Testing PDR models against ISO fine structure line data for extragalactic sources

TL;DR

This study compares observed far-infrared fine structure line fluxes from extragalactic sources with PDR model predictions, highlighting the impact of [O i] 63 micron self-absorption on line ratio discrepancies.

Contribution

It evaluates PDR models against ISO data and incorporates self-absorption effects to improve model-observation agreement.

Findings

Observed line ratios show offsets from models, partly due to [O i] 63 micron self-absorption.

Self-absorption reduces [O i] 63 micron line strength by 20-80%.

High PDR densities and radiation fields best match observed ratios.

Abstract

Far-infrared [C ii] 158 micron, [O i] 145 micron and [O i] 63 micron fine structure emission line fluxes were measured from archival Infrared Space Observatory Long Wavelength Spectrometer spectra of 46 extragalactic sources, with 28 sources providing detections in all three lines. For 12 of the sources, the contribution to the [C ii] 158 micron line flux from H ii regions could be estimated from their detected [N ii] 122 micron line fluxes. The measured [C ii]/[O i] and [O i] 63/145 micron line flux ratios were compared with those from a grid of PDR models previously computed using the UCL PDR code. Persistent offsets between the observed and modelled line ratios could be partly attributed to the effects of [O i] 63 micron self-absorption. Using the SMMOL code, we calculated model [O i] line profiles and found that the strength of the [O i] 63 micron line was reduced by 20-80%, depending on the PDR parameters. We conclude that high PDR densities and radiation field strengths, coupled with the effects of [O i] 63 micron self-absorption, are likely to provide the best match to the observed line flux ratios.