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

TL;DR

This study uses ALMA observations at ~0.1 pc resolution to investigate cold methanol gas in the Low Metallicity environment of the Large Magellanic Cloud, focusing on the N79 region near an SSC candidate. Two dense cores, N79S-1 and N79S-2, exhibit CH3OH emission along with other species, enabling rotation-diagram based gas temperatures of 13–15 K and CH3OH abundances of (2.1 ± 1.1)×10^-9 and (4.5 ± 2.5)×10^-10, respectively. Most regions in N79 show non-detections for CH3OH, suggesting widespread inhibition of organic-molecule formation, while the two cores represent exceptional sites of efficient CH3OH production. The results illuminate methanol chemistry in a low-metallicity ISM, with implications for the survival and formation of complex organics in early-universe-like conditions and for understanding the environment around potential SSC progenitors.

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.