Dense gas in nearby galaxies: XV. Hot ammonia in NGC253, Maffei2 and IC342

TL;DR

This study reports the detection of high-excitation ammonia lines in the central regions of nearby galaxies, revealing warm molecular gas components and providing insights into their physical conditions and chemical processes.

Contribution

First detection of ammonia inversion lines up to (6,6) and tentative (9,9) in NGC253, Maffei2, and IC342, highlighting the presence of warm molecular gas in these galaxy centers.

Findings

Warm (100-140K) molecular gas detected in all three galaxies.

Ammonia abundances are similar to those in the Galactic center.

Different gas conditions observed in M82, with cooler, dense, and shielded gas.

Abstract

The detection of NH3 inversion lines up to the (J,K)=(6,6) level is reported toward the central regions of the nearby galaxies NGC253, Maffei2, and IC342. The observed lines are up to 406K (for (J,K)=(6,6)) and 848K (for the (9,9) transition) above the ground state and reveal a warm (T_kin= 100 - 140 K) molecular component toward all galaxies studied. The tentatively detected (J,K)=(9,9) line is evidence for an even warmer (>400K) component toward IC342. Toward NGC253, IC342 and Maffei2 the global beam averaged NH3 abundances are 1-2 10^-8, while the abundance relative to warm H2 is around 10^-7. The temperatures and NH3 abundances are similar to values found for the Galactic central region. C-shocks produced in cloud-cloud collisions can explain kinetic temperatures and chemical abundances. In the central region of M82, however, the NH3 emitting gas component is comparatively cool (~ 30K). It must be dense (to provide sufficient NH3 excitation) and well shielded from dissociating photons and comprises only a small fraction of the molecular gas mass in M82. An important molecular component, which is warm and tenuous and characterized by a low ammonia abundance, can be seen mainly in CO. Photon dominated regions (PDRs) can explain both the high fraction of warm H_2 in M82 and the observed chemical abundances.