Dissecting the Red Sequence. II. Star Formation Histories of Early-Type Galaxies Throughout the Fundamental Plane

This analysis uses spectra of ~16,000 nearby Sloan Digital Sky Survey quiescent galaxies to track variations in galaxy star formation histories (SFHs) along and perpendicular to the fundamental plane (FP). We sort galaxies by their FP properties ( sigma , Re , and Ie ) and construct high signal-to-noise ratio mean galaxy spectra that span the breadth and thickness of the FP. From these spectra, we determine mean luminosity-weighted ages, [Fe/H], [Mg/H], and [Mg/Fe] based on single stellar population models using the method described in Graves and Schiavon. In agreement with previous work, the SFHs of early-type galaxies are found to form a two-parameter family. The major trend is that mean age, [Fe/H], [Mg/H], and [Mg/Fe] all increase with sigma . However, no stellar population property shows any dependence on Re at fixed sigma , suggesting that sigma and not dynamical mass (M dyn sigma 2 Re ) is the better predictor of past SFH. In addition to the main trend with sigma , galaxies also show a range of population properties at fixed sigma that are strongly correlated with surface brightness residuals from the FP ( Delta log Ie ), such that higher surface brightness galaxies have younger mean ages, higher [Fe/H], higher [Mg/H], and lower [Mg/Fe] than lower surface brightness galaxies. These latter trends are a major new constraint on SFHs.