2005ApJ...629..767O

We present interferometric maps of ammonia (NH₃) of the nearby starburst galaxy NGC 253. The observations have been taken with the Australia Telescope Compact Array and include the para-NH₃(1, 1) and (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. Rotational temperatures are derived toward selected individual positions, as well as for the entire southeastern and northwestern molecular complexes. 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 southwest and northeast, respectively. The temperatures under which ammonia was formed in the past are with ≳30 K also warmer toward the southwest than toward the northeast (∼15-20 K). This is indicated by the ortho-ammonia-to-para-ammonia ratio, which is ∼1 and 1.5-2.5 toward the southwest and northeast, respectively. Ammonia column densities in the brightest complexes are in the range of (6-11)x10¹⁴/cm2, which adds up to a total ammonia mass of ∼20 M_☉, about evenly distributed toward both sides of the nucleus. Ammonia abundances relative to H₂are ∼3x10^–8. In the southwestern complex, the ammonia abundances increase from the starburst center to larger galactocentric radii. Toward the center of NGC 253, NH₃(1, 1), (2, 2), and (6, 6) are 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 NH₃maser. Evidence for an expanding shell in the southwestern complex is provided. The shell, with a dynamical age of ∼1.3 Myr, is centered on an X-ray point source that must be located within the dense gas of NGC 253. The shell and X-ray properties can be reproduced by the energy input of a highly obscured young stellar cluster with a mass of ∼10⁵ M_☉, which also heats the dense gas. A current star formation rate of ∼2.8 M_☉/yr is derived for the nuclear starburst in NGC 253 based on its 1.2 cm continuum emission.