QSO absorption systems detected in Ne VIII: Evidence for high-metallicity, multiphase gas far from galaxies

Details QSO absorption systems detected in Ne VIII: Evidence for high-metallicity, multiphase gas far from galaxies QSO absorption systems detected in Ne VIII: Evidence for high-metallicity, multiphase gas far from galaxies

2012-01-04

arxiv.org/abs/1201.0939v2

Astronomy and Astrophysics

Astrophysics

Cosmology and Extragalactic Astrophysics

14 pages, submitted to ApJ, comments welcome

Scientific paper

Using high signal-to-noise spectra (S/N 30) taken with the Cosmic Origins Spectrograph (COS) of the z=0.9754 quasar PG1148+549, we report on the physical conditions of Ne VIII + OVI absorption line systems at z=0.6838, 0.7015, 0.7248. At these redshifts, absorption lines from multiple ionization stages of oxygen (O II, O III, O IV, O VI) are available to constrain the ionization and physical conditions of the gas. A single-phase model with only photoionization or collisional ionization fails to reproduce the observed column density ratios. Even with extreme modifications to the ionizing background, single-phase photoionzation models fail to produce enough Ne VIII. We thus conclude that the O VI and Ne VIII are produced via collisional ionization. The metallicities of these systems are determined to be [O/H] >-0.5, higher than the typically assumed value for O VI absorbers of [O/H] ~ -1.0. Densities of n(H) ~ 10^-4 cm-3 are determined for the photoionized phase, indicating that these systems reside in regions of overdensity \Delta ~ 80. The collisionally ionized component of the gas traced by O VI and Ne VIII bears temperatures of T ~ 10^5.7 K. We determine the redshift density of Ne VIII absorbers dN/dz ~ 7 (+7,-3), similar to the redshift density of O VI absorbers of comparable equivalent width to these systems. Based on the redshift density and galaxy luminosity function at z~0.7, we estimate the cross section radius of the O VI + Ne VIII absorbers to be ~70-150 kpc. We find a star forming ~L* galaxy at the redshift of the zabs=0.7248 system, at an impact parameter of 217 kpc.

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