Vol. 554
In section 6. Interstellar and circumstellar matter

A Herschel [CII] Galactic plane survey I: the global distribution of ISM gas components

Our understanding of the distribution of hydrogen in the interstellar medium of our own and external galaxies has been based on the assumption that CO is a good tracer of molecular hydrogen. While it has always been known that this assumption was doubtful, there have been few quantitative studies aimed at understanding how much H2 is hidden in layers that are invisible in CO. In the article highlighted in this issue, Pineda et al. use the Herschel observations of the 157 micron line of ionized carbon to estimate the hidden H2 in the Galaxy, finding that the ratio of CO-dark H2 to total H2 varies from 20 percent in the inner Galaxy to 80 percent at a radius of 10 kpc.