Mining the Milky Way

Astronomers have released the results from a new survey of the contents of the Milky Way as seen at radio wavelengths. The survey, led by Prof Melvin Hoare at the University of Leeds, was conducted with the Very Large Array in New Mexico and is the most detailed done to date. Radio waves have the advantage that they can be used to look right through the plane of our Galaxy without any obscuration from the clouds of interstellar material that affects other forms of radiation.

From the outset, the radio survey was conceived to compliment surveys being conducted at other wavebands. In particular, it matched the area covered by NASA’s Spitzer satellite, which had already produced images an order of magnitude better than any previous ones in the infrared waveband. This co-ordinated approach was designed to give a comprehensive view of where all the stars much more massive than our Sun are being born in our Galaxy. Such stars form out of the gravitational collapse of large clouds of interstellar gas and dust that then gets heated to high temperatures by the newly formed star. The hot gas radiates strongly at radio wavelengths like a continuous hiss, whilst the hot dust emits in the infrared similar to the glow seen in thermal imaging cameras.

Given the multi-wavelength approach, the new survey is known as the Co-Ordinated Radio ‘N’ Infrared Survey for High-mass star formation or CORNISH survey. This also reflects the origins of Prof Hoare and fellow Cornishman and team member Prof Phil Diamond, who now heads the global team designing the world’s next generation radio telescope, the Square Kilometre Array. The infrared satellite survey was led by Prof Ed Churchwell, of the University of Wisconsin, USA, who commented “Having matching radio data will really help us determine the nature of some of the tens of millions of infrared sources, whether or not they are detected in the radio”.

A catalogue of over 3000 of the most reliable sources has been released online. About 10% of the sources are identified as the type of nebulae associated with the birth of massive stars similar to the famous nebula in Orion that can be seen with the naked eye. Prof Hoare says that “Having such a comprehensive sample of these objects across the Galaxy means that we can really start to test theories of how these stars form”.

Understanding exactly how material accumulates onto these stars that can be a hundred times more massive than our Sun and put out a million times more radiation has challenged astronomers. Although these stars are rare their prodigious output of energy can drive the evolution of whole galaxies. In addition to studying the birth of massive stars, the combination of radio and infrared studies is also good for a systematic census of the death of stars similar to our Sun. When our star begins to run out of fuel in five billion years’ time its outer layers of gas and dust will be ejected and heated by the hot core left behind. Newly discovered examples of these so-called planetary nebulae are also a significant population among the sources in the new radio survey. Other examples of radio emitting star-like objects have been found that are completely unknown. Mining this new resource will keep the thirty-strong international team from around the world busy for years to come.

Milky Way surveys

The above image shows an example section of the Milky Way surveys in the infrared (image) and radio (green contours in insets) showing three very different types of sources. In the middle is a region where massive stars are being born where it appears that the pressure from an older, larger nebula has caused the formation of a younger, smaller radio source. On the left is an example of the other end of stellar life where a star like our Sun is ejecting its outer layers in a bipolar nebula and the hot stellar core is heating up the gases to give rise to the radio emission. The radio source on the right is not in the Milky Way at all, but a very distant radio galaxy powered by a super-massive black hole. Such objects are not seen in the infrared.

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