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

Abstract

[abridged] We present interferometric maps of ammonia (NH3) of the nearby starburst galaxy NGC 253 [star formation rate: ~2.8 Mo yr^(-1)]. The observations have been taken with the Australia Telescope Compact Array and include the para-NH3 (1,1), (2,2), and the ortho-NH3 (3,3) and (6,6) inversion lines. Six major complexes of dense ammonia are identified, three of them on either side of the starburst center, out to projected galactocentric radii of \~250 pc. [...] The application of radiative transfer large velocity gradient models reveals that the bulk of the ammonia molecules is embedded in a one-temperature gas phase. Kinetic temperatures of this gas are ~200 and 140 K toward 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 K also warmer toward the south-west than toward the north-east (~15-20 K). This is indicated by the ortho-to-para ammonia ratio which is ~1 and 1.5-2.5 toward the south-west and north-east, respectively. Ammonia column densities in the brightest complexes are in the range of 6-11x10^(14) cm^(-2), which adds up to a total ammonia mass of ~20 Mo, about evenly distributed toward both sides of the nucleus. [...] Toward the center of NGC 253, NH3 (1,1), (2,2), and (6,6) is detected in absorption against an unresolved continuum source. At the same location, however, ammonia (3,3) is found in emission which indicates maser activity. This would be the first detected extragalactic NH3 maser. Evidence for an expanding shell in the south-western complex is provided. [...] The shell and X-ray properties can be reproduced by the energy input of a highly obscured young stellar cluster with a mass of ~10^5 Mo which also heats the dense gas.