High-ion absorption in the proximate damped Lyman-alpha system toward Q0841+129

TL;DR

This study uses VLT/UVES spectroscopy of a quasar with proximate DLAs to analyze high-ion absorption lines, modeling their origins through photoionization and turbulent mixing layer theories, revealing complex ionization structures.

Contribution

It provides a detailed case study of high-ion absorption in a proximate DLA, comparing models to explain the origin of high ions in such environments.

Findings

High ions show multiple velocity components spanning ~160 km/s.

Both photoionization and turbulent mixing models can reproduce ionic ratios.

Turbulent mixing layers may explain high-ion ratios in a broader DLA sample.

Abstract

We present VLT/UVES spectroscopy of the quasar Q0841+129, whose spectrum shows a proximate damped Lyman-alpha (PDLA) absorber at z=2.47621 and a proximate sub-DLA at z=2.50620, both lying close in redshift to the QSO itself at z_em=2.49510+/-0.00003. This fortuitous arrangement, with the sub-DLA acting as a filter that hardens the QSO's ionizing radiation field, allows us to model the ionization level in the foreground PDLA, and provides an interesting case-study on the origin of the high-ion absorption lines Si IV, C IV, and O VI in DLAs. The high ions in the PDLA show at least five components spanning a total velocity extent of ~160 km/s, whereas the low ions exist predominantly in a single component spanning just 30 km/s. We examine various models for the origin of the high ions. Both photoionization and turbulent mixing layer models are fairly successful at reproducing the observed ionic ratios after correcting for the non-solar relative abundance pattern, though neither model can explain all five components. We show that the turbulent mixing layer model, in which the high ions trace the interfaces between the cool PDLA gas and a hotter phase of shock-heated plasma, can explain the average high-ion ratios measured in a larger sample of 12 DLAs.