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Rare ‘Green Pea’ galaxies crank out stars

YALE (US)—A newly discovered group of galaxies known as Green Peas—for their size and color—appear to be growing and forming stars at an incredibly high rate. Astronomers are hopeful the compact galaxies will offer insights into the early universe.

Working with the scientists through an online project called the Galaxy Zoo, a group of volunteers came across a number of objects that stuck out because of their small size and bright green color which they dubbed the Green Peas.

“These are among the most extremely active star-forming galaxies we’ve ever found,” says Carolyn Cardamone, lead author of a paper reporting the findings and a Yale University astronomy graduate student.

Of the one million galaxies that make up Galaxy Zoo’s image bank, the team found only 250 Green Peas.

“No one person could have done this on their own,” Cardamone explains. “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 recognized them as a unique class of galaxies.”

The galaxies, which are between 1.5 billion and 5 billion light years away, are 10 times smaller than our own Milky Way galaxy and 100 times less massive. But surprisingly, given their small size, they are forming stars 10 times faster than the Milky Way.

“They’re growing at an incredible rate,” says Kevin Schawinski, a postdoctoral associate 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.”

Galaxy Zoo volunteers—many of whom had no previous astronomy background or experience—were asked to refine the sample of objects they detected in order to determine which were bona fide Green Peas and which were not, based on color. By analyzing their light, Cardamone was able to determine how much star formation is taking place within the galaxies.

“This is a genuine citizen science project, where the users were directly involved in the analysis,” Schawinski says, adding that 10 Galaxy Zoo volunteers 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.”

The Galaxy Zoo project was launched in 2007 by a team of astronomers in the United Kingdom and the United States. To date, 230,000 volunteers from all over the world have helped classify one million images of galaxies taken by the Sloan Digital Sky Survey. Galaxy Zoo 2, which launched in February 2009, lets users more fully analyze 250,000 of the brightest galaxies.

Contributing to the paper were researchers from the University of Hertfordshire, University of Nottingham, University of California-Santa Barbara, University of Oxford, University of Alabama, Drexel University, University of Portsmouth, and Johns Hopkins University. Findings will be published in an upcoming issue of the Monthly Notices of the Royal Astronomical Society.

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