We study the radial dependence in stellar populations of 33 nearby early-type galaxies with central stellar velocity dispersions \sigma _ { \ast } \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { \raise 2. % 0 pt \hbox { $ > $ } } } } 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 ( \sim 10 Gyr ) , relatively metal poor ( [ Fe/H ] \approx - 0.5 ) , and \alpha -enhanced ( [ Mg/Fe ] \approx 0.3 ) .
The stars were made rapidly at z \approx 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 \approx 1.5 - 2 ) .