The Temperature Distribution of Dense Molecular Gas in the Center of NGC 253

[abridged] We present interferometric maps of ammonia (NH3) of the nearbystarburst galaxy NGC 253 [star formation rate: ~2.8 Mo yr^(-1)]. Theobservations have been taken with the Australia Telescope Compact Array andinclude the para-NH3 (1,1), (2,2), and the ortho-NH3 (3,3) and (6,6) inversionlines. Six major complexes of dense ammonia are identified, three of them oneither side of the starburst center, out to projected galactocentric radii of\~250 pc. [...] The application of radiative transfer large velocity gradientmodels reveals that the bulk of the ammonia molecules is embedded in aone-temperature gas phase. Kinetic temperatures of this gas are ~200 and 140 Ktoward the south-west and north-east [of the nucleus of NGC 253], respectively.The temperatures under which ammonia was formed in the past are with >~30 Kalso warmer toward the south-west than toward the north-east (~15-20 K). Thisis indicated by the ortho-to-para ammonia ratio which is ~1 and 1.5-2.5 towardthe south-west and north-east, respectively. Ammonia column densities in thebrightest complexes are in the range of 6-11x10^(14) cm^(-2), which adds up toa total ammonia mass of ~20 Mo, about evenly distributed toward both sides ofthe nucleus. [...] Toward the center of NGC 253, NH3 (1,1), (2,2), and (6,6) isdetected in absorption against an unresolved continuum source. At the samelocation, however, ammonia (3,3) is found in emission which indicates maseractivity. This would be the first detected extragalactic NH3 maser. Evidencefor an expanding shell in the south-western complex is provided. [...] Theshell and X-ray properties can be reproduced by the energy input of a highlyobscured young stellar cluster with a mass of ~10^5 Mo which also heats thedense gas.Comment: 42 pages including 13 figures, accepted for publication in Ap