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) 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 > 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 LLS = 0.44 +0.12 -0.11 at d 160 kpc and 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 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 (C III) 0.1 = 0.75 +0.08 -0.13 for W r (977) 0.1 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.