HST/COS observations of a new population of associated QSO absorbers

TL;DR

This study identifies a new population of associated quasar absorbers with high ionization states, revealing their physical properties and ionization mechanisms through HST/COS spectra analysis.

Contribution

First detection of Na IX absorption in associated quasar absorbers, expanding understanding of their ionization conditions and metallicity.

Findings

40% incidence rate of Ne VIII absorbers among quasars

High ionization parameters and metallicities observed in the absorbers

Detection of Na IX in high-velocity systems, indicating complex ionization processes

Abstract

(Abridged) We present a sample of new population of associated absorbers, detected through Ne VIII \lambda\lambda 770,780 absorption, in HST/COS spectra of intermediate redshift (0.45 < z < 1.21) quasars (QSOs). Our sample comprised of total 12 associated Ne VIII systems detected towards 8 lines of sight (none of them are radio bright). The incidence rate of these absorbers is found to be 40%. Majority of the Ne VIII systems at small ejection velocities (v_ej) show complete coverage of the background source, but systems with higher v_ej show lower covering fractions (i.e. f_c < 0.8) and systematically higher values of N(Ne VIII). We detect Mg X \lambda\lambda 609,624 absorption in 7 out of the 8 Ne VIII systems where the expected wavelength range is cover by our spectra and is free of any strong blending. We report the detections of Na IX \lambda\lambda 681,694 absorption, for the first time, in three highest ejection velocity (e.g. v_ej > 7,000 km/s) systems in our sample. All these systems show very high N(Ne VIII) (i.e. > 10^{15.6} cm^{-2}), high ionization parameter (i.e. log U > 0.5), high metallicity (i.e. Z > Z_{\odot}), and ionization potential dependent f_c 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.