Charting the evolution of the ages and metallicities of massive galaxies since z=0.7

The stellar populations of intermediate-redshift galaxies can shed light onto the growth of massive galaxies in the last 8 billion years. We perform deep, multi-object rest-frame optical spectroscopy with IMACS/Magellan of ~70 galaxies in the E-CDFS with redshift 0.6522.7 and stellar mass >10^{10}Msun. Following the Bayesian approach adopted for previous low-redshift studies, we constrain the stellar mass, mean stellar age and stellar metallicity of individual galaxies from stellar absorption features. We characterize for the first time the dependence of stellar metallicity and age on stellar mass at z~0.7 for all galaxies and for quiescent and star-forming galaxies separately. These relations for the whole sample have a similar shape as the z=0.1 SDSS analog, but are shifted by -0.28 dex in age and by -0.13 dex in metallicity, at odds with simple passive evolution. We find that no additional star formation and chemical enrichment are required for z=0.7 quiescent galaxies to evolve into the present-day quiescent population. However, this must be accompanied by the quenching of a fraction of z=0.7 Mstar>10^{11}Msun star-forming galaxies with metallicities comparable to those of quiescent galaxies, thus increasing the scatter in age without affecting the metallicity distribution. However rapid quenching of the entire population of massive star-forming galaxies at z=0.7 would be inconsistent with the age/metallicity--mass relation for the population as a whole and with the metallicity distribution of star-forming galaxies only, which are on average 0.12 dex less metal-rich than their local counterparts. This indicates chemical enrichment until the present in at least a fraction of the z=0.7 massive star-forming galaxies.[abridged]Comment: accepted for publication on ApJ, 26 pages, 13 figure

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

[Heavily Abbreviated] In this paper we analyze previously published spectra with high signal-to-noise ratios of E/S0 galaxies in the rich cluster CL1358+62 at z=0.33, and introduce techniques for fitting stellar population models to the data. Here we focus on the 19 E and S0 galaxies with an 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. We find: (1) no differences between the stellar population parameters of Es and S0s, at fixed sigma; (2) the stars in the Es and S0s are uniformly old, consistent with previously published results using M/L ratios; (3) a significant correlation of [Z/H] with sigma, in a manner consistent with the observed B-V colors of the galaxies; (4) no significant correlation of [alpha/Fe] with sigma; and (5) a significant anti-correlation of [alpha/N] with [Z/H], which we interpret as the signature of secondary nitrogen. Neither [alpha/C], nor [alpha/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 sigma does reproduce the B-V colors of the galaxies, as well as the slope of the local Mg-sigma relation. In matching the inferred population trends with published data on nearby galaxies, the line strength-line width relations match well, save for the narrow iron indices. 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.Comment: Accepted for publication in ApJ. 15 figures. (the new version has had some very minor changes, and some more typographical errors fixed