Dense gas in nearby galaxies. XVII. The distribution of ammonia in NGC 253, Maffei 2, and IC 342

Context. The central few 100 pc of galaxies often contain large amounts of molecular gas. The chemical and physical properties of these extragalactic star formation regions differ from those in galactic disks, but are poorly constrained.
Aims: This study aims to develop a better knowledge of the spatial distribution and kinetic temperature of the dense neutral gas associated with the nuclear regions of three prototypical spiral galaxies, NGC 253, IC 342, and Maffei 2.
Methods: VLA CnD and D configuration measurements have been made of three ammonia (NH₃) inversion transitions.
Results: The (J,K) = (1, 1) and (2, 2) transitions of NH₃ were imaged toward IC 342 and Maffei 2. The (3, 3) transition was imaged toward NGC 253. The entire flux obtained from single-antenna measurements is recovered for all three galaxies observed. Derived lower limits to the kinetic temperatures determined for the giant molecular clouds in the centers of these galaxies are between 25 and 50 K. There is good agreement between the distributions of NH₃ and other H₂ tracers, such as rare CO isotopologues or HCN, suggesting that NH₃ is representative of the distribution of dense gas. The "Western Peak" in IC 342 is seen in the (6, 6) line but not in lower transitions, suggesting maser emission in the (6, 6) transition.

Appendix A is available in electronic form at http://www.aanda.org