GEPI

De la conception instrumentale
à l’exploitation des observables
Accueil > Seminar on 2021.03.01 by Eduardo (...) > Tracing the Transition from Atomic (...)

Tracing the Transition from Atomic to Molecular ISM

Di Li (FAST, China)

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.