Carbon monoxide in the starburst of NGC 253 revisited

Abstract

We present sensitive measurements of 12CO, 13CO and C18O in the J = 1 → 0, 2 → 1 and 3 → 2 lines, and C17O in the J = 1 → 0 and 2→1 lines towards the nucleus of the starburst galaxy NGC 253. Our C18O measurements are consistent with those reported by Harrison et al., but our 13CO J = 3 → 2 measurement is inconsistent with that of Wall et al. The resulting similarity between the 13CO J = 3 → 2/J = 2 → 1 and C18O 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 13CO is co-existent with C18O 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 16O/18O ratio to be ∼ 150 and the 18O/17O ratio to be ∼ 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 × 1018 cm-2 as the beam-averaged column density of CO in the central 23 arcsec. The beam filling factor of CO is ∼ 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 2.5×107 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(H2)/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 12CO 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.