Characterizing circumgalactic gas around massive ellipticals at z ~ 0.4-I. Initial results

We present a new Hubble Space Telescope Cosmic Origins Spectrograph (COS) absorptionline survey to study halo gas around 16 luminous red galaxies (LRGs) at z = 0.21-0.55. The LRGs are selected uniformly with stellar mass M_star > 10¹¹M_⊙ and no prior knowledge of the presence/absence of any absorption features. Based on observations of the full Lyman series, we obtain accurate measurements of neutral hydrogen column density N(H I) and find that high-N(H I) gas is common in these massive quiescent haloes with a median of 〈 log N(H I)〉 = 16.6 at projected distances d ≲ 160 kpc. We measure a mean covering fraction of optically thick gas with log N(H I) ≳ 17.2 of 〈 κ 〉 _LLS = 0.44 _-0.11 ^+0.12 at d ≲ 160 kpc and 〈κ 〉_LLS = 0.7 _-0.20 ^1+0.11 at d ≲ 100 kpc. The line-of-sight velocity separations between the HI absorbing gas and LRGs are characterized by a mean and dispersion of 〈 v_gas-gal〉 = 29 km s^-1 and σ〈 v_gas-gal〉 = 171 km s^-1. Combining COS far-ultraviolet and ground-based echelle spectra provides an expanded spectral coverage for multiple ionic transitions, from low-ionization MgII and Si II, to intermediate-ionization SiIII and C III, and to high-ionization OVI absorption lines. We find that intermediate ions probed by C III and Si III are the most prominent UV metal lines in LRG haloes with a mean covering fraction of 〈κ(C III)〉_0.1 = 0.75 _-0.13 ^+0.08 for W_r(977)≥ 0.1 Å at d < 160 kpc, comparable to what is seen for CIII in L* and sub-L* star-forming and red galaxies but exceeding MgII or OVI in quiescent halos. The COS-LRG survey shows that massive quiescent haloes contain widespread chemically enriched cool gas and that little distinction between LRG and star-forming haloes is found in their HI and C III content.