Molecular Cloud Formation out of the Atomic Phase

This thesis is dedicated to study observationally the formation of molecular cloud formation out of the atomic phase. In the framework of 'The HI, OH, Recombination Line survey of the Milky Way' (THOR), we used the Very Large Array (VLA) to map the 21 cm HI line, 4 OH transitions, up to 19 H alpha recombination lines and the continuum from 1 to 2 GHz of a significant fraction of the Milky Way. The goal of this thesis is to characterize and study the atomic and molecular content of molecular clouds. Observations of the giant molecular cloud complex associated with the W43 star formation complex revealed large quantities of optically thick HI emission. We used strong continuum sources to measure the optical depth and used this information to correct the HI column density and hence the HI mass. Without these corrections, the mass estimate is at least a factor of 2.4 too low. In addition, we observed HI column densities up to N(HI) ~ 150 M_sun/pc^2, which is in contradiction to current cloud formation models. Furthermore, we present a catalog of ~4400 continuum sources, extracted from the first half of the THOR survey. Due to the broad bandwidth from 1 to 2 GHz, we are able to determine a reliable spectral index for ~1800 bright sources, which allows us to distinguish between thermal and non-thermal radiation. Using this information, we can confirm four super nova remnant candidates. Beside the direct scientific studies of these evolved sources, this catalog is the basis for prospective absorption studies of the HI and OH lines. By extracting HI self absorption (HISA) features of the molecular filament GMF38.1-32.4, we examined directly the cold and dense atomic hydrogen and compared it to the molecular counterpart. We studied the kinematics and column density probability distribution functions (PDFs) and found a log-normal shape for them, which indicates turbulent motion. Furthermore, we might observe several evolutionary stages within the filament, which will be helpful to validate theoretical models and simulations