We present the first results of an ongoing spectroscopic follow-up of close luminous red galaxy ( LRGs ) and Mg II \lambda \lambda 2796,2803 absorber pairs for an initial sample of 15 photometrically selected LRGs at physical projected separations \rho \lesssim 350 h ^ { -1 } ~ { } { kpc } ~ { } Â from a QSO sightline .
Our moderate-resolution spectra confirm a physical association between the cool gas ( T \sim 10 ^ { 4 } K ) revealed by the presence of Mg II 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 , \gtrsim 10 ^ { 13 } h ^ { -1 } { M } _ { \odot } Â dark matter halos hosting LRGs at z \sim 0.5 .
This study is performed using a sample of foreground LRGs that are located at \rho < 400 h ^ { -1 } ~ { } { kpc } ~ { } Â from a QSO sightline .
The LRGs are selected to have a robust photometric redshift \sigma _ { z } / ( 1 + z _ { ph } ) \simeq 0.03 .
We determine \kappa _ { max } based on the incidence of Mg II absorption systems that occur within z _ { ph } \pm 3 \sigma _ { 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 ) \geq 1.0 Ã Â and \kappa _ { max } \approx 0.18 at W _ { r } ( 2796 ) \geq 0.5 Ã , averaged over 400 h ^ { -1 } ~ { } { kpc } ~ { } Â radius .
Our study shows that while cool gas is present in \gtrsim 10 ^ { 13 } h ^ { -1 } { M } _ { \odot } Â halos , the mean covering fraction of strong absorbers is no more than 7 % .