The transition from diffuse, largely atomic gas to dense, molecular gas is a key step in the life cycle of the interstellar medium. The results of this transition set up the conditions of star formation. The conditions of diffuse, atomic ISM have been well studied by observing the emission from its dominant component, HI. The study of dense, molecular ISM has been carried out mostly through tracers, especially CO. The transition phase is not friendly to either probe. In fact, this transition gas has been postulated to be a component of the so-called ’’dark gas’’. The total mass of dark gas could be multiples of known CO molecular gas, thus changing fundamental parameters for galaxy evolution, such as the star formation efficiency.
We studied this transition phase with three spectroscopic probes, ionized carbon emission (C+ ; Herschel and SOFIA), atomic carbon emission (CI ; SWAS), HI narrow self-absorption (HINSA ; Arecibo and GBT). These studies provide evidence for the existence of dark gas, the clumpy nature of star forming clouds, and a direct measurement of molecular clouds forming time scale.
I will also give a brief introduction of the on-going FAST project aiming to build the largest single dish telescope, which could enable significant progresses in studying HI absorption.