2010A&A...522A..62M

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. We present high-sensitivity observations of CCH and two of its ¹³C isotopologues, C¹³CH and ¹³CCH, as well as the optically thin emission from C¹⁸O and ¹³C¹⁸O. We assume the column density ratio between isotopologues is representative of the ¹²C/¹³C isotopic ratio. From CCH, lower limits to the ¹²C/¹³C isotopic ratio of 138 in M 82, and 81 in NGC 253, are derived. Lower limits to the ¹²C/¹³C ratios from CO isotopologues support these. ¹³C¹⁸O is tentatively detected in NGC 253, which is the first reported detection in the extragalactic ISM. Based on these limits, we infer ratios of ¹⁶O/¹⁸O>350 and >300 in M 82 and NGC 253, respectively, and ³²S/³⁴S>16 in NGC 253. The derived CCH fractional abundances toward these galaxies of ≲1.1x10^–8 agree well with those of molecular clouds in the Galactic disk. Our lower limits to the ¹²C/¹³C 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 ¹²C/¹³C isotopic ratio of the molecular ISM in these starburst galaxies suggest that the gas has been recently accreted toward their nuclear regions.