Journal Article

The stellar populations of bright coma cluster galaxies

J. Price, S. Phillipps, A. Huxor, R. J. Smith and J. R. Lucey

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 411, issue 4, pages 2558-2585
Published in print March 2011 | ISSN: 0035-8711
Published online March 2011 | e-ISSN: 1365-2966 | DOI:
The stellar populations of bright coma cluster galaxies

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In this paper we study the stellar populations of 356 bright, Mr≤−19, Coma cluster members located in a 2° field centred on the cluster core using Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) spectroscopy. We find ∼31 per cent of the sample have significant emission in Hβ, [O iii]5007, Hα or [N ii]6585, due to star formation or active galactic nuclei (AGN)/LINER activity. The remaining portion of the sample we describe as passive or quiescent. Using line-ratio diagnostics, we find the fraction of galaxies displaying AGN/LINER type emission increases with increasing galaxy luminosity while the star-forming fraction decreases. For the quiescent galaxies we find strong correlations between absorption-line index strength and velocity dispersion (σ) for CN2, C4668, Mgb and Hβ. Employing a planar analysis technique that factors out index correlations with σ, we find significant cluster-centric radial gradients in Hβ, Mgb and C4668 for the passive galaxies. We use state-of-the-art stellar population models and the measured absorption-line indices to infer the single-stellar-population-equivalent (SSP-equivalent) age and [Fe/H] for each galaxy, as well as their abundance patterns in terms of [Mg/Fe], [C/Fe], [N/Fe] and [Ca/Fe]. For the passive galaxy subsample we find strong evidence for ‘archaeological downsizing’, with age ∝σ0.90 ± 0.06. This trend is shown to be robust against variations in sample selection criteria (morphologically early-type versus spectroscopically quiescent), emission-line detection thresholds, index velocity broadening corrections and the specific SSP model employed. Weaker positive correlations are obtained between σ and all other measured stellar population parameters. We recover significant cluster-centric radial stellar population gradients for the passive sample in SSP-equivalent age, [Mg/Fe], [C/Fe] and [N/Fe]. These trends are in the sense that, at fixed velocity dispersion, passive galaxies on the outskirts of the cluster are 24 ± 9 per cent younger with lower [Mg/Fe] and [N/Fe] but higher [C/Fe] than those in the cluster core. We find no significant increase in cluster-centric radial stellar population gradients when fitting to a passive galaxy subset selected to cover the cluster core and south-west region, which contains the NGC 4839 subgroup. Thus, we conclude that the NGC 4839 infall region is not unique, at least in terms of the stellar populations of bright galaxies. We speculate that the more pronounced cluster-centric radial gradients seen by other recent studies may be attributed to the luminosity range spanned by their samples, rather than to limited azimuthal coverage of the cluster. Finally, for our passive sample we have found an age–metallicity anti-correlation which cannot be accounted for by correlated errors.

Keywords: surveys; galaxies: clusters: individual: Coma; galaxies: evolution; galaxies: stellar content

Journal Article.  20142 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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