Carbon monoxide in the starburst of NGC 253 revisited

We present sensitive measurements of (CO)-C-12, (CO)-C-13 and (CO)-O-18 in the J = 1 --> 0, 2 --> 1 and 3 --> 2 lines, and (CO)-O-17 in the J = 1 --> 0 and 2 --> 1 lines towards the nucleus of the starburst galaxy NGC 253. Our (CO)-O-18 measurements are consistent with those reported by Harrison et al., but our (CO)-C-13 J = 3 --> 2 measurement is inconsistent with that of Wall et al. The resulting similarity between the (CO)-C-13 J = 3 --> 2/J = 2 --> 1 and (CO)-O-18 J = 3 --> 2/J = 2 --> 1 integrated line intensity ratios indicates that there is little or no evidence for isotope-selective photodissociation of CO in NGC 253, that (CO)-C-13 is co-existent with (CO)-O-18 in the central 23 arcsec (300 pc) of the galaxy, and that the relative abundances of various CO isotopomers can be derived. We have determined the isotopic ratios of oxygen towards the centre of NGC 253. We find the O-16/O-18 ratio to be similar to 150 and the O-18/O-17 ratio to be similar to 6.5. The latter value confirms the results of Sage, Mauersberger & Henkel, and requires that the interstellar medium (ISM) in the core of NGC 253 is substantially enriched by the ejecta from massive stars. We derive 3.5+/-1.5 x 10(18) cm(-2) as the beam-averaged column density of CO in the central 23 arcsec. The beam filling factor of CO is similar to 0.1. This indicates that the average C/CO abundance ratio is 1.4+/-0.7 in this region. We use measurements of the principal gas-phase sinks of carbon to determine that 2.5x10(7) M. is the mass of the ISM within this region. This is consistent with the results of Mauersberger et al., implying that the X = N(H-2)/I(CO) conversion factor in NGC 253 is < 20 per cent of the value determined for the disc of the Milky Way. Our measurements also highlight that there is a radial fall-off in the average molecular gas density in NGC 253, in agreement with that reported by Wall et al. The isophotes of our (CO)-C-12 data show a radial change in ellipticity. Such a change is consistent with the existence of barred orbits in NGC 253, recently postulated by Peng et al.