ATOMIC CARBON AND CARBON-MONOXIDE IN THE NUCLEAR REGION OF NGC-253

We have observed neutral carbon, [CI], and isotopic carbon monoxide (( CO)-O-18 3-->2 and (CO)-O-18 2-->1) towards the nucleus of the starbur st galaxy NGC 253. [CI] is bright and we derive a mean abundance ratio C-0/CO similar to 0.5 across the central similar to 300 pc of the sta rburst. Observations of (CO)-C-13, by Wall et al., indicate that the b ulk of the molecular gas in the starburst region is hot (T similar to 100 K) and dense (n > 10(4) cm(-3)). OUT observations show that (CO)-O -18 traces the smaller fraction of cold molecular gas and, thus, is a poor tracer of the column density of molecular gas in NGC 253. The eff ects of selective photodissociation on CO and its isotopomers means th at it is unwise to use CO to derive reliable estimates of the intrinsi c O-16/O-18 ratio in starburst nuclei. Standard models of photon-domin ated regions (PDRs) can only account for about 5 per cent of the obser ved C-0 emission. As most of the molecular gas is at low visual extinc tion and is hot, the enhanced cosmic ray flux expected in the nucleus of NGC 253 should play little or no part in enhancing the carbon emiss ion with respect to other PDR tracers and CO. The nature of the bulk o f the molecular gas in NGC 253, and the size of the region(similar to 300 pc), rules out time-dependent chemistry across the nucleus as the explanation for the large abundance of atomic carbon. The most plausib le explanation for the brightness of [Cr] emission is that the C/O ele mental ratio is higher in NGC 253 than in the models with which we com pare our observations. NGC 253 shows evidence for a dense torus of gas around the nucleus (R similar to 50 pc) but does not have any inner L indblad resonances. The most likely explanation for the existence of t he torus is pressure confinement of the gas by a superwind.