Sulfur chemistry and isotopic ratios in the starburst galaxy NGC 253

On the basis of observations of the most abundant sulfur-bearing molecules (H2S, CS, NS, SO, H2CS, OCS, and SO2) carried out with the IRAM 30 m telescope and the Swedish-ESO Submillimetre Telescope (SEST), we present the first analysis of the sulfur chemistry of an extragalactic source, the nuclear region of the starburst galaxy NGC 253. This is the first time that H2S and, tentatively, H2CS are detected toward the nucleus of a starburst galaxy. Source-averaged fractional abundances of these molecules are a few times 10(-9), except for CS and OCS, which are more abundant (10(-8)). Sulfur isotopic ratios, S-32/S-34 similar to 8 +/- 2 and S-34/S-33 > 9, are measured through observations of (CS)-C-13, (CS)-S-34, and (CS)-S-33. A comparison with the observed relative abundances toward different prototypical Galactic sources suggests that the chemical composition of NGC 253 is similar to that found toward molecular cloud complexes like Sgr B2 in the nuclear region of the Milky Way. The large overabundance of OCS compared with the predictions of time-dependent sulfur chemistry models supports the idea that OCS is likely to have been injected into the gas phase from grain mantles by low-velocity shocks.