2013MNRAS.431.2885M

We present a sample of new population of associated absorbers, detected through Neviii λλ770, 780 absorption, in the Hubble Space Telescope (HST)/Cosmic Origins Spectrograph (COS) spectra of intermediate-redshift (0.45 < z < 1.21) quasars (QSOs). Our sample comprised of total 12 associated Neviii systems detected towards eight lines of sight (none of them are radio bright). The incidence rate of these absorbers is found to be 40percent. Majority of the Neviii systems at small ejection velocities (vej) show complete coverage of the background source, but systems with higher vej show lower covering fractions (i.e. fc ≤ 0.8) and systematically higher values of N(Ne VIII). We detect Mgx λλ609, 624 absorption in seven out of the eight Neviii systems where the expected wavelength range is covered by our spectra and is free of any strong blending. We report the detections of Naix λλ681, 694 absorption, for the first time, in three highest ejection velocity (e.g. {verbar}vej{verbar} ≳ 7000 km s-1) systems in our sample. All these systems show very high N(Ne VIII) (i.e. >1015.6 cm-2), high ionization parameter (i.e. logU ≳ 0.5), high metallicity (i.e. Z ≳ Z☉) and ionization-potential-dependent fc values. The observed column density ratios of different ions are reproduced by multiphase photoionization (PI) and/or collisional ionization (CI) equilibrium models. While solar abundance ratios are adequate in CIE, enhancement of Na relative to Mg is required in PI models to explain our observations.

The column density ratios of highly ionized species (i.e. Ovi, Neviii, Mgx, etc.) show a very narrow spread. Moreover, the measured N(Ne VIII)/N(O VI) ratio in the associated absorbers is similar to what is seen in the intervening absorbers. All these suggest a narrow range of ionization parameter in the case of PI or a narrow temperature range (i.e. T ∼ 105.9±0.1 K) in the case of CIE models. The present data do not distinguish between these two alternatives. However, detection of absorption line variability with repeat HST/COS observations will allow us to (i) distinguish between these alternatives, (ii) establish the location of the absorbing gas and (iii) understand the mechanism that provides stability to the multiphase medium. These are important for understanding the contribution of associated Neviii absorbers to the AGN feedback.