The Stellar Halos of Massive Elliptical Galaxies II: Detailed Abundance Ratios at Large Radius

TL;DR

This study examines the stellar populations of 33 nearby massive elliptical galaxies, revealing that their outer regions consist of old, metal-poor, alpha-enhanced stars likely acquired through minor mergers with early-truncated disk galaxies.

Contribution

It provides detailed measurements of abundance ratios at large radii in massive ellipticals, linking stellar population properties to galaxy assembly history and merger processes.

Findings

Outer stars are old (~10 Gyr) and metal-poor ([Fe/H]~-0.5).

Radial gradients in [C/Fe] follow [Fe/H], indicating rapid star formation.

Outer stars resemble Milky Way thick disk stars, suggesting minor mergers with early-truncated disks.

Abstract

We study the radial dependence in stellar populations of 33 nearby early-type galaxies with central stellar velocity dispersions sigma* > 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 2 R_e is old (~10 Gyr), relatively metal poor ([Fe/H] -0.5), and alpha-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 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).