2011MNRAS.418.2730G

We present a spectroscopic sample of z ≈ 0.5 luminous red galaxies (LRGs) that are located within physical projected distances ρ ≲350H^–1 of a quasi-stellar object (QSO) sightline. Of the 37 LRGs in our sample, eight have associated Mg II absorbers with rest-frame equivalent width W_r(2796)>0.3 Å and velocity separation |Δν|≲350 km/s and 29 do not have associated Mg II absorbers to a 2σ limit of W_r(2796)=0.3 Å. We perform a stellar population synthesis analysis using stacked spectra of the Mg II absorbing and non-absorbing LRG subsamples. We find that LRGs with or without associated Mg II absorbers share similar star formation histories and are best described by old stellar population models (≳ 1 Gyr). Younger stellar populations ( ≲ 1 Gyr) fail to reproduce their spectra. These findings are consistent with the lack of [O II] 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 Mg II 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 #954;= 14±6 per cent in the LRG haloes for absorbers of W_r(2796)>0.3 Å. Finally, we speculate on the origin of the observed Mg II absorbers around the LRGs.

This paper includes data gathered with the 2.5-m du Pont telescope located at Las Campanas Observatory, Chile, and with the Apache Point Observatory 3.5-m telescope, which is owned and operated by the Astrophysical Research Consortium.