NOAO Fundamental Plane Survey. II. Age and Metallicity along the Red Sequence from Line-Strength Data

We present spectroscopic line-strength data for 4097 red-sequence galaxies in 93 low-redshift galaxy clusters and use these to investigate variations in average stellar populations as a function of galaxy mass. Our analysis includes an improved treatment of nebular emission contamination, which affects ~10% of the sample galaxies. Using the stellar population models of D. Thomas and collaborators, we simultaneously fit 12 observed line-strength-σ relations in terms of common underlying trends of age, [Z/H] (total metallicity), and [α/Fe] (α-element enhancement). We find that the observed line-strength-σ relations can be explained only if higher mass red-sequence galaxies are, on average, older, more metal-rich, and more α-enhanced than lower mass galaxies. Quantitatively, the scaling relations are age ∝ σ0.59±0.13, Z/H ∝ σ0.53±0.08, and α/Fe ∝ σ0.31±0.06, where the errors reflect the range obtained using different subsets of indices. Our conclusions are not strongly dependent on which Balmer lines are used as age indicators. The derived age-σ relation is such that if the largest (σ ~ 400 km s-1) galaxies formed their stars ~13 Gyr ago, then the mean age of low-mass (σ ~ 50 km s-1) objects is only ~4 Gyr. The data also suggest a large spread in age at the low-mass end of the red sequence, with 68% of the galaxies having ages between 2 and 8 Gyr. We conclude that although the stars in giant red galaxies in clusters formed early, most of the galaxies at the faint end joined the red sequence only at recent epochs. This "downsizing" trend is in good qualitative agreement with observations of the red sequence at higher redshifts but is not predicted by semianalytic models of galaxy formation.