Spiral Galaxies May Die From Within

The bar-shaped structure of stars that spans the center of many spiral galaxies, including our own Milky Way, may play a role in destroying those galaxies, a new study suggests.

Researchers found that spiral galaxies where star formation has stopped are about twice as likely to have central bars ? huge linear formations ? as galaxies that are still actively birthing stars. The researchers also contend that those bars are the main suspects for choking off star formation, though how this could happen remains a mystery. [Photo of a barred spiral galaxy.]

It is also possible that the bars are simply a side effect of the mysterious star-snuffing mechanism, researchers said. In any event, the observation could help astronomers better understand the future of our own Milky Way, which has a prominent central bar.

An army of volunteers

To help perform their analysis, the researchers enlisted thousands of volunteers via the online Galaxy Zoo 2 project.

The original Galaxy Zoo asked volunteers to help classify galaxies by shape ? for example, as elliptical or spiral. Galaxy Zoo 2 is building on this work by asking members of the public to look more closely at 250,000 galaxies, including determining if they have a central bar or not.

With this information ? the largest-ever sample of galaxies with visual bar identifications ? researchers found that red spiral galaxies are about twice as likely as blue spirals to host bars.

The colors are significant. Blue galaxies get their hue from the hot young stars they contain, implying that they are forming stars in large numbers. In red galaxies, this star formation has stopped, leaving behind the cooler, long-lived stars that give them their reddish color.

"For some time, data have hinted that spirals with more old stars are more likely to have bars, but with such a large number of bar classifications we?re much more confident about our results," said study leader Karen Masters of the University of Portsmouth in a statement. "And all of this is thanks to the dedication of the volunteers who provide the raw 'clicks.'"

The research will be detailed in an upcoming issue of the journal Monthly Notices of the Royal Astronomical Society.

Mysterious bars

About half of all spiral galaxies have a bar structure. These bars are important in the evolution of galaxies, providing a way to move material in and out in the spiral disk, and possibly helping to spark star formation in the central regions, researchers said.

Bars may even help feed the central massive black hole that seems to be present in almost all galaxies, researchers said. But astronomers are still unclear on why some galaxies have bars and others do not.

It's also unknown if the bars somehow cause galaxies to stop forming stars, or if they're just a consequence of some as-yet unknown process, researchers said.

?It?s not yet clear whether the bars are some side effect of an external process that turns spiral galaxies red, or if they alone can cause this transformation," Masters said. "We should get closer to answering that
question with more work on the Galaxy Zoo data set.?

Masters expressed appreciation for the Galaxy Zoo 2 users' help. For their part, many of the volunteers were happy to be of assistance.

"I had fun doing my bit, and my high school students were involved, too," said one Galaxy Zoo 2 volunteer, schoolteacher Mike Tracey. "It is great to be part of a real-life project which can produce real science."

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