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Galaxies going green

Nearby compact galaxies may shine light on early star formation

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6:31pm, July 28, 2009
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For galaxies, it’s not easy being green. Most of them appear blue or red from Earth.

Indeed, after combing through an online image bank of 1 million galaxies, volunteers for the Galaxy Zoo project have found a mere 250 galaxies with an unusual, greenish color. These compact bodies, dubbed the Green Peas, are only about one-tenth the size of the Milky Way.

Now, a team of astronomers working with the volunteers has discovered that the Green Peas are hamming it up, forming stars at an enormous rate — about 10 times faster than the Milky Way. Spectra of the galaxies taken by the Sloan Digital Sky Survey — the source of the online images — indicate that the greenish hue comes from the glow of ionized oxygen gas heated by newborn stars, says astronomer Carolin Cardamone of Yale University. 

High rates of star formation are common among some remote galaxies, which hail from the early universe, but the Green Peas are relatively nearby — between 1.5 billion and 5 billion light-years from Earth. The Green Peas may represent a closer, and therefore easier to observe, analog of those distant galaxies, report Cardamone and her colleagues in an upcoming Monthly Notices of the Royal Astronomical Society. The team has also posted the findings online at arxiv.org.

Galaxies from further back in time, when the universe was one-third to one-fourth its current age, forged many more stars than the typical galaxy of today does, notes Alice Shapley of the University of California, Los Angeles. That’s because these galaxies were pulling in a fresh supply of gas, the raw material for making stars, at a much higher rate than galaxies do today.

Those few galaxies that exhibit high star formation rates today are usually undergoing a rare, major merger with another galaxy. But the Green Peas appear to be loners, devoid of mergers. “If the Green Peas are really isolated systems, then the origin of their high star formation rates is a real mystery,” Shapley says.

Shapley says there are both similarities and differences between the Green Peas and the remote population, which are known as Lyman-break galaxies.

She agrees that the Green Peas appear to be distinct from typical, nearby star-forming galaxies and concurs that the Peas are forging stars at a prodigious rate similar to the Lyman-break galaxies. However, these remote galaxies are considerably heavier and their abundance of metals, defined in astronomical parlance as any element heavier than helium, seems higher on average than those of the Green Peas.

In her team’s new study, Shapley and her collaborators have delineated several properties that contribute to the high star formation rate in the remote Lyman-break galaxies. The team’s study, posted online at arxiv.org, will appear in the August 10 Astrophysical Journal. The researchers were able to study the pattern of emissions from hydrogen, nitrogen, oxygen and sulfur atoms in two remote galaxies in great detail because their light was greatly magnified by a gravitational lens — a massive foreground object that bends and focuses light from background bodies.

The team found that the star-forming regions of the two remote Lyman-break galaxies have densities 10 to 100 times higher than those of most galaxies in the nearby universe. The density of photons energetic enough to ionize hydrogen atoms is also significantly higher, and the researchers infer that the star-forming regions in the Lyman-breaks are under higher pressure.

These factors may dictate that gas collapsing to form stars in these galaxies coalesces into slightly heavier stars on average than the gas in galaxies today does, she suggests.

Shapley says she looks forward to seeing some of these additional measurements made for the newly discovered galaxies.

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