A large 12C/13C isotopic ratio in M 82 and NGC 253

Aims. Our aim is to derive carbon isotopic ratios from optically thin tracers in the central regions of the starburst galaxies M 82 and NGC 253. Methods. We present high-sensitivity observations of CCH and two of its C-13 isotopologues, (CCH)-C-13 and (CCH)-C-13, as well as the optically thin emission from (CO)-O-18 and (CO)-C-13-O-18. We assume the column density ratio between isotopologues is representative of the C-12/C-13 isotopic ratio. Results. From CCH, lower limits to the C-12/C-13 isotopic ratio of 138 in M 82, and 81 in NGC 253, are derived. Lower limits to the C-12/C-13 ratios from CO isotopologues support these. (CO)-C-13-O-18 is tentatively detected in NGC 253, which is the first reported detection in the extragalactic ISM. Based on these limits, we infer ratios of O-16/O-18> 350 and > 300 in M 82 and NGC 253, respectively, and S-32/S-34 > 16 in NGC 253. The derived CCH fractional abundances toward these galaxies of less than or similar to 1.1 x 10(-8) agree well with those of molecular clouds in the Galactic disk. Conclusions. Our lower limits to the C-12/C-13 ratio from CCH are a factor of 2-3 larger than previous limits. The results are discussed in the context of molecular and nucleo-chemical evolution. The high C-12/C-13 isotopic ratio of the molecular ISM in these starburst galaxies suggest that the gas has been recently accreted toward their nuclear regions.