ALMA Observations of Cold Methanol Gas in the Large Magellanic Cloud (LMC): N79 South GMC

Abstract

We report ALMA continuum and molecular line observations at 0.1 pc resolution toward the super star cluster (SSC) candidate H72.97-69.39 in the N79 region of the LMC. The continuum emission has a sharp peak around the SSC candidate but is also widely distributed. We identify two continuum sources at the northern (N79S-1) and northwestern (N79S-2) positions of the SSC continuum peak, associated with CH${}_3$OH emission. In addition to CH${}_3$OH, we also detect H${}_2$CO, H${}_2$CS, CS, SO, CO, CN, and CCH at the positions of N79S-1 and N79S-2. The rotation diagram analysis of CH${}_3$OH and SO lines yields an average gas temperature of 13 $\pm$ 0.4 K for N79S-1 and 15 $\pm$ 0.9 K for N79S-2. Most emission lines exhibit line widths of less than 2.8 km s${}^{-1}$, consistent with emissions from cold, dense molecular cloud cores. The abundance of cold CH${}_3$OH gas is estimated to be (2.1 $\pm$ 1.1)$\times$ 10${}^{-9}$ at N79S-1 and (4.5 $\pm$2.5)$\times$ 10${}^{-10}$ at N79S-2. Despite the lower metallicity in the LMC, the CH${}_3$OH abundance at N79S-1 is comparable to that of similar cold sources in our Galaxy. However, the formation of organic molecules is inhibited throughout the N79 regions, as can be seen in the non-detection of CH${}_3$OH in most of the regions. The two positions N79S-1 and N79S-2 would be exceptional positions, where CH${}_3$OH production is efficient. The possible origins of cold CH${}_3$OH gas in these dense cores are discussed, along with a possible explanation for the non-detection of CH${}_3$OH in the SSC candidate.

Accepted for publication in The Astrophysical Journal (ApJ)