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Massive galactic cloud survey begins

Galactic gas Astronomers have begun the mammoth task of mapping the largest structures in the Milky Way galaxy, the giant molecular clouds where stars are born.

The survey, led by Professor Michael Burton of the University of New South Wales, could answer long standing mysteries about how these clouds are created, and whether they're the missing source of high energy gamma rays.

"One of the largest unresolved mysteries in galactic astronomy is how these giant, diffuse clouds form in the interstellar medium," says Burton.

"These clouds are the centres of the galactic ecosystem. They play a key role in the cosmic life cycle of the birth and death of stars."

Galactic clouds contain molecular hydrogen and other gases, the building blocks of new stars, at temperatures of just a few degrees above absolute zero (-273°C). Only gas at this temperature moves slowly enough to form the molecules that will eventually form new stars.

In the Milky Way, a new star is born about once a year on average. Stars that die and explode enrich these clouds with new elements.

Each of these clouds can be up to a hundred light years wide, covering vast expanses of interstellar space, and containing up to a million times the mass of the Sun.

"But we don't yet have a clear view of where these molecular clouds lie," says Burton.

"We're making a new map, scanning along the southern Milky Way galaxy at a higher resolution than has ever been seen before. We only have a fairly murky view of what it looks like at the moment."

Cold molecular hydrogen is hard to detect, because it doesn't emit energy at such cold temperatures. So instead, Burton's team are looking for carbon monoxide, which does emit energy.

"It is the second most abundant molecule and the easiest to see, so we can use it to trace where these clouds are located." says Burton

To find these clouds, Burton's team is using the 22-metre Mopra millimetre wavelength radio telescope at Coonabarabran in outback New South Wales, which survived the devastating bushfires which ravaged the region earlier this year.

"Despite being the most massive objects in our galaxy, it's still a big mystery as to how they [the clouds] come together," says Burton.

"They must form out of atomic gas, but the process has been completely invisible."

The dark side

Burton's team will also be looking for "dark" galactic gas clouds, which contain very little carbon monoxide and which are thought to be mostly made up of molecular hydrogen.

They'll use telescopes in Chile and at the South Pole to search for the presence of carbon atoms, rather than carbon molecules, a process Burton calls "following the galactic carbon trail".

Dark clouds could be the missing source of gamma rays, produced when high-energy cosmic rays generated by extremely powerful events such as supernovae, collide with the nuclei of gas atoms or molecules.

"The source of more than 30 per cent of gamma rays remains unidentified - another big mystery our research could throw light on," says Burton.

"Telescopes have started to make the first maps of our galaxy in gamma rays, and it's turning out that these maps are looking roughly similar to the molecular cloud maps."

"So there's clearly an association between the production of gamma rays and where the molecular clouds are," says Burton.

"Part of what we are trying to do is understand what that association is."

The results of the initial 'pilot' stage of the survey appears in the journal Publications of the Astronomical Society of Australia.