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The Small Magellanic Cloud (SMC) provides the only laboratory to study the detailed physics of star formation and the interstellar medium relevant to the high redshift universe. Not only is the transition from warm to cold neutral gas a rate limiting step to the formation of molecular gas, but it appears to influence star formation efficiency globally and we still do not understand how metallicity affects this transition. We present first results from a new HI and OH absorption line study using the ATCA to measure the warm-to-cold atomic fraction and the atomic-to-molecular transition in the SMC. The survey includes 55 sources in the SMC, which doubles the number of existing observations with up to 5x greater sensitivity and 4x higher spectral resolution than previous absorption line studies. From these observations, we are able to measure the optical depth, average spin temperature, individual cloud temperatures, and cold atomic gas fraction. These measurements of the optical depth allow us to constrain the amount of ’CO-faint’ gas that is optically thick HI. We find very cold (~20 K) temperatures for individual cold gas clouds and an average cold HI cloud temperature of 30 K, lower than in the Milk Way, and a cold atomic gas fraction of 25% for the SMC, which is similar to the Milky Way and the Local Group. ALMA ACA CO observations of a subset of sources have only revealed detectable CO emission near one source with cold HI indicating that many regions with cold atomic gas have little CO emission and possibly no molecular gas. I will also present preliminary results from a new large (1 deg x 0.5 deg) ALMA ACA CO map of the Southwest Bar of the SMC, which shows previously undetectable small (~ 2 pc) molecular gas clumps are found throughout the region. |
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