The stellar halos of massive elliptical galaxies. II. Detailed abundance ratios at large radius.
GREENE J.E., MURPHY J.D., GRAVES G.J., GUNN J.E., RASKUTTI S., COMERFORD J.M. and GEBHARDT K.
Abstract (from CDS):
We study the radial dependence in stellar populations of 33 nearby early-type galaxies with central stellar velocity dispersions σ* ≳ 150 km/s. We measure stellar population properties in composite spectra, and use ratios of these composites to highlight the largest spectral changes as a function of radius. Based on stellar population modeling, the typical star at 2Re is old (∼10 Gyr), relatively metal-poor ([Fe/H] ~ -0.5), and α-enhanced ([Mg/Fe] ~ 0.3). The stars were made rapidly at z ~ 1.5-2 in shallow potential wells. Declining radial gradients in [C/Fe], which follow [Fe/H], also arise from rapid star formation timescales due to declining carbon yields from low-metallicity massive stars. In contrast, [N/Fe] remains high at large radius. Stars at large radius have different abundance ratio patterns from stars in the center of any present-day galaxy, but are similar to average Milky Way thick disk stars. Our observations are thus consistent with a picture in which the stellar outskirts are built up through minor mergers with disky galaxies whose star formation is truncated early (z ~ 1.5-2).
galaxies: abundances - galaxies: elliptical and lenticular, cD - galaxies: evolution