This project thoroughly maps the spatial distribution of cold atomic hydrogen gas (H I) in the molecular clouds in the interstellar medium of our Galaxy. The formation of cold clouds from the warm ambient interstellar medium is thought to be a vital step to make molecular clouds, without which star formation would not occur. The PI has developed a method to extract detailed structure data of cold HI clouds from existing high-resolution H I surveys using 21cm-line H I self-absorption (HISA) 3-D shadows. The work provides a census of all cold atomic clouds in the Galaxy that are detectable as HISA shadow clouds, and investigate cloud spatial and velocity distribution, relationship to spiral structure, statistical association with CO emission clouds, and other physical properties (such as H2 molecule formation) in cold, quiescent regions that are traditionally difficult to observe. In order to test competing theories of cloud evolution, star formation, and Galactic structure. The observations are compared to numerical simulations of HISA on scales ranging from small interstellar clouds to whole spiral arms, as the HISA observations probe cloud evolution and large-scale Galactic structure. The extracted HISA data cubes, maps, cloud catalogs, and the analysis tools will be made available to the public at the end of the project.
