Radiation from the gaseous disk of a galaxy like the Milky Way principally occurs via fine structure transitions of carbon, oxygen, and nitrogen, all of which occur at submillimeter wavelengths at which the earth's atmosphere is opaque. Indeed, more than ten percent of all the energy radiated by the stars and gas in the Milky Way comes out in the 1900 GHz line of once ionized carbon! This line can only be observed from space, and it can only be observed with arcsecond angular resolution with a space-based interferometer. The international space station Freedom provides a stable platform, with the right dimension, for the necessary submillimeter interferometer.
IAU Colloq. 140: Astronomy with Millimeter and Submillimeter Wave Interferometry
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