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

Abstract

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-circle dot 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 less than or similar to 160 kpc. We measure a mean covering fraction of optically thick gas with log N(H I) greater than or similar to 17.2 of <kappa >(LLS) = 0.44(-0.11)(+0.12) at d less than or similar to 160 kpc and <kappa >(LLS) = 0.71(-0.20)(+0.11) at d less than or similar to 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 sigma (< vgas-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 <kappa(C III)>(0.1) = 0.75(-0.13)(+0.08) for W-r(977) >= 0.1 angstrom 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.