Joseph D. Meiring, Todd M. Tripp, Jessica K. Werk, Christopher Howk, Edward B. Jenkins, Jason X. Prochaska, Nicholas Lehner, Kenneth R. Sembach
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]\sim -1.0. Densities of
n(H) \sim 10^-4 cm-3 are determined for the photoionized phase, indicating that
these systems reside in regions of overdensity \Delta \sim 80. The
collisionally ionized component of the gas traced by O VI and Ne VIII bears
temperatures of T\sim 10^5.7 K. We determine the redshift density of Ne VIII
absorbers dN/dz \sim 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\sim0.7, we estimate the
cross section radius of the O VI + Ne VIII absorbers to be \sim70-150 kpc. We
find a star forming \simL\star galaxy at the redshift of the zabs=0.7248
system, at an impact parameter of 217 kpc.
View original:
http://arxiv.org/abs/1201.0939
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