Determine the CNM–UNM transition temperature in the LMC

Determine the temperature boundary between the cold neutral medium and thermally unstable neutral medium in the Large Magellanic Cloud, accounting for its lower metallicity and stronger ultraviolet radiation field.

Background

The paper classifies absorption components with spin temperatures below 250 K as cold neutral medium and those above 250 K as thermally unstable neutral medium, following a convention developed for Milky Way observations. However, the authors note that the LMC’s lower metallicity and stronger ultraviolet radiation field may shift the CNM–UNM boundary to lower temperatures.

This uncertainty affects the classification of intermediate-temperature components: the authors identify 34 components with spin temperatures above 100 K, including several near 100–200 K, but regard only the two components near 600 K as robustly thermally unstable. Establishing the appropriate transition temperature would improve measurements of the relative CNM and UNM content in the LMC and clarify how the thermal phase balance changes with metallicity and radiation environment.

References

However, this criterion may not be directly applicable to the LMC. As shown in Figure~\ref{fig:cdf_comp}, the spin temperatures in the LMC are systematically lower than those in the Milky Way, suggesting that the boundary between CNM and UNM may also shift to lower temperatures in a lower metallicity and higher UV radiation environment. Quantifying this shift is challenging. In our sample, we identify 34 components with $T_s > 100$ K, including 4 with $T_s \sim 200$ K and 2 with $T_s \sim 600$ K. While the classification of components with moderate temperatures ($T_s \sim 100$–200 K) remains uncertain, the two components with $T_s \sim 600$ K are well above any plausible CNM–UNM boundary and can be robustly identified as UNM.

— The GASKAP-HI Survey towards the Magellanic Clouds: Cold Atomic Gas Survival and Evolution in the Large Magellanic Cloud  (2608.21115 - Chen et al., 21 Aug 2026) in Section 4.4, “Unstable neutral medium” (Section \ref{sec:unm})