Since the H molecule is the homonuclear diatomic molecule,
the H molecule has no electric dipole.
Thus, the electric dipole radiation is not expected for this molecule.
While the second abundant molecule CO is the heteronuclear diatomic
molecules and has electric dipole.
Lower levels of rotational transition of CO are
relatively easily excited even in the cold interstellar medium of
Therefore, the rotational transition of CO molecule is the first
possibility to observe cold molecular gas.
The -factor represents the ratio of the column density of H
molecules to the integrated intensity of CO line
There are several empirical but physical estimations of the -factor:
Using -rays, -factor is estimated as
(Strong et al. 1988).
This is based on the idea that the emissivity of -ray emission
is proportional to the cosmic-ray intensity times the number of
The -ray intensity is proportional to the column density
if the cosmic-ray intensity is uniform.
From the distribution of -ray intensity,
Strong et al. (1988). estimated total column density
using some model.
This gives the value of -factor.
Estimations using the Virial mass of the interstellar cloud
and the visual extinction .