Atomic And Molecular Gas Properties During Cloud Formation

Context. Molecular clouds, which harbor the birthplaces of stars, form out of the atomic phase of the interstellar medium (ISM). To understand this transition process, it is crucial to investigate the spatial and kinematic relationships between atomic and molecular gas.Aims. We aim to characterize the atomic and molecular phases of the ISM and set their physical properties into the context of cloud formation processes.Methods. We studied the cold neutral medium (CNM) by means of HI self-absorption (HISA) toward the giant molecular filament GMF20.0-17.9 (distance = 3.5 kpc, length similar to 170 pc) and compared our results with molecular gas traced by (CO)-C-13 emission. We fitted baselines of HISA features to HI emission spectra using first and second order polynomial functions.Results. The CNM identified by this method spatially correlates with the morphology of the molecular gas toward the western region. However, no spatial correlation between HISA and (CO)-C-13 is evident toward the eastern part of the filament. The distribution of HISA peak velocities and line widths agrees well with (CO)-C-13 within the whole filament. The column densities of the CNM probed by HISA are on the order of 10(20) cm(-2) while those of molecular hydrogen traced by (CO)-C-13 are an order of magnitude higher. The column density probability density functions (N-PDFs) of HISA (CNM) and HI emission (tracing both the CNM and the warm neutral medium, WNM) have a log-normal shape for all parts of the filament, indicative of turbulent motions as the main driver for these structures. The H-2 N-PDFs show a broad log-normal distribution with a power-law tail suggesting the onset of gravitational contraction. The saturation of HI column density is observed at similar to 25 M-circle dot pc(-2).Conclusions. We conjecture that different evolutionary stages are evident within the filament. In the eastern region, we witness the onset of molecular cloud formation out of the atomic gas reservoir while the western part is more evolved, as it reveals pronounced H-2 column density peaks and signs of active star formation.