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We present a model of the photodissociation regions of NGC 4038, which ispart of the Antennae galaxies. We have considered one-dimensional slabs ofuniform density all having a maximum $A_V=10\,{\rm mag}$, interacting withplane-parallel radiation. The density range in our simulations spans fourorders of magnitude ($100\le n\le 10^6\,{\rm cm}^{-3}$) and the UV fieldstrength spans more than three orders of magnitude ($10\le\chi\le10^{4.5}$multiples of the ${\rm Draine}$ field), from which we generated a grid of about1400 simulations. We compare our results with Herschel SPIRE-FTS, CSO andISO-LWS observations of eight CO transition lines ($J=1-0$ to $8-7$) and the[CI] 609\mu m and [OI] 146\mu m fine structure lines. We find that themolecular and atomic emission lines trace different gas components of NGC 4038,thus single emission models are insufficient to reproduce the observed values.In general, low-$J$ CO transition lines correspond to either low densityregions interacting with low UV field strengths, or high density regionsinteracting with high UV field strengths. Higher $J$ CO transition lines areless dependent on the UV field strength and are fitted by gas with density$n\sim10^{4.5}-10^{5.2}\,{\rm cm}^{-3}$. We find that the observed finestructure line ratio of [CI] 609\mu m/[OI] 146\mu m is reproduced by cloudssubject to weaker UV fields compared to the CO lines. We make estimates of the$X_{\rm CO}$ factor which relates the CO emission with the column density ofmolecular hydrogen, and find that it is less than the canonical Milky Wayvalue.

A Photodissociation Region study of NGC 4038