Atomic and molecular gas properties during cloud formation

Abstract

Molecular clouds, which harbor the birthplaces of stars, form out of the atomic phase of the interstellar medium (ISM). We aim to characterize the atomic and molecular phases of the ISM and set their physical properties into the context of cloud formation processes. We studied the cold neutral medium (CNM) by means of $\mathrm{HI}$ self-absorption (HISA) toward the giant molecular filament GMF20.0-17.9 and compared our results with molecular gas traced by ${}^{13}\mathrm{CO}$ emission. We fitted baselines of HISA features to $\mathrm{HI}$ emission spectra using first and second order polynomial functions. 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 ${}^{13}\mathrm{CO}$ is evident toward the eastern part of the filament. The distribution of HISA peak velocities and line widths agrees well with ${}^{13}\mathrm{CO}$ within the whole filament. The column density probability density functions (N-PDFs) of HISA (CNM) and $\mathrm{HI}$ 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 ${\mathrm{H}}_2$ N-PDFs show a broad log-normal distribution with a power-law tail suggesting the onset of gravitational contraction. The saturation of $\mathrm{HI}$ column density is observed at $\sim$25${\mathrm{M}}_{\odot }\mathrm{p}{\mathrm{c}}^{-2}$. 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 ${\mathrm{H}}_2$ column density peaks and signs of active star formation.

21 pages, 19 PDF figures

References in corpus (28)

• Theory of Star Formation
• Trigonometric Parallaxes Of High-Mass Star Forming Regions: Our View Of The Milky Way
• The Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey
• The Statistics of Supersonic Isothermal Turbulence
• The Atomic to Molecular Transition in Galaxies. II: HI and H_2 Column Densities
• On the Star Formation Efficiency of Turbulent Magnetized Clouds
• CO Isotopologues in the Perseus Molecular Cloud Complex: the X-Factor and Regional Variations
• What determines the density structure of molecular clouds ? A case study of Orion B with Herschel
• On the Density Distribution in Star-forming Interstellar Clouds
• HI-to-H2 Transitions and H I Column Densities in Galaxy Star-Forming Regions
• The Atomic to Molecular Transition in Galaxies. I: An Analytic Approximation for Photodissociation Fronts in Finite Clouds
• CO-dark gas and molecular filaments in Milky Way type galaxies
• FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45-m telescope (FUGIN) I: Project Overview and Initial Results
• The Bones of the Milky Way
• Unfolding the Laws of Star Formation: The Density Distribution of Molecular Clouds
• Molecular clouds have power-law probability distribution functions
• ATLASGAL-selected massive clumps in the inner Galaxy: I. CO depletion and isotopic ratios
• An application of an optimal statistic for characterising relative orientations
• The HI/OH/Recombination line survey of the inner Milky Way (THOR): data release 2 and HI overview
• Turbulent Mixing in the Interstellar Medium -- an application for Lagrangian Tracer Particles
• The Interstellar Medium and star formation on kpc size scales
• CO-dark gas and molecular filaments in Milky Way-type galaxies - II: The temperature distribution of the gas
• The evolution of Giant Molecular Filaments
• An Improved Technique for Measurement of Cold HI in Molecular Cloud Cores
• The HI Probability Distribution Function and the Atomic-to-Molecular Transition in Molecular Clouds
• HI-to-H2 Transition Layers in the Star-Forming Region W43
• Cloud formation in the atomic and molecular phase: HI self absorption (HISA) towards a Giant Molecular Filament
• The Carina Nebula and Gum 31 molecular complex: II. The distribution of the atomic gas revealed in unprecedented detail