The 2.6 mm line

The molecule of Hydrogen is homopolar, and therefore it has no electric dipole moment. For this reason it does not emit either vibrational or rotational radiation. The most abundant molecule after $H_2$ is expected to be carbon monoxide, CO, with emits strong line radiation at 2.6 millimeter, corresponding to the J = $0\rightarrow1$ rotational transition.
Strong CO radiation has been detected throughout the Galaxy and provides important information about dense molecular regions. The importance of CO observations is that it can be assumed that wherever there are CO molecules there must also exist $H_2$ molecules, and hence CO is used as a tracer for $H_2$ molecules. Indeed, the excitation mechanism for the CO molecules is collisions with hydrogen molecules. The CO observations hence provide a measure of the number density of $H_2$ molecules in these clouds.

Figure 1.4: Distribution of molecular clouds in the galactic region close to the Sun [Dame et al., 1987].