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.
|