THE INCIDENCE OF COOL GAS IN ∼1013 Msun HALOS

We present the first results of an ongoing spectroscopic follow-up of close luminous red galaxies (LRGs) and Mg II λλ2796,2803 absorber pairs for an initial sample of 15 photometrically selected LRGs at physical projected separations ρ ∼< 350 h -1 kpc from a QSO sight line. Our moderate-resolution spectra confirm a physical association between the cool gas (T ∼ 104 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 (κmax) of cool gas in massive, ∼>1013 h -1 Msun dark matter halos hosting LRGs at z ∼ 0.5. This study is performed using a sample of foreground LRGs that are located at ρ < 400 h -1 kpc from a QSO sight line. The LRGs are selected to have a robust photometric redshift σz/(1 + zph) ≅ 0.03. We determine κmax based on the incidence of Mg II absorption systems that occur within zph ± 3 σz in the spectra of the background QSOs. Despite the large uncertainties in zph, this experiment provides a conservative upper limit to the covering fraction of cool gas in the halos of LRGs. We find that κmax ∼ 0.07 at Wr (2796) ≥ 1.0 A and κmax ∼ 0.18 at Wr (2796) ≥ 0.5 A, averaged over 400 h -1 kpc radius. Our study shows that while cool gas is present in ∼>1013 h -1 Msun halos, the mean covering fraction of strong absorbers is no more than 7%.