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

Abstract

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 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, d {N}/dz\sim 7^{+7}_{-3}, requires a large number of these clouds for every L > 0.1 L* galaxy and a large effective absorption cross section (gsim 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.

Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program GO11741.