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 relations in terms of common underlying trends of age, [Z/H] (total metallicity), and [/Fe] (-element enhancement). We find that the observed line-strength relations can be explained only if higher mass red-sequence galaxies are, on average, older, more metal-rich, and more -enhanced than lower mass galaxies. Quantitatively, the scaling relations are age0.59±0.13, Z/H0.53±0.08, and /Fe0.31±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- relation is such that if the largest (400 km s-1) galaxies formed their stars 13 Gyr ago, then the mean age of low-mass (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

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 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 ~8 sigma significance) and stronger Hbeta and Hgamma absorption (~3 sigma, ~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 ~1 R_200 have on average younger ages (by 15+/-4%) and lower alpha-element abundance ratios (by 10+/-2%), than galaxies of the same velocity dispersion but located near the cluster centre. For the total metallicity, Z/H, no significant gradient is found (2+/-3% larger at R_200 than in the core). The cluster-centric age and alpha/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.Comment: Twenty pages, thirteen figures; accepted by MNRAS; corrected labels to left-hand panel of Figure

NOAO fundamental plane survey. I. Survey design, redshifts, and velocity dispersion data

We introduce the NOAO Fundamental Plane Survey (NFPS), a wide-field imaging/spectroscopic study of rich, low-redshift galaxy clusters. The survey targets X-rayselected clusters at 0.010 < z < 0.067, distributed over the whole sky, with imaging and spectroscopic observations obtained for 93 clusters. This data set will be used in investigations of galaxy properties in the cluster environment and of large-scale velocity fields through the fundamental plane. In this paper, we present details of the cluster sample construction and the strategies employed to select early-type galaxy samples for spectroscopy. Details of the spectroscopic observations are reported. From observations of 5479 red galaxies, we present redshift measurements for 5388 objects and internal velocity dispersions for 4131. The velocity dispersions have a median estimated error 7%. The NFPS has 15% overlap with previously published velocity dispersion data sets. Comparisons to these external catalogs are presented and indicate typical external errors of 8%