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 Iself 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 the13CO emission. The peak velocities of the HISA and13CO observations agree well on the eastern side of the filament, whereas a velocity offset of approximately 4 km s−1is 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 1020to 1021cm−2. The column density of molecular hydrogen, traced by13CO, is an order of magnitude higher. The N-PDFs from HISA (CNM), H Iemission (the warm and cold neutral medium), and13CO (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.
Highlights
Stars, one of the key components of our Universe, form in molecular clouds which are composed mainly of molecular hydrogen (e.g., Larson 2003; Stahler & Palla 2005; McKee & Ostriker 2007; Dobbs et al 2014; Tan et al 2014), yet the formation process of molecular clouds is still under debate
Our goal is to study the kinematics of this giant molecular filaments (GMFs) in the molecular and atomic hydrogen traced by 13CO emission and H I self absorption (HISA), respectively
Kinematics we discuss the kinematic properties of the HISA features
Summary
One of the key components of our Universe, form in molecular clouds which are composed mainly of molecular hydrogen (e.g., Larson 2003; Stahler & Palla 2005; McKee & Ostriker 2007; Dobbs et al 2014; Tan et al 2014), yet the formation process of molecular clouds is still under debate. The first one is to use absorptionfree H I emission spectra, located close to the absorption feature (e.g., Gibson et al 2000), referred to as “off-positions” This method assumes that the H I background emission stays spatially constant over the absorption feature, which might be true for spatially small HISA features. These off-positions were chosen to be regions without significant 1.4 GHz continuum emission and without 13CO emission. These spectra reveal large variations, which makes it difficult to use them as a common off-position
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