The incidence of cool gas in ~ 1e13 Msun halos
  We present the first results of an ongoing spectroscopic follow-up of close
luminous red galaxy (LRGs) and MgII {\lambda}{\lambda} 2796,2803 absorber pairs
for an initial sample of 15 photometrically selected LRGs at physical projected
separations {\rho} \le 350 kpc/h from a QSO sightline. Our moderate-resolution
spectra confirm a physical association between the cool gas (T ~ 1e4 K)
revealed by the presence of MgII absorption features and the LRG halo in five
cases. In addition, we report an empirical estimate of the maximum covering
fraction (\kappa_max) of cool gas in massive, \ge 1e13 Msun/h dark matter halos
hosting LRGs at z ~ 0.5. This study is performed using a sample of foreground
LRGs that are located at {\rho} < 400 kpc/h from a QSO sightline. The LRGs are
selected to have a robust photometric redshift \sigma_z/(1+z_ph) \approx 0.03.
We determine \kappa_max based on the incidence of MgII absorption systems that
occur within z_ph +/- 3sigma_z in the spectra of the background QSOs. Despite
the large uncertainties in z_ph, this experiment provides a conservative upper
limit to the covering fraction of cool gas in the halos of LRGs. We find that
\kappa_max \approx 0.07 at W_r(2796) \ge 1.0 A and \kappa_max \approx 0.18 at
W_r(2796) \ge 0.5 A, averaged over 400 kpc/h radius. Our study shows that while
cool gas is present in \ge 1e13 Msun/h halos, the mean covering fraction of
strong absorbers is no more than 7%.
