We present VLT/UVES spectroscopy of the quasar Q0841+129 , whose spectrum shows a proximate damped Ly- \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 \pm 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 \approx 160 km s ^ { -1 } , whereas the low ions exist predominantly in a single component spanning just 30 km s ^ { -1 } .
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 .