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

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 difficult 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 to examine 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 are 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 13 CO emission. The peak velocities of the HISA and 13 CO 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 10 20 to 10 21 cm −2 . The column density of molecular hydrogen, traced by 13 CO, is an order of magnitude higher. The N-PDFs from HISA (CNM), H i emission (the warm and cold neutral medium), and 13 CO (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.

Dense gas in a giant molecular filament

“…Riener et al (2020) estimated the 13 CO(1-0) optical depth for GMF24, and found a median τ value of ∼0.5. Wang et al (2020) found that the GRS 13 CO(1-0) optical depth for another filament, GMF38a (l ∼ 33 • −37 • ), is mostly smaller than 1 with a median value of ∼0.4. If we apply this 0.4 to GMF54, the column density increases by ∼21%.…”

“…The 13 CO N-PDF can be better described with a log-normal function, but we still could not recover the log-normal peak of the N-PDFs. Wang et al (2020) studied the N-PDFs of the atomic and molecular gas in the giant molecular filament GMF38a, and found that the N-PDF of H I emission has the smallest width, followed by the cold neutral media traced by H I self absorption (HISA), and the 13 CO N-PDF has the largest width. It seems that the width of the N-PDF is correlated with the density it traces.…”

“…Studies of nearby molecular clouds (MCs) show that the dense regions of MCs are permeated with filaments that contain sites of star formation (e.g., André et al 2014). Recent observations identified a class of large ( 50 pc) and massive ( 10 5 M ) filaments, known as giant molecular filaments (GMFs;Jackson et al 2010;Goodman et al 2014;Ragan et al 2014;Zucker et al 2015;Wang et al 2015Wang et al , 2016Wang et al , 2020Li et al 2016;Abreu-Vicente et al 2016). Many of these giant filaments lie along, or extremely close to spiral arms in the position-position-velocity space, suggesting they may trace the dense "spine" of the spiral arms and the mid-plane of the gravitational potential in the Milky Way (MW; Goodman et al 2014;Zucker et al 2015).…”

Atomic and molecular gas properties during cloud formation

“…The 13 CO emission peaks at different velocities are not associated with GMF20.0-17.9 as their velocities are attributed to neighboring spiral arm structures (e.g., Vallée 2008;Reid et al 2014Reid et al , 2019. For the analysis of the physical properties of HISA, we followed the derivation by Gibson et al (2000) and Wang et al (2020a). A comprehensive discussion of the radiative transfer of HISA clouds is given in Gibson et al (2000), Kavars et al (2003), and Li & Goldsmith (2003).…”

“…Observations of H I 21cm line A68, page 1 of 21 A&A 642, A68 (2020) emission are generally attributed to both warm neutral medium (WNM) and CNM. To separate the WNM from the CNM, we make use of the presence of H I self-absorption (HISA; see e.g., Riegel & Crutcher 1972;Knapp 1974;van der Werf et al 1988;Feldt 1993;Gibson et al 2000Gibson et al , 2005aKavars et al 2003;Dénes et al 2018;Wang et al 2020a) to trace the cold atomic phase. H I self-absorption is found throughout the Milky Way in various environments.…”