Artist's impression of molecular gas across the Milky Way's plane. Credit: ESA – C. Carreau Enlarge

( —A survey from Herschel has revealed that the reservoir of molecular gas in the Milky Way is hugely underestimated – almost by one third – when it is traced with traditional methods. Monitoring the emission from ionised carbon, the new study identified molecular gas in the intermediate evolutionary stage between diffuse, atomic gas and the densest star-forming molecular clouds. The discovery not only indicates that there is more raw material for the formation of new stars in the Galaxy, but also that it extends farther than astronomers knew.

In the Milky Way, as well as in other galaxies, stars are born from the collapse of the densest and coldest  of matter in a molecular cloud. These clouds are gigantic star-forming complexes consisting mainly of molecular hydrogen (H2), a gas that does not emit any light at the low temperatures found in .

Astronomers investigating the early stages of star formation are not only interested in how molecular clouds fragment to form stars, but also in the processes that take place even earlier and initially cause molecular clouds to take shape from diffuse,  gas. For this purpose, astronomers study the distribution and properties of H2 across the Galaxy – but without the benefit of direct observations, they must resort to alternative methods to trace it.

The most widely used proxy to track down molecular gas in star-forming regions is  (CO). A mere contaminant in molecular clouds, CO radiates much more efficiently than H2 and can be detected easily. However, such indirect tracers can be biased, since there is no guarantee that all portions of a cloud containing H2 also contain CO, in which case observations of CO would miss these regions entirely.

To achieve a more complete picture of the Milky Way's molecular content, astronomers in the past decades have combined observations of CO with other tracers of H2. These include the emission from dust – another contaminant in molecular clouds – and the gamma rays that are produced when cosmic ray particles interact with atomic and  in the interstellar medium (ISM).

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