Di Li (Chief Scientist (Radio Astronomy Division, National Astronomical Observatories of China))

Di Li Colloquium: Dark Gas in the Milky Way

Marsfield lecture theatre

Abstract

HI and CO emission have been the standard tracer of the total gas content in the ISM. Caveats, though, exist for both measurements. Small opacity has been a necessary assumption in obtaining HI mass from the 21cm line. A stable [CO/H2 ]~1E-4 abundance has been commonly used in interpreting CO observations. The recent decades have seen growing evidence, particularly diffuse gamma ray and IR surveys, of previously under-counted ISM contents, giving rise to the term 'dark molecular gas' (DMG). I report here our recent results in probing DMG through absorption measurements. OH and CH are found to be better tracers of H2 in all but the densest cores. The great potential of FAST in such experiment will be briefly mentioned.