First Detection of Ammonia in the Large Magellanic Cloud: The Kinetic Temperature of Dense Molecular Cores in N159W

TL;DR

This study reports the first detection of ammonia in the Large Magellanic Cloud, revealing a surprisingly low kinetic temperature and ammonia abundance, which provides insights into the molecular environment of star-forming regions in low-metallicity galaxies.

Contribution

It presents the first detection of ammonia in the Magellanic Clouds and characterizes the physical conditions of dense molecular cores in N159W.

Findings

Detected ammonia lines in N159W with a kinetic temperature of ~16K.

Ammonia abundance is significantly lower than in Galactic regions.

The low ammonia abundance is influenced by low nitrogen levels and high UV flux.

Abstract

The first detection of ammonia (NH3) is reported from the Magellanic Clouds. Using the Australia Telescope Compact Array, we present a targeted search for the (J,K) = (1,1) and (2,2) inversion lines towards seven prominent star-forming regions in the Large Magellanic Cloud (LMC). Both lines are detected in the massive star-forming region N159W, which is located in the peculiar molecular ridge south of 30 Doradus, a site of extreme star formation strongly influenced by an interaction with the Milky Way halo. Using the ammonia lines, we derive a kinetic temperature of ~16K, which is 2-3 times below the previously derived dust temperature. The ammonia column density, averaged over ~17" is ~6x10^{12} cm^{-2} <1.5x10^{13} cm^{-2} over 9" in the other six sources) and we derive an ammonia abundance of ~4x10^{-10} with respect to molecular hydrogen. This fractional abundance is 1.5-5 orders of magnitude below those observed in Galactic star-forming regions. The nitrogen abundance in the LMC (~10% solar) and the high UV flux, which can photo-dissociate the particularly fragile NH3 molecule, must both contribute to the low fractional NH3 abundance, and we likely only see the molecule in an ensemble of the densest, best shielded cores of the LMC.