Cold Atomic filaments in the Galactic plane as traced by HI self-absorption

Jonas Syed

Tuesday, Dec. 6th, 12:50CET

Stars form in the dense interiors of molecular clouds. The dynamics and physical properties of the atomic interstellar medium (ISM) set the conditions under which molecular clouds and eventually stars will form. It is therefore critical to investigate the relationship between the atomic and molecular gas phase of the ISM to understand the global star formation process. Using the high angular resolution data from The HI/OH/Recombination line survey of the Milky Way (THOR), we aim at constraining the kinematic and physical properties of the atomic hydrogen gas toward the inner Galactic plane. HI self-absorption (HISA) has proven to be a viable method to detect cold atomic hydrogen clouds in the Galactic plane. With the help of a newly developed self-absorption extraction routine (astroSABER), we have extended upon previous case studies to identify HI self-absorption toward a sample of Giant Molecular Filaments (GMFs). We find the cold atomic gas to be spatially correlated with the molecular gas in some regions within these filaments. In other regions, however, the molecular gas appears to be decoupled from the cold atomic clouds as no significant spatial correlation is evident. The column densities of the cold atomic gas traced by HISA are usually on the order of 10**20 cm-2 while those of molecular hydrogen traced by 13CO are two orders of magnitude higher. The HISA column densities are attributed to the cold gas that accounts for a fraction of the total atomic gas budget within the clouds. Probing the cold phase of the atomic interstellar medium by the means of self-absorption can better our understanding of the atomic-molecular gas phase transition during cloud formation.