%0 Generic
%A Syed, Jonas Ibrahim
%C Heidelberg
%D 2023
%F heidok:33552
%R 10.11588/heidok.00033552
%T Atomic cloud formation processes - Constraining the properties of the atomic interstellar medium by means of HI emission and HI self-absorption
%U https://archiv.ub.uni-heidelberg.de/volltextserver/33552/
%X Atomic hydrogen gas (HI) is an integral constituent of the interstellar medium (ISM) and thus plays a critical role in the assembly of molecular clouds, the sites of star formation. An important physical agent in controlling the transition from atomic to molecular gas is cold HI. HI emission, that traces a wide range of hydrogen properties, is found throughout the Galactic plane and exhibits complex kinematic signatures that are imposed by the Galactic rotation. The unfavorable viewing geometry from our vantage point in the Galaxy requires a new set of tools that allows us to disentangle HI structures along our lines of sight. In this thesis, I constrain the properties of the interstellar hydrogen in our Milky Way galaxy and present a way to isolate cold hydrogen from HI emission using HI self-absorption (HISA). The outer Galaxy offers a less confused view on HI emission, and we exploit this circumstance with unprecedented detail using the high-angular resolution data of The HI/OH/Recombination line survey of the inner Milky Way (THOR). We discover the “Maggie” filament, one of the largest, almost purely atomic filaments in the Milky Way. Maggie has a length of 1.2 kpc and is located on the far side of the Galaxy at a distance of 12 kpc from the Galactic center. Optical depth measurements suggest that Maggie is in a largely cold HI phase and molecular gas is only found on the smallest spatial scales. When targeting molecular clouds in the inner Galactic plane, we detect pronounced HISA as a tracer of cold hydrogen. While the kinematic correlation is less surprising due to the selection bias of our sample, the spatial distribution of cold HI gas is also found to be well correlated with that of the molecular gas. The detection of HISA furthermore extends to the whole of our survey. We frequently find absorption signatures without any associated molecular counterpart. While cold atomic gas is rendered vitally important to the formation process of molecular clouds, the cold phase of atomic hydrogen marks a distinct gas phase in the ISM that exists throughout the Galaxy, independent of the occurrence of molecular gas.
%Z put forward by Jonas Ibrahim Syed; referees: Prof. Dr. Henrik Beuther, Prof. Dr. Cornelis P. Dullemond