Atomic and molecular gas properties during cloud formation

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: syed@mpia.de
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ National Radio Astronomy Observatory, PO Box O, 1003 Lopezville Road, Socorro, NM 87801, USA
⁴ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
⁵ Department of Astronomy, University of Massachusetts, Amherst, MA01003, USA
⁶ Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Str. 2, 69120 Heidelberg, Germany
⁷ Universität Heidelberg, Interdisziplinäres Zentrum für Wissenschaftliches Rechnen, INF 205, 69120 Heidelberg, Germany
⁸ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
⁹ Centre for Astrophysics and Planetary Science, University of Kent, Canterbury CT2 7NH, UK
¹⁰ School of Physics and Astronomy, Cardiff University, Queen’s Buildings, The Parade, Cardiff CF24 3AA, UK
¹¹ School of Physics and Astronomy, E.C. Stoner Building, The University of Leeds, Leeds LS2 9JT, UK

Received: 19 May 2020
Accepted: 12 August 2020

Abstract

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 H I self-absorption (HISA) toward the giant molecular filament GMF20.0-17.9 (distance = 3.5 kpc, length ~170 pc) and compared our results with molecular gas traced by ¹³CO emission. We fitted baselines of HISA features to H I 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 ¹³CO is evident toward the eastern part of the filament. The distribution of HISA peak velocities and line widths agrees well with ¹³CO within the whole filament. The column densities of the CNM probed by HISA are on the order of 10²⁰ cm⁻² while those of molecular hydrogen traced by ¹³CO are an order of magnitude higher. The column density probability density functions (N-PDFs) of HISA (CNM) and H I 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 H₂ N-PDFs show a broad log-normal distribution with a power-law tail suggesting the onset of gravitational contraction. The saturation of H I column density is observed at ~25 M_⊙ 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 H₂ column density peaks and signs of active star formation.