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

Far-infrared [C ii] 158-mu m, [O i] 145-mu m and [O i] 63-mu m 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-mu m line flux from H ii regions could be estimated from their detected [N ii] 122-mu m line fluxes. The measured [C ii]/[O i] and [O i] 63/145-mu m line flux ratios were compared with those from a grid of photodissociation region (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-mu m self-absorption. Using the Spherical Multi-Mol (smmol) code, we calculated model [O i] line profiles and found that the strength of the [O i] 63-mu m line was reduced by 20-80 per cent, depending on the PDR parameters. We conclude that high PDR densities and radiation field strengths, coupled with the effects of [O i] 63-mu m self-absorption, are likely to provide the best match to the observed line flux ratios.