International Summer Institute for Modeling in Astrophysics

In this document, we report our recent study on the turbulence inside the multi-phase ISM. First, we quantify the turbulence inside the molecular phase ISM by adopting a pixel-by-pixel line fitting strategy and studying the statistics of the fittings results of the ¹³CO lines. The histogram of the ¹³CO line amplitude and ¹³CO line FWHM show power-law behavior, indicative of turbulence. Especially, the histogram of the FWHM of the ¹³CO line show a dN/dv proportional to v^-2.45 at high velocity end, which seems to be universal. By plotting the 2D histogram of the ¹³CO line strength versus the ¹³CO line width, a lower limit of the ¹³CO line width for a given ¹³CO line strength can be identified. We argue that such a lower limit is due to the self-gravity of the molecular cloud. Second, with the combination of ¹³CO data from the GRS survey and the Hi 21cm data from the VGPS survey, we study the connection between the molecular gas probed with the ¹³CO line emission and the cold Hi gas probed by the Hi 21cm self-absorption feature. We found that the Hi gas that is associated with the molecular clouds is also turbulent. The molecular clouds that have detectable Hi envelope are more turbulent than clouds that do not show such envelope. Our results support the idea the molecular cloud turbulence is driven from outside, e.g. by the large-scale converging flow.