2013ApJ...767...49M

Using high-resolution, high signal-to-noise ultraviolet spectra of the z_em= 0.9754 quasar PG1148+549 obtained with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope, we study the physical conditions and abundances of Ne VIII+O VI absorption line systems at z_abs= 0.68381, 0.70152, 0.72478. In addition to Ne VIII and O VI, absorption lines from multiple ionization stages of oxygen (O II, O III, O IV) are detected and are well aligned with the more highly ionized species. We show that these absorbers are multiphase systems including hot gas (T ~ 10^5.7 K) that produces Ne VIII and O VI, and the gas metallicity of the cool phase ranges from Z = 0.3 Z_☉ to supersolar. The cool (~10⁴ K) phases have densities n_H~ 10^–4/cm3 and small sizes (<4 kpc); these cool clouds are likely to expand and dissipate, and the Ne VIII may be within a transition layer between the cool gas and a surrounding, much hotter medium. The Ne VIII redshift density, dN'/dz ∼ 7_–3⁺⁷, requires a large number of these clouds for every L > 0.1 L* galaxy and a large effective absorption cross section ( ≳ 100 kpc), and indeed, we find a star-forming ∼L ^{sstarf} galaxy at the redshift of the z_abs = 0.72478 system, at an impact parameter of 217 kpc. Multiphase absorbers like these Ne VIII systems are likely to be an important reservoir of baryons and metals in the circumgalactic media of galaxies.