The Incidence of Cool Gas in ∼10¹³m☉halos

We present the first results of an ongoing spectroscopic follow-up of close luminous red galaxy (LRGs) and MgII {λ}{λ} 2796,2803 absorber pairs for an initial sample of 15 photometrically selected LRGs at physical projected separations {ρ} ≤ 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 (κ_max) of cool gas in massive, ≥ 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 {ρ} < 400 kpc/h from a QSO sightline. 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 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 κ_max ≈ 0.07 at W_r(2796) ≥ 1.0 A and κ_max ≈ 0.18 at W_r(2796) ≥ 0.5 A, averaged over 400 kpc/h radius. Our study shows that while cool gas is present in ≥ 1e13 Msun/h halos, the mean covering fraction of strong absorbers is no more than 7%.