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Sulfur chemistry and isotopic ratios in the starburst galaxy NGC 253

Based on observations of the most abundant sulfur-bearing molecules (H2S, CS, NS, SO, H2CS, OCS, and SO2) carried out with the IRAM 30m telescope and 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 towards the nucleus of a starburst galaxy. Source averaged fractional abundances of these molecules are a few 10^-9, except for CS and OCS which are more abundant (10^-8). Sulfur isotopic ratios, 32S/34S~8+-2 and 34S/33S>9, are measured through observations of 13CS, C34S, and C33S. A comparison with the observed relative abundances towards different prototypical Galactic sources suggests that the chemical composition of NGC 253 is similar to that found towards the molecular clouds complexes like Sgr B2 in the nuclear region of the Milky Way. The large overabundance of OCS compared to the predictions of time-dependent sulfur chemistry models supports the idea that OCS is likely injected into the gas phase from the grain mantles by low velocity shocks.