We present a systematic investigation of the circumgalactic medium ( CGM ) within projected distances d < 160 kpc of luminous red galaxies ( LRGs ) .
The sample comprises 16 intermediate-redshift ( z = 0.21 - 0.55 ) LRGs of stellar mass M _ { \mathrm { star } } > 10 ^ { 11 } \mathrm { M } _ { \odot } .
Combining far-ultraviolet Cosmic Origin Spectrograph spectra from the Hubble Space Telescope and optical echelle spectra from the ground enables a detailed ionization analysis based on resolved component structures of a suite of absorption transitions , including the full \ion HI Lyman series and various ionic metal transitions .
By comparing the relative abundances of different ions in individually-matched components , we show that cool gas ( T \sim 10 ^ { 4 } K ) density and metallicity can vary by more than a factor of ten in an LRG halo .
Specifically , metal-poor absorbing components with < 1 / 10 solar metallicity are seen in 50 % of the LRG halos , while gas with solar and super-solar metallicity is also common .
These results indicate a complex multiphase structure and poor chemical mixing in these quiescent halos .
We calculate the total surface mass density of cool gas , \Sigma _ { \mathrm { cool } } , by applying the estimated ionization fraction corrections to the observed \ion HI column densities .
The radial profile of \Sigma _ { \mathrm { cool } } is best-described by a projected Einasto profile of slope \alpha = 1 and scale radius r _ { s } = 48 kpc .
We find that typical LRGs at z \sim 0.4 contain cool gas mass of \mathrm { \mathit { M } _ { cool } = ( 1 - 2 ) \times 10 ^ { 10 } \mathrm { M _ { \odot } } } at d < 160 kpc ( or as much as \mathrm { \mathit { M } _ { cool } \approx 4 \times 10 ^ { 10 } \mathrm { M _ { \odot } } } at d < 500 kpc ) , comparable to the cool CGM mass of star-forming galaxies .
Furthermore , we show that high-ionization \ion OVI and low-ionization absorption species exhibit distinct velocity profiles , highlighting their different physical origins .
We discuss the implications of our findings for the origin and fate of cool gas in LRG halos .