First Detection of Ammonia in the Large Magellanic Cloud: The Kinetic Temperature of Dense Molecular Cores in N 159 W

The first detection of ammonia (NH_3) 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 toward seven prominent star-forming regions in the Large Magellanic Cloud (LMC). Both lines are detected in the massive star-forming region N 159 W, 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 ~16 K, which is 2-3 times below the previously derived dust temperature. The ammonia column density, averaged over ~17", is ~6 × 10^(12) cm^(–2) (<1.5 × 10^(13) cm^(–2) over 9" in the other six sources) and we derive an ammonia abundance of ~4 × 10^(–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 NH_3 molecule, both must 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.