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We present a systematic investigation of the circumgalactic medium (CGM)within projected distances d<160 kpc of luminous red galaxies (LRGs). Thesample comprises 16 intermediate-redshift (z=0.21-0.55) LRGs of stellar massM_star>1e11 M_sun. Combining far-ultraviolet Cosmic Origin Spectrograph spectrafrom the Hubble Space Telescope and optical echelle spectra from the groundenables a detailed ionization analysis based on resolved component structuresof a suite of absorption transitions, including the full HI Lyman series andvarious ionic metal transitions. By comparing the relative abundances ofdifferent ions in individually-matched components, we show that cool gas (T~1e4K) density and metallicity can vary by more than a factor of ten in in an LRGhalo. Specifically, metal-poor absorbing components with <1/10 solarmetallicity are seen in 50% of the LRG halos, while gas with solar andsuper-solar metallicity is also common. These results indicate a complexmultiphase structure and poor chemical mixing in these quiescent halos. Wecalculate the total surface mass density of cool gas, \Sigma_cool, by applyingthe estimated ionization fraction corrections to the observed HI columndensities. The radial profile of \Sigma_cool is best-described by a projectedEinasto profile of slope \alpha=1 and scale radius r_s=48 kpc. We find thattypical LRGs at z~0.4 contain cool gas mass of M_cool= (1-2) x1e10 M_sun atd<160 kpc (or as much as 4x1e10 M_sun at d<500 kpc), comparable to the cool CGMmass of star-forming galaxies. Furthermore, we show that high-ionization OVIand low-ionization absorption species exhibit distinct velocity profiles,highlighting their different physical origins. We discuss the implications ofour findings for the origin and fate of cool gas in LRG halos.

Characterizing Circumgalactic Gas around Massive Ellipticals at z~0.4 - II. Physical Properties and Elemental Abundances