Journal
ASTROPHYSICAL JOURNAL
Volume 693, Issue 1, Pages 486-506Publisher
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/693/1/486
Keywords
galaxies: abundances; galaxies: elliptical and lenticular, cD
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We use a sample of similar to 16,000 non-emission-line galaxies from the Sloan Digital Sky Survey to investigate the physical parameters underlying the well known color-magnitude and color-sigma relations. Galaxies are sorted in terms of velocity dispersions (sigma), luminosity (L), and color, and their spectra are stacked to obtain very high S/N mean spectra for stellar population analysis. This allows us to map mean luminosity-weighted ages, [Fe/H], [Mg/H], and [Mg/Fe] in sigma-L-color space. Our first result is that there are many different red sequences, with age, [Fe/H], [Mg/H], and [Mg/Fe] showing different amounts of slope and scatter when plotted versus sigma, L, or color. These behaviors are explained if the star formation histories of the galaxies populate a two-dimensional parameter space. One parameter is the previously well known increase in age, [Fe/H], [Mg/H], and [Mg/Fe] with sigma. In addition to this, we find systematic variations at fixed sigma, such that more luminous galaxies are younger, more Fe-rich, but have lower [Mg/Fe] than their fainter counterparts. The main sigma trends support a paradigm in which more-massive galaxies form their stars more rapidly and at earlier times than less-massive galaxies. The trends at fixed sigma are consistent with scatter in the duration of star formation for galaxies at a given sigma. The covariation of stellar population properties and L residuals at fixed sigma that we present here has a number of implications: it explains the differing behavior of stellar population indicators when investigated versus sigma as compared to L, and it reveals that L is not as efficient as sigma for indicating galaxy size in stellar population studies.
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