A Photodissociation Region study of NGC 4038
  We present a model of the photodissociation regions of NGC 4038, which is
part of the Antennae galaxies. We have considered one-dimensional slabs of
uniform density all having a maximum $A_V=10\,{\rm mag}$, interacting with
plane-parallel radiation. The density range in our simulations spans four
orders of magnitude ($100\le n\le 10^6\,{\rm cm}^{-3}$) and the UV field
strength 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 about
1400 simulations. We compare our results with Herschel SPIRE-FTS, CSO and
ISO-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 the
molecular 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 density
regions interacting with low UV field strengths, or high density regions
interacting with high UV field strengths. Higher $J$ CO transition lines are
less 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 fine
structure line ratio of [CI] 609\mu m/[OI] 146\mu m is reproduced by clouds
subject 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 of
molecular hydrogen, and find that it is less than the canonical Milky Way
value.
