Dynamical cloud formation traced by atomic and molecular gas
  Context: Atomic and molecular cloud formation is a dynamical process.
However, kinematic signatures of these processes are still observationally
poorly constrained. Methods: Targeting the cloud-scale environment of the
prototypical infrared dark cloud G28.3, we employ spectral line imaging
observations of the two atomic lines HI and [CI] as well as molecular lines
observations in 13CO in the 1--0 and 3--2 transitions. The analysis comprises
investigations of the kinematic properties of the different tracers, estimates
of the mass flow rates, velocity structure functions, a Histogram of Oriented
Gradients (HOG) study as well as comparisons to simulations. Results: The
central IRDC is embedded in a more diffuse envelope of cold neutral medium
(CNM) traced by HI self-absorption (HISA) and molecular gas. The spectral line
data as well as the HOG and structure function analysis indicate a possible
kinematic decoupling of the HI from the other gas compounds. Spectral analysis
and position-velocity diagrams reveal two velocity components that converge at
the position of the IRDC. Estimated mass flow rates appear rather constant from
the cloud edge toward the center. The velocity structure function analysis is
consistent with gas flows being dominated by the formation of hierarchical
structures. Conclusions: The observations and analysis are consistent with a
picture where the IRDC G28 is formed at the center of two converging gas flows.
While the approximately constant mass flow rates are consistent with a
self-similar, gravitationally driven collapse of the cloud, external
compression by, e.g., spiral arm shocks or supernovae explosions cannot be
excluded yet. Future investigations should aim at differentiating the origin of
such converging gas flows.
