The Star Formation History of Luminous Red Galaxies Hosting MgII Absorbers

TL;DR

This study investigates the properties of luminous red galaxies near quasars, revealing that their old stellar populations are unlikely to produce MgII absorbers through starburst outflows, and constrains the cool gas covering fraction in their halos.

Contribution

It provides the first spectroscopic analysis comparing MgII absorbing and non-absorbing LRGs, showing similar old stellar populations and constraining the cool gas covering fraction.

Findings

LRGs with and without MgII absorbers share similar old stellar populations.

The cool gas covering fraction in LRG halos is approximately 14%.

Starburst outflows are unlikely to be the source of MgII absorbers around LRGs.

Abstract

We present a spectroscopic sample of z \approx 0.5 luminous red galaxies (LRGs) that are located within physical projected distances \rho \leq 350 kpc/h of a QSO sightline. Of the 37 LRGs in our sample, eight have associated MgII absorbers with rest-frame equivalent width Wr(2796)>0.3A and velocity separation |dv| \leq 350 km/s and 29 do not have associated MgII absorbers to a 2-sigma limit of Wr(2796)=0.3A. We perform a stellar population synthesis analysis using stacked spectra of the MgII absorbing and non-absorbing LRG subsamples. We find that LRGs with or without associated MgII absorbers share similar star formation histories and are best described by old stellar population models (\geq 1 Gyr). Younger stellar populations (\leq 1 Gyr) fail to reproduce their spectra. These findings are consistent with the lack of [OII] emission features in the LRG spectra. The primarily old stellar populations in the LRGs indicate that starburst driven outflows are unlikely to explain the observed MgII absorbers at large distances from the LRGs. In addition, the spectroscopic LRG sample allows us to derive a sensitive constraint for the cool gas covering fraction of <\kappa>=14 +/- 6% in the LRG halos for absorbers of Wr(2796)>0.3A. Finally, we speculate on the origin of the observed MgII absorbers around the LRGs.