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Paper: Age-Dating the Red Sequence in Low-Redshift Clusters
Volume: 379, Cosmic Frontiers
Page: 117
Authors: Lucey, J.R.; Smith, R.J.; Hudson, M.J.; Nelan, J.E.; Wegner, G.A.
Abstract: The colour-magnitude relation (or red sequence) is a ubiquitous characteristic of rich galaxy clusters. Here we summarize recent results from the NOAO Fundamental Plane Survey that has investigated the origin of the red sequence via absorption line diagnostics for a sample of ~4000 galaxies in nearby rich clusters. Linestrength indices of red-sequence galaxies obey welldefined trends with velocity dispersion, reflecting systematic changes in their stellar populations as a function of mass. The observed relations are best explained if the higher mass red-sequence galaxies are, on average, older, more metal rich, and more α-enhanced than lower mass galaxies. The data also suggest a large spread in age for low-mass red-sequence galaxies. We infer that although the stellar populations in the massive galaxies formed early on, the lower mass galaxies, on average, must have only joined the red sequence at recent epochs, i.e.~4 Gyrs. This “down-sizing” trend is in good qualitative agreement with some observations of the red sequence at higher redshifts. We also investigate the pattern of cluster-centric gradients and find evidence that red-sequence galaxies located at ~1 R200 are younger and have lower α-element ratios than galaxies of the same σ located near the cluster centres. No significant gradient in total metallicity is found.
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