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. These absorption features contain valuable information about the intervening gas clouds which absorbed the radio signals. However, the number of known molecular absorption systems seen in millimeter/submillimeter waveband has been very limited: only about 30 in the Milky Way galaxy and a limited number in other galaxies.
By examining data from 36 calibrator sources, the team discovered 3 new absorption systems and confirmed one previously known system. For one calibrator source, J1717-337, they found absorptions caused by 10 different molecules, eg C3H2, CS, and HCS+. In addition, they found absorption signals caused by HCO molecules for 2 of the calibrator sources, J1717-337 and NRAO530. The HCO absorption signal is very rare; only 3 other examples are known in the Milky Way galaxy.
Absorption systems allow researchers to investigate very tenuous gas clouds. A gas cloud too diffuse to emit sufficient radio waves to be detected can still absorb enough radio waves to produce a detectable radio shadow. The team estimated that the amount of HCO in the cloud backlit by NRAO530 is only half that of other known systems. This shows that it is one of the most diffuse gas clouds ever discovered in the Milky Way galaxy. Even though astronomers assume that tenuous gas clouds account for a considerable fraction of the total mass of the Milky Way galaxy, very little is known about them.
The absorption signals help us determine the environment around the foreground gas clouds. HCO molecules are thought to be formed in special environments full of intense UV from giant young stars. The diffuse gas clouds backlit by J1717-337 and NRAO530 show chemical composition similar to the gas in active star forming regions, indicating that the diffuse gas is bathed in strong UV light which affects the properties of diffuse clouds. The HCO absorption systems found by ALMA provide an opportunity to verify that idea. http://www.eurekalert.org/pub_releases/2015-12/nion-rsr120415.php
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