Astronomers describe bar scene at beginning of universe

Tag : shape bar

The thought of spiral galaxies invokes images of star-studded armstrailing off of spinning disks. But more than two-thirds of spiralgalaxies, including our own Milky Way, have a bar-shaped paththrough their middles. Barred galaxies are shaped more like atiger's eye, with two starry arms trailing off either end of along, dark stardust lane. They take shape as stellar orbits in adisk become unstable and deviate from a circular path.
"The formation of a bar may be the final important act in theevolution of a spiral galaxy," says Sheth, a Spitzer staffscientist and lead author on a study examining the evolution ofbarred galaxies. "Galaxies are thought to build themselves upthrough mergers with other galaxies. After settling down, the onlyother dramatic way for galaxies to evolve is through the action ofbars."
According to new observations of over 2,000 spiral galaxies, madewith NASA's Hubble Space Telescope, the bar scene was dramaticallydifferent seven billion years ago, when the universe was half asold as it is today. The study is part of the Cosmic EvolutionSurvey (COSMOS), Hubble's largest survey ever, in which Sheth andhis team of 15 scientists is examining how galaxies form andevolve.
COSMOS covers an area of sky nine times larger than the full moon,surveying 10 times more spiral galaxies than previous studies,which Sheth says typically yielded ambiguous clues to barred galaxyevolution.
The astronomers discovered that while spiral galaxies were aroundin the distant past, only around 20 percent of them possessed thebars that are so common in their modern counterparts. The triplingrate does not proceed in an even-handed way, either. "They areforming mostly in the small, low-mass galaxies," says Sheth, addingthat among the most massive galaxies, the proportion of bars to nobars is the same as it is today.
"We know that evolution is generally faster for more massivegalaxies--they form their stars early and fast and then fade intored disks," Sheth explains. "Low-mass galaxies were also known toform more slowly, but now we see that they also made their barsslower."
Survey team member Bruce Elmegreen, an astrophysicist with IBM'sResearch Division, describes how a bar grows after stellar orbitsin a spiral galaxy begin to deviate from a circular path. "It locksmore and more of these elongated orbits into place, making the bareven stronger. Eventually a high fraction of the stars in the innerdisk join the bar."
Bars are perhaps the most important catalysts for changing agalaxy, Sheth says. They force a large amount of gas towards thegalactic center, fueling new star formation, buildingbulges--spheres in the centers of galaxies made only of stars--andfeeding massive black holes.
Indeed, bars may even contribute to the growth of black holes, saysNicholas Scoville, Caltech's Moseley Professor of Astronomy andCOSMOS principal investigator. "They pull stars and gas out oftheir normal circular orbits into the central regions, perhaps evenfunneling gas to the central supermassive black hole. Without thisfueling, the black holes would be starved and the central regionsof galaxies devoid of young stars."
"The new observations suggest that instabilities are faster in moremassive galaxies, perhaps because their inner disks are denser andtheir gravity is stronger," adds team member Lia Athanassoula ofthe Laboratoire d'Astrophysique de Marseille.
The Milky Way, possibly the best-known barred galaxy, is a massiveone whose bar probably formed somewhat early, like the bars inother massive galaxies, Sheth suggests. "Understanding how thisoccurred in the most distant galaxies will eventually shed light onhow it occurred here, in our own backyard," he adds. -CaliforniaInstitute of Technology