Characterizing circumgalactic gas around massive ellipticals at z ∼ 0.4 - II. Physical properties and elemental abundances

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_star> 10^{11} 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 H I 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 ∼ 10⁴ K) density and metallicity can vary by more than a factor of 10 in an LRG halo. Specifically, metal-poor absorbing components with <1/10 solar metallicity are seen in 50 per cent of the LRG haloes, 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 haloes. We calculate the total surface mass density of cool gas, Σ_cool, by applying the estimated ionization fraction corrections to the observed H I column densities. The radial profile of Σ_cool is best described by a projected Einasto profile of slope α = 1 and scale radius r_s = 48 kpc. We find that typical LRGs at z ∼ 0.4 contain cool gas mass of \mathit {M_{cool} = (1-2)× 10^{10} M_\odot } at d < 160 kpc (or as much as \mathit {M_{cool} ≈ 4× 10^{10} M_\odot } at d < 500 kpc), comparable to the cool CGM mass of star-forming galaxies. Furthermore, we show that high-ionization O VI 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 haloes.