Large-scale shocks in the starburst galaxy NGC 253. Interferometer mapping of a ~ 600 pc SiO/H13CO+ circumnuclear disk

This paper presents the first high-resolution SiO map made in an external galaxy. The nucleus of the nearby barred spiral NGC 253 has been observed simultaneously in the v=0, J=2-1 line of SiO and in the J=1-0 line of H¹³CO⁺ with the IRAM interferometer, with a resolution of 7.5arcsecx 2.6arcsec . Emission from SiO and H¹³CO⁺ is extended in the nucleus of NGC 253. The bulk of the SiO/H¹³CO⁺ emission arises from a (600pcx250pc) circumnuclear disk (CND) with a double ringed structure. The inner ring, of radius r ~ 60pc(4arcsec ), viewed edge-on along PA=51^deg, hosts the nuclear starburst; the outer pseudo-ring opens out as a spiral-like arc up to r ~ 300pc(20arcsec ). The kinematics of the gaseous disk, characterized by strong non-circular motions, is interpreted in terms of the resonant response of the gas to the barred potential. The inner ring would correspond to the inner Inner Lindblad Resonance(iILR), whereas the outer region is linked to the onset of a trailing spiral wave across the outer ILR(oILR). This scenario accounts for the unlike morphology of the maps observed in different molecules. Most notably, we report the detection of a molecular gas counterpart of the giant outflow of hot gas, previously seen in X-ray and optical lines, and tentatively identified as a dust chimney in the the 450mu continuum band. Two gas filaments appear in the SiO map to come out of the plane in the NGC 253 nucleus at r+/-60pc, delimiting the working surfaces of gas entrained by the hot wind. The SiO shows a high average fractional abundance in the CND of < X(SiO)> ~ 1.5x10^-10. This is more than an order of magnitude above the predicted value of a PDR. Moreover, X(SiO) varies at least by an order of magnitude between the inner starburst region, which dominates the global emission, where we derive X(SiO) ~ 1-2x10^-10, and the outer region, where X(SiO) reaches a few 10^-9. SiO abundance is also significantly enhanced in the outflow (X(SiO) ~ 3-5x10^-10). Different mechanisms are explored to explain the unlike chemical processing of molecular gas within the nucleus. Large-scale shocks induced by the crowding of clouds orbits across the oILR in the outer region and by the outflow of hot gas seem to be by large the most efficient mechanisms in rising the Silicon abundance.