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 e t 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 relativ e 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 rat io to be similar to 6.5. The latter value confirms the results of Sage, Mau ersberger & 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 thi s 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. Thi s 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. Su ch a change is consistent with the existence of barred orbits in NGC 253, r ecently postulated by Peng et al.