Milky Way May Fizzle Out Sooner Than Expected

Mardi, 09 Novembre 2010 20:30

Milky Way May Fizzle Out Sooner Than Expected

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A thick bar of stars, gas and dust spanning across the Milky Way’s center could be speeding star formation and, as supplies run out, our host galaxy’s eventual death.

A new study, the first to

trickle out of Galaxy Zoo’s second crowd-sourced scientific effort, buoys the idea that bars somehow encourage galaxies to form big, blue and short-lived stars, as well as funnel gas and dust to supermassive black holes lurking at their cores. In the process, bars may quickly consume star-making materials to leave behind only a “dead” galaxy of red and fading stars.

“Basically, as you go from the really youthful galaxies to the dead ones, more and more frequently we see bars in them,” said Kevin Schawinski, an astronomer at Yale University and co-author of the study, set to appear in an upcoming edition of the Monthly Notices of the Royal Astronomical Society. “Our immediate suspicion is that bars are involved in speeding galaxy evolution.”

Schawinski said the work isn’t causal proof that bars shorten galaxies’ star-forming lifespans — for one, it’s tricky to compare galaxies across space and time. But he said the data certainly backs the idea, which is shared among many astronomers.

“Bars seem to help exhaust supplies of gas, pushing galaxies to a passive state and no longer forming any stars. This is inline with our results and what others are saying,” Schawinski said. “The Milky Way, which is more or less agreed to be a barred spiral, may be an example of a galaxy in transition from an active state to something anemic and passive.”

George Djorgovski, an astronomer at Caltech who described his team as “in a friendly competition of sorts” with Galaxy Zoo, said the new research is interesting and does support existing ideas in the field.

“More than anything it illustrates how citizen science approach can be used very effectively, both in research and outreach,” Djorgovski said. “It’s a pretty exciting way of doing science, and Galaxy Zoo is certainly the most successful to date.”

Launched in July 2007, Galaxy Zoo enlisted the help of web citizens to classify a million galaxies photographed by the Sloan Digital Sky Survey, or SDSS. The effort produced at least 15 published studies, with more under review.

During the second phase from February 2009 through April 2010, called Galaxy Zoo 2, volunteers analyzed a quarter million of the brightest, previously classified disk-shaped galaxies in more detail. In all, 200,000 volunteers made 60 million classifications, or about 240 assessments per galaxy. Each object is between 140 million and 875 million light-years away.

“Our volunteers essentially became the world’s biggest and best pattern-recognizing supercomputer,” Schawinski said. “They were able to measure something that’s very, very hard to teach a computer, which is to recognize bars and other details in [astronomical images].”

Of 13,665 disk-shaped galaxies, Galaxy Zoo 2 discovered about 30 percent of them have bars. What’s more, 10 to 20 percent of the blue star-forming galaxies have bar, while about 50 percent of passive reddish galaxies have them. In contrast, Schawinski said only about 6 percent of non-barred galaxies are “red and dead.”

“Stars of all colors are being born at the same time, but blue stars are the ones that die very quickly. If star formation stops, all you have are red stars.” Schawinski said, adding that football-shaped elliptical galaxies don’t have great mechanisms to churn star formation, leaving most of them red-and-dead. “Spiral galaxies, on the other hand, still have a lot of gas in their disks, and stars are being born in them all of the time, including in our Milky Way,” he said.

If spiral galaxies are left to their own demise, Djorgovski said it takes a few billion years for star formation to fizzle out (the process consumes 99.9 percent gas and dust supplies, black holes just a shred at 0.01 percent). Yet many such galaxies have lasted longer than that, especially those similar to the Milky Way.

“Galaxies keep accreting fresh gas from space as they move through it, otherwise they would have run out long ago,” Djorgovski said, adding that many galaxies have corralled hundreds of billions of stars for about 10 billion years. “There’s still plenty of gas out there.”

When the Milky Way does run out of available stellar fuel and succumbs to its reddish death, which is extremely difficult to precisely predict, all may not be lost. The nearby Andromeda galaxy is expected to collide with our galaxy in about 4.5 billion years.

“When the Andromeda galaxy collides and merges with the Milky Way, it’s going to be spectacular fireworks of star formation,” Schawinski said, noting how gravity-induced chaos should stir up diffuse gas and dust. “Maybe even the galaxies’ black holes will start feeding again, too.”

Images: 1) Astronomers’ best guess of what the Milky Way galaxy looks like from above. NASA/JPL-Caltech (hi-res). 3) Images Galaxy Zoo 2 volunteers classified. Top: A red barred spiral galaxy. Bottom: A blue spiral galaxy with no bar. Galaxy Zoo/SDSS. 2) The barred spiral galaxy NGC 1300. NASA/ESA/Hubble Heritage (hi-res).

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