Calibrator sources have flat radio spectra. Molecules in the intervening gas clouds absorb radio waves at specific frequencies determined by the type of molecules. Credit: R. Ando (The University of Tokyo), ESO/José Francisco Salgado
Astronomers using ALMA have discovered the most tenuous molecular gas ever observed. They detected the absorption of radio waves by gas clouds in front of bright radio sources. This radio shadow revealed the composition and conditions of diffuse gas in the Milky Way galaxy.
To calibrate its systems, ALMA looks at objects emitting strong radio waves (radio ‘bright’ objects). On rare occasions, the signals from distant calibrator sources have specific radio frequencies absorbed out of them by foreground gas...
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