AGES AND ABUNDANCES OF RED SEQUENCE GALAXIES AS A FUNCTION OF LINER EMISSION-LINE STRENGTH

摘要

Although the spectrum of a prototypical early-type galaxy is assumed to lack emission lines, a substantial fraction (likely as high as 30%) of nearby red sequence galaxy spectra contain emission lines with line ratios characteristic of LINERs. We use spectra of ～6000 galaxies from the SDSS in a narrow redshift slice (0.06 ＜ z ＜ 0.08) to compare the stellar populations of red sequence galaxies with and without LINER-like emission. The spectra are binned by internal velocity dispersion (σ) and by emission properties to produce high-S/N stacked spectra. The recent stellar population models of R. Schiavon make it possible to measure ages, [Fe/H], and individual elemental abundance ratios [Mg/Fe], [C/Fe], [N/Fe], and [Ca/Fe] for each of the stacked spectra. We find that red sequence galaxies with strong LINER-like emission are systematically 2-3.5 Gyr (10%-40%) younger than their emission-free counterparts at the same σ. This suggests a connection between the mechanism powering the emission (whether AGN, post-AGB stars, shocks, or cooling flows) and more recent star formation in the galaxy. We find that mean stellar age and [Fe/H] increase with σ for all galaxies. Elemental abundance [Mg/Fe] increases modestly with σ in agreement with previous results, and [C/Fe] and [N/Fe] increase more strongly with σ than does [Mg/Fe]. [Ca/Fe] appears to be roughly solar for all galaxies. At fixed σ galaxies with fainter r-band luminosities have lower [Fe/H] and older ages but similar abundance ratios compared to brighter galaxies.