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Galaxy Zoo hunters discover "Green Peas"

Posted: July 28, 2009

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A rare group of galaxies dubbed the Green Peas have been uncovered by astronomers and volunteers using the online galaxy bank Galaxy Zoo, and could offer insight into how stars form in the early Universe.

Galaxy Zoo and its successor Galaxy Zoo 2 asks volunteers to classify galaxies in the extensive Sloan Digital Sky Survey, and since its launch in 2007, nearly a quarter of a million people have classified an impressive one million galaxies.

The distinct green colour and compact morphology of the Galaxy Zoo "Peas" (left three images) are easily distinguishable from the classical red elliptical galaxies (right). Image: Cardamone et al.

In a forthcoming paper to be published in the Monthly Notices of the Royal Astronomical Society, astronomers announced the discovery of a new class of compact galaxy forming stars at an incredibly high rate.

"These are among the most extremely active star-forming galaxies we've ever found," says Carolin Cardamone, an astronomy graduate student at Yale and lead author of the paper.

Galaxy Zoo volunteers first noticed the objects because of their small size and bright green colour, lending themselves to the Green Pea moniker. By analysing the light emitted by the galaxies, Cardamone and colleagues could determine the rate of star formation. Despite being ten times smaller than our own Milky Way Galaxy and one hundred times less massive, they are still generating stars ten times faster than the Milky Way.

"They're growing at an incredible rate," says Kevin Schawinski, also of Yale and one of Galaxy Zoo's founders. "These galaxies would have been normal in the early Universe, but we just don't see such active galaxies today. Understanding the Green Peas may tell us something about how stars were formed in the early Universe and how galaxies evolve."

The Green Peas are located between 1.5 and five billion light years away, and out of the one million galaxies surveyed only 251 Green Peas were spotted. "No one person could have done this on their own," says Cardamone. "Even if we had managed to look through 10,000 of these images, we would have only come across a few Green Peas and wouldn't have recognised them as a unique class of galaxies."

Schawinski adds that Galaxy Zoo is a genuine citizen science project where the users are directly involved in the analysis. Ten Galaxy Zoo users are acknowledged in the paper as having made a particularly significant contribution. "It's a great example of how a new way of doing science produced a result that wouldn't have been possible otherwise," he says.

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