Large-scale shocks in the starburst galaxy NGC253. Interferometer mapping of a 600 pc SiO/H13CO+ circumnuclear disk

摘要

This paper presents the first high-resolution SiO map made in an externalgalaxy. The nucleus of the nearby barred spiral NGC253 has been observedsimultaneously 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 7.5"x2.6". Emission from SiOand H13CO+ is extended in the nucleus of NGC253. The bulk of the SiO/H13CO+emission arises from a 600pcx250pc circumnuclear disk (CND) with a doubleringed structure. The inner ring, of radius r=60pc, viewed edge-on alongPA=51deg, hosts the nuclear starburst; the outer pseudo-ring opens out as aspiral-like arc up to r=300pc. The kinematics of the gaseous disk,characterized by strong non-circular motions, is interpreted in terms of theresonant response of the gas to the barred potential. The inner ring wouldcorrespond to the inner Inner Lindblad Resonance(iILR), whereas the outerregion is linked to the onset of a trailing spiral wave across the outer ILR(oILR). Most notably, we report the detection of a molecular gas counterpart ofthe giant outflow of hot gas, previously seen in X-ray and optical lines, andtentatively identified as a dust chimney in the the 450mu continuum band.TheSiO shows a high average fractional abundance in the CND of X(SiO)=1.5x10E-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 innerstarburst region, which dominates the global emission, where we deriveX(SiO)=1--2x10E-10, and the outer region, where X(SiO) reaches a few 10E-9. SiOabundance is also significantly enhanced in the outflow (X(SiO)=3--5x10E-10).Different mechanisms are explored to explain the unlike chemical processing ofmolecular gas within the nucleus.