Spiral galaxies with bar-shaped arms may not have been born that way. Astronomers have found that the number of bars has tripled in the past 7 billion years, indicating that spiral galaxies evolve as they age.
Barred galaxies are shaped like a tiger's eye, with two starry arms trailing off either end of a long, dark stardust lane. They take shape as stellar orbits in a disk become unstable and deviate from a circular path.
"The formation of a bar may be the final important act in the evolution of a spiral galaxy," said study leader Kartik Sheth of NASA's Spitzer Science Center at Caltech. "Galaxies are thought to build themselves up through mergers with other galaxies. After settling down, the only other dramatic way for galaxies to evolve is through the action of bars."
More than two-thirds of spiral galaxies, including our own Milky Way, have a bar-shaped path through their middles.
According to new observations of over 2,000 spiral galaxies, made as a part of the Hubble Space Telescope's Cosmic Evolution Survey (COSMOS), the bar scene was dramatically different seven billion years ago, when the universe was half as old as it is today.
The astronomers involved in the study, detailed in a recent issue of The Astrophysical Journal, discovered that while spiral galaxies were around in the distant past, only around 20 percent of them possessed the bars that are so common in their modern counterparts.
The tripling rate does not proceed in a uniform way, either. "They are forming mostly in the small, low-mass galaxies," Sheth said, adding that among the most massive galaxies, the proportion of bars to no bars is the same as it is today.
"We know that evolution is generally faster for more massive galaxies ? they form their stars early and fast and then fade into red disks," Sheth explained. "Low-mass galaxies were also known to form more slowly, but now we see that they also made their bars slower."
Bars grow in galaxies after stellar orbits in a spiral galaxy begin to deviate from a circular path, said survey team member Bruce Elmegreen, an astrophysicist with IBM's Research Division.
"It locks more and more of these elongated orbits into place, making the bar even stronger. Eventually a high fraction of the stars in the inner disk join the bar," Elmegreen added.
Bars are perhaps the most important catalysts for changing a galaxy, Sheth said. They force a large amount of gas towards the galactic center, fueling new star formation, building bulges ?spheres in the centers of galaxies made only of stars ? and feeding massive black holes.
"They pull stars and gas out of their normal circular orbits into the central regions, perhaps even funneling gas to the central supermassive black hole," said Nicholas Scoville of Caltech and a COSMOS principal investigator. "Without this fueling, the black holes would be starved and the central regions of galaxies devoid of young stars."
The Milky Way, possibly the best-known barred galaxy, is a massive one whose bar probably formed somewhat early, like the bars in other massive galaxies, Sheth suggests.
"Understanding how this occurred in the most distant galaxies will eventually shed light on how it occurred here, in our own backyard," he adds.
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