. Here we focus on the 19 elliptical and lenticular galaxies with an homogeneous set of eight blue Lick/IDS indices. These early-type galaxies follow very narrow line strength-line width relations using Balmer and metal lines, indicating a high degree of uniformity in their formation and enrichment histories. We explore these histories using recently published, six-parameter stellar population models (Thomas, Maraston, & Bender 2003a; Thomas, Maraston, & Korn 2004), and describe a novel approach for fitting these models differentially, such that the largest sources of systematic error are avoided. The results of the model fitting are accurate relative measures of the stellar population parameters, with typical formal errors of < ∼ 0.1 dex. The best-fit models yield a mean χ of 1.2 per degree of freedom, indicating that the models provide good descriptions of the underlying stellar populations. We find: (1) no significant differences between the best-fit stellar population parameters of Es and S0s at fixed velocity dispersion; (2) the stellar populations of the Es and S0s are uniformly old, consistent with results previously published using the fundamental plane; (3) a significant correlation of [Z/H] with galaxy velocity dispersion, in a manner consistent with the observed B−V colors of the galaxies, and indicating that dust is not a significant contributor to the colors of early-type galaxies; (4) a possible, modest anti-correlation of [α/Fe] with velocity dispersion, with < 2σ significance, and discrepant with the correlation inferred from data on nearby galaxies at the < 3σ level; and (5) a significant anti-correlation of [α/N] with galaxy velocity dispersion, which we interpret as a correlation of nitrogen enhancement with mean metallicity. Neither [α/C], nor [α/Ca] shows significant variation. 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, as well as the slope of the local Mg-σ relation. Our tests indicate that the inferred population trends do describe real galaxies quite well, and matching our results with published data on nearby galaxies, we infer that the discrepancy stems largely from the historical treatment of broadening corrections to the narrow indices. The data also strongly indicate that secondary nitrogen is an important component in the chemistry of elliptical and lenticular galaxies. Taken together, these results reduce early-type galaxies in clusters to a family with one-parameter, velocity dispersion, greatly simplifying scenarios for their formation and evolution. More specifically, our data conclusively show that cluster S0s did not form their stars at significantly later epochs than cluster ellipticals 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. Subject headings: galaxies: clusters: individual (CL1358+62), galaxies: stellar content, galaxies: elliptical and lenticular, galaxies: evolution