The Distribution of the Dense Clouds in the Starburst Nucleus of NGC 253

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Abstract

We present 2'' ? 4'' resolution aperture synthesis observations of HCN J = 1 ? 0 emission from the starburst galaxy NGC 253. Because of its large critical density, HCN traces dense (nH2 104 cm-3) molecular gas. The emission lies along a central bar in several unresolved (r 30 pc) complexes. At 7'' resolution the HCN J = 1 ? 0/CO J = 1 ? 0 intensity ratio, a measure of density, has a maximum value of 0.2. Excitation models suggest that the HCN-emitting clouds in NGC 253 have high temperatures (100 K), densities (nH2 ~ 104-105 cm-3), and beam-averaged column densities (NH2 ~ 3-10 ? 1022 cm-2). We find a rough association between the star-forming regions in NGC 253 (radio continuum emission) and the molecular cloud complexes. Most of the radio sources have been interpreted as supernova remnants, but several have flat spectral indices typical of thermal emission from H II regions. The radio continuum sources have up to 100 times the radio luminosity of W49, the most luminous H II region complex in the Milky Way. The high cloud temperatures we infer for NGC 253 are most likely due to intense ultraviolet radiation from massive star formation and enhanced cosmic-ray heating by supernovae. The HCN distributions in NGC 253 and the Galactic center are remarkably similar when compared on the same spatial scale. They are asymmetric with longitude, and the peaks lie 100-200 pc away from the nucleus. However, the HCN luminosity and HCN/CO intensity ratio are several times higher in NGC 253 than in the Milky Way. Therefore, the average gas densities and the mass of dense gas are larger in the starburst nucleus of NGC 253.

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