Other radio-wavelengths

We've seen how we expect gas to become mostly molecular in high density regions. And so we cannot observe these with 21-cm observations, since also the hydrogen will become molecular. Fortunately, many molecules also emit radiation at radio-wavelengths. Typically these are caused by rotational transitions.^4.7

Which transition is strongest depends a lot on the density of the cloud. The most commonly studied transition is from the CO molecule, with wavelength 2.6cm. At higher densities, other molecules such as CH, OH and CS become observable.

The result of many years of investigation is that the properties within gas clouds can vary widely. In most of the outer parts of the disk, densities are low, and most gas is in the form of HI with typical density of order 1cm$^{-3}$. Closer in, we find a variety of clouds within the HI gas, which differ in total mass, and density. Giant Molecular Clouds, or GMCs for short, are enormous complexes of gas and dust, with total masses up to $10^6\hbox{$M_\odot$}$, temperatures $T\sim 20$K, and densities 100-300cm$^{-3}$. These clouds often have sub-condensations which are much denser. Thousands of GMCs are known in the MW, mostly in the spiral arms.

It is thought that most, if not all, star formation in the MW occurs in GMCs. To understand what determines whether a cloud can remain stable, or will become unstable and undergo star formation, brings us to the next concept: the Jeans mass.