Work

The Stellar Halos of Massive Elliptical Galaxies. Ii. Detailed Abundance Ratios at Large Radius

Year: 2013

Type: article

Abstract: We study the radial dependence in stellar populations of 33 nearby early-type galaxies with central stellar velocity dispersions σ * ≳ 150 km s −1 . 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 2 R e 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).

Source: The Astrophysical Journal

DOI: 10.1088/0004-637x/776/2/64