THE STELLAR HALOS OF MASSIVE ELLIPTICAL GALAXIES. II. DETAILED ABUNDANCE RATIOS AT LARGE RADIUS

Abstract

We study the radial dependence in stellar populations of 33 nearby early-type\ngalaxies with central stellar velocity dispersions sigma* > 150 km/s. We\nmeasure stellar population properties in composite spectra, and use ratios of\nthese composites to highlight the largest spectral changes as a function of\nradius. Based on stellar population modeling, the typical star at 2 R_e is old\n(~10 Gyr), relatively metal poor ([Fe/H] -0.5), and alpha-enhanced\n([Mg/Fe]~0.3). The stars were made rapidly at z~1.5-2 in shallow potential\nwells. Declining radial gradients in [C/Fe], which follow [Fe/H], also arise\nfrom rapid star formation timescales due to declining carbon yields from\nlow-metallicity massive stars. In contrast, [N/Fe] remains high at large\nradius. Stars at large radius have different abundance ratio patterns from\nstars in the center of any present-day galaxy, but are similar to Milky Way\nthick disk stars. Our observations are thus consistent with a picture in which\nthe stellar outskirts are built up through minor mergers with disky galaxies\nwhose star formation is truncated early (z~1.5-2).\n