THE INCIDENCE OF COOL GAS IN ∼1013 M⊙ HALOS

We present the first results of an ongoing spectroscopic follow-up of close luminous red galaxies (LRGs) and Mg II lambda lambda 2796,2803 absorber pairs for an initial sample of 15 photometrically selected LRGs at physical projected separations rho less than or similar to 350 h(-1) kpc from a QSO sight line. Our moderate-resolution spectra confirm a physical association between the cool gas (T similar to 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, greater than or similar to 10(13) h(-1)M(circle dot) dark matter halos hosting LRGs at z similar to 0.5. This study is performed using a sample of foreground LRGs that are located at rho < 400 h(-1) kpc from a QSO sight line. The LRGs are selected to have a robust photometric redshift sigma(z)/(1 + z(ph)) similar or equal to 0.03. We determine kappa(max) based on the incidence of Mg II absorption systems that occur within z(ph) +/- 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) approximate to 0.07 at W-r(2796) >= 1.0 angstrom and kappa(max) approximate to 0.18 at W-r(2796) >= 0.5 angstrom, averaged over 400 h(-1) kpc radius. Our study shows that while cool gas is present in greater than or similar to 10(13) h(-1) M-circle dot halos, the mean covering fraction of strong absorbers is no more than 7%.