The NOAO Fundamental Plane Survey - III. Variations in the stellar populations of red-sequence galaxies from the cluster core to the virial radius

We analyse absorption line-strength indices for ∼3000 red-sequence galaxies in 94 nearby clusters to investigate systematic variations of their stellar content with location in the host cluster. The data are drawn from the National Optical Astronomy Observatory (NOAO) Fundamental Plane Survey. Our adopted method is a generalization of that introduced by Nelan et al. to determine the global age-mass and metallicity-mass relations from the same survey. We find strong evidence for a change in galaxy properties, at fixed mass, over a range from the cluster centre to the virial radius, R 200. For example, red-sequence galaxies further out in the clusters have weaker Mgb5177 (at ∼8σ significance) and stronger Hβ and Hγ absorption (∼3σ, ∼4σ) than galaxies of the same velocity dispersion in the cluster cores. The Fe5270 and Fe5335 indices show only very weak trends with radius. Using a total of 12 indices, the pattern of cluster-centric gradients is considered in light of their different dependences on stellar age and chemical composition. The measured gradients for all 12 indices can be reproduced by a model in which red-sequence galaxies at ∼1-R 200 have on average younger ages (by 15 ± 4 per cent) and lower α-element abundance ratios (by 10 ± 2 per cent) than galaxies of the same velocity dispersion but located near the cluster centres. For the total metallicity, Z/H, no significant gradient is found (2 ± 3 per cent larger at R 200 than in the cores). There are hints that the age trend may be stronger for galaxies of lower mass and/or for galaxies with more discy morphology. We show, however, that the trends cannot be driven primarily by changes in the morphological mix as a function of radius. The cluster-centric age and [α/Fe] gradients are in the sense expected if galaxies in the cluster core were accreted at an earlier epoch than those at larger radii, and if this earlier accretion contributed to an earlier cessation of star formation. The size of the observed age trend is comparable to predictions from semi-analytic models of hierarchical galaxy formation.