SIMBAD references

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 AV = 10mag, interacting with plane-parallel radiation. The density range in our simulations spans four orders of magnitude (100 ≤ n ≤ 10⁶/cm3) and the UV field strength spans more than three orders of magnitude (10 ≤ χ ≤ 10^4.5 multiples of the 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] 609µm and [Oi] 146µm 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 ∼ 10^4.5-10^5.2cm- 3. We find that the observed fine-structure line ratio of [Ci] 609µm/[Oi] 146µm is reproduced by clouds subject to weaker UV fields compared to the CO lines. We make estimates of the XCO 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.