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QSO Absorption Systems Detected in Ne VIII: High-Metallicity Clouds with a Large Effective Cross Section

Using high resolution, high signal-to-noise ultraviolet spectra of the z = 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 NeVIII+OVI absorption line systems at z(abs) =0.68381, 0.70152, 0.72478. In addition to NeVIII and OVI, absorption lines from multiple ionization stages of oxygen (OII, OIII, OIV) 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 NeVIII and OVI, and the gas metallicity of the cool phase ranges from Z = 0.3 Z_{solar} to supersolar. The cool (~10^{4} K) phases have densities n_{H} ~ 10^{-4} cm^{-3} and small sizes (< 4kpc); these cool clouds are likely to expand and dissipate, and the NeVIII may be within a transition layer between the cool gas and a surrounding, much hotter medium. The NeVIII redshift density, dN/dz = 7^{+7}_{-3}, requires a large number of these clouds for every L > 0.1L* galaxy and a large effective absorption cross section (>~ 100 kpc), and indeed, we find a star forming ~L* galaxy at the redshift of the z(abs)=0.72478 system, at an impact parameter of 217 kpc. Multiphase absorbers like these NeVIII systems are likely to be an important reservoir of baryons and metals in the circumgalactic media of galaxies.