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

We present a new Hubble Space Telescope Cosmic Origins Spectrograph (COS) absorption-line 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>10^{11} M_{⊙} 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^{+0.12}_{-0.11} at d ≲ 160 kpc and < κ > _LLS=0.71^{+0.11}_{-0.20} at d ≲ 100 kpc. The line-of-sight velocity separations between the H I 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 O VI 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^{+0.08}_{-0.13} for W_r(977) ⩾ 0.1 Å at d < 160 kpc, comparable to what is seen for C III in L_* and sub-L_* star-forming and red galaxies but exceeding Mg II or O VI 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.