The deep2 galaxy redshift survey: Mean ages and metallicies of red field galaxies at z ∼ 0.9 from stacked keck deimos spectr

As part of the DEEP2 galaxy redshift survey, we analyze absorption line strengths in stacked Keck DEIMOS spectra of red field galaxies with weak to no emission lines, at redshifts 0.7 ≤ z ≤ 1. Comparison with models of stellar population synthesis shows that red galaxies at z ∼ 0.9 have mean luminosity-weighted ages of the order of only 1 Gyr and at least solar metallicities. These ages cannot be reconciled with a scenario in which all stars evolved passively after forming at very high z. Rather, a significant fraction of stars can be no more than 1 Gyr old, which means that some star formation in the stacked populations continued to at least z ∼ 1.2. Furthermore, a comparison of these distant galaxies with a local SDSS sample, using stellar population synthesis models, shows that the drop in the equivalent width of Hδ from z ∼ 0.9 to 0.1 is less than that predicted by passively evolving models. This admits two interpretations: either each individual galaxy experiences continuing low-level star formation, or the red-sequence galaxy population from z ∼ 0.9 to 0.1 is continually being added to by new galaxies with younger stars.