The Star Formation History of Luminous Red Galaxies Hosting MgII
  Absorbers
  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.
