2010ApJ...716.1263G

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 (κ_max) of cool gas in massive, ≳10¹³ h ^–1 M_☉ 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 σ_z/(1 + z_ph) =~ 0.03. We determine κ_max based on the incidence of Mg II absorption systems that occur within z_ph ±3 σ_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 κ_max ≈ 0.07 at W_r(2796) ≥ 1.0 Å and κ_max ≈ 0.18 at W_r(2796) ≥ 0.5 Å, averaged over 400 h^–1 kpc radius. Our study shows that while cool gas is present in ≳10¹³ h^–1 M_☉halos, the mean covering fraction of strong absorbers is no more than 7%.