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NOAO Fundamental Plane Survey -- II. Age and Metallicity along the Red Sequence

We present spectroscopic linestrength 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 twelve observed linestrength-sigma relations in terms of common underlying trends of age, [Z/H] (total metallicity) and a/Fe (alpha-element enhancement). We find that the observed linestrength-sigma relations can be explained only if higher-mass red-sequence galaxies are, on average, older, more metal rich, and more alpha-enhanced than lower-mass galaxies. Quantitatively, the scaling relations are age=sigma^(0.59+/-0.13), Z/H=sigma^(0.53+/-0.08) and a/Fe=sigma^(0.31+/-0.06), where the errors reflect the range obtained using different subsets of indices. 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 "down-sizing" trend is in good qualitative agreement with observations of the red sequence at higher redshifts, but is not predicted by semi-analytic models of galaxy formation.