Characterizing Circumgalactic Gas around Massive Ellipticals at z ~ 0.4
  I. Initial Results
  We present a new Hubble Space Telescope (HST) 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
Mstar>1e11 Msun 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(HI) and find that
high-N(HI) gas is common in these massive quiescent halos with a median of <log
N(HI)> = 16.6 at projected distances d<~160 kpc. We measure a mean covering
fraction of optically-thick gas with log N(HI)>~17.2 of
<kappa>LLS=0.44^{+0.12}_{-0.11} at d<~160 kpc and
<kappa>LLS=0.71^{+0.11}_{-0.20} 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 and \sigma_v_{gas-gal}=171 km/s.
Combining COS FUV and ground-based echelle spectra provides an expanded
spectral coverage for multiple ionic transitions, from low-ionization MgII and
SiII, to intermediate ionization SiIII and CIII, and to high-ionization OVI
absorption lines. We find that intermediate ions probed by CIII and SiIII are
the most prominent UV metal lines in LRG halos with a mean covering fraction of
<kappa(CIII)>_{0.1}=0.75^{+0.08}_{-0.13} for W(977)>=0.1 Ang 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 halos contain widespread chemically-enriched cool gas
and that little distinction between LRG and star-forming halos is found in
their HI and CIII content.
