Why is it so hard to detect carbon monoxide in irregular galaxies?
Irregular galaxies have been surveyed for carbon monoxide gas in their interstellar medium, but factors of 1/6 to 1/20 less have been found compared to similar gas in the Milky Way. Irregular galaxies often have a 10 times lower heavy element abundance that the Milky Way so the constituent elements for molecules such as carbon monoxide are somewhat less abundant than in the Milky Way. This means that per atom of hydrogen intercepted by a telescope beam, the intensity of the CO will be a factor of 1/6 to 1/20 fainter that for the Milky Way. Also, CO requires a source of excitation, and in the Milky Way this is often provided by luminous, massive stars. There are fewer of these in a typical irregular galaxy whose star formation processes seem to favor episodes every few billion years. Massive stars ARE produced since supernovae remnants have been detected in the Large Magellanic Cloud, however, these massive stars only live a few 10s of millions of years so an irregular galaxy spends mos
