2015MNRAS.446.2444H

We report the detection of Neviii in a zabs = 0.599 61 absorber towards the QSO PG1407+265 (zem= 0.94). Besides Neviii, absorption from Hi Lyman series lines (Hi λ1025-λ915), several other low (Cii, Nii, Oii and Sii), intermediate (Ciii, Niii, Niv, Oiii, Siv and Sv) and high (Svi, Ovi and Neviii) ionization metal lines are detected. Disparity in the absorption line kinematics between different ions implies that the absorbing gas comprises of multiple ionization phases. The low and the intermediate ions (except Sv) trace a compact ( ∼ 410 pc), metal-rich (Z ∼ Z_☉) and overdense (lognH ∼ -2.6) photoionized region that sustained star formation for a prolonged period. The high ions, Neviii and Ovi, can be explained as arising in a low density (-5.3 ≤ lognH ≤ -5.0), metal-rich (Z ≳ Z_☉) and diffuse ( ∼ 180 kpc) photoionized gas. The Sv, Svi and Civ [detected in the Faint Object Spectrograph (FOS) spectrum] require an intermediate photoionization phase with -4.2 < lognH < -3.5. Alternatively, a pure collisional ionization model, as used to explain the previous known Neviii absorbers, with 5.65 < logT < 5.72, can reproduce the Svi, Ovi and Neviii column densities simultaneously in a single phase. However, even such models require an intermediate phase to reproduce any observable Sv and/or Civ. Therefore, we conclude that when multiple phases are present, the presence of Neviii is not necessarily an unambiguous indication of collisionally ionized hot gas.