We analyse absorption line-strength indices for \sim 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 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 \sim 8 \sigma significance ) and stronger H \beta and H \gamma absorption ( \sim 3 \sigma , \sim 4 \sigma ) 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 twelve 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 twelve indices can be reproduced by a model in which red-sequence galaxies at \sim 1 R _ { 200 } have on average younger ages ( by 15 \pm 4 % ) and lower \alpha -element abundance ratios ( by 10 \pm 2 % ) , 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 \pm 3 % 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 disky morphology .
We show , however , that the trends can not be driven primarily by changes in the morphological mix as a function of radius .
The cluster-centric age and [ \alpha /Fe ] Â 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 .