Cloud formation in the atomic and molecular phase: H I self absorption (HISA) towards a giant molecular filament^★

¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: wang@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 Chalmers University of Technology, Department of Space, Earth and Environment, SE-412 93 Gothenburg, Sweden
⁵ I. Physik. Institut, University of Cologne, Zülpicher Str. 77, 50937 Cologne, Germany
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 Research School of Astronomy and Astrophysics, The Australian National University, Canberra, ACT, Australia
⁹ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹⁰ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
¹¹ School of Physics and Astronomy, Cardiff University, Queen’s Buildings, The Parade, Cardiff, CF24 3AA, UK
¹² Department of Physics and Astronomy, West Virginia University, Morgantown, WV 26506, USA
¹³ Adjunct Astronomer at the Green Bank Observatory, PO Box 2, Green Bank WV 24944, USA
¹⁴ Center for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, Morgantown, WV 26505, USA
¹⁵ School of Physical Sciences, University of Kent, Ingram Building, Canterbury, Kent CT2 7NH, UK
¹⁶ Department of Physics, Indian Institute of Science, Bengaluru 560012, India
¹⁷ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK
¹⁸ Argelander Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
¹⁹ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK

Received: 10 May 2019
Accepted: 10 January 2020

Abstract

Molecular clouds form from the atomic phase of the interstellar medium. However, characterizing the transition between the atomic and the molecular interstellar medium (ISM) is a complex observational task. Here we address cloud formation processes by combining H I self absorption (HISA) with molecular line data. Column density probability density functions (N-PDFs) are a common tool for examining molecular clouds. One scenario proposed by numerical simulations is that the N-PDF evolves from a log-normal shape at early times to a power-law-like shape at later times. To date, investigations of N-PDFs have been mostly limited to the molecular component of the cloud. In this paper, we study the cold atomic component of the giant molecular filament GMF38.1-32.4a (GMF38a, distance = 3.4 kpc, length ~ 230 pc), calculate its N-PDFs, and study its kinematics. We identify an extended HISA feature, which is partly correlated with the ¹³CO emission. The peak velocities of the HISA and ¹³CO observations agree well on the eastern side of the filament, whereas a velocity offset of approximately 4 km s⁻¹ is found on the western side. The sonic Mach number we derive from the linewidth measurements shows that a large fraction of the HISA, which is ascribed to the cold neutral medium (CNM), is at subsonic and transonic velocities. The column density of the CNM part is on the order of 10²⁰ to 10²¹ cm⁻². The column density of molecular hydrogen, traced by ¹³CO, is an order of magnitude higher. The N-PDFs from HISA (CNM), H I emission (the warm and cold neutral medium), and ¹³CO (molecular component) are well described by log-normal functions, which is in agreement with turbulent motions being the main driver of cloud dynamics. The N-PDF of the molecular component also shows a power law in the high column-density region, indicating self-gravity. We suggest that we are witnessing two different evolutionary stages within the filament. The eastern subregion seems to be forming a molecular cloud out of the atomic gas, whereas the western subregion already shows high column density peaks, active star formation, and evidence of related feedback processes.