The incidence of cool gas in ~ 1e13 Msun halos
Abstract
We present the first results of an ongoing spectroscopic follow-up of close luminous red galaxy (LRGs) and MgII λλ 2796,2803 absorber pairs for an initial sample of 15 photometrically selected LRGs at physical projected separations 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 (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 < 400 kpc/h from a QSO sightline. The LRGs are selected to have a robust photometric redshift σz/(1+zph) ≈ 0.03. We determine max based on the incidence of MgII absorption systems that occur within zph +/- 3sigmaz 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 max ≈ 0.07 at Wr(2796) 1.0 A and max ≈ 0.18 at Wr(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%.