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(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 Ovi, absorption lines from multiple ionization stages of oxygen (O ii, 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 approximate to 10(5.7) K) that produces Ne viii and Ovi, and the gas metallicity of the cool phase ranges from Z = 0.3 Z(circle dot) to supersolar. The cool (approximate to 10(4) K) phases have densities n(H) approximate to 10(-4) cm(-3) 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 similar to 7(-3)(+7), requires a large number of these clouds for everyL > 0.1L* galaxy and a large effective absorption cross section (greater than or similar to 100 kpc), and indeed, we find a star-forming similar to L* 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.