WHY are supermassive black holes so, well, supermassive? It has long been a mystery how enough matter can reach these cosmic gluttons to swell them to such large sizes. Now it seems the answer could be connected to a starry disc at the heart of the Andromeda galaxy. Although they may be hard to see, such discs may be common.
Black holes millions or billions times as massive as the sun reside at the heart of most galaxies, including our own. These black holes have been fattened up by huge amounts of gas. But astronomers don鈥檛 know how that gas makes it through a final hurdle, migrating the last dozens or hundreds of light years to be eaten.
Philip Hopkins and Eliot Quataert of the University of California, Berkeley, suggest that the formation of a skewed ring of stars facilitates the flow of gas, by sapping its momentum so that it spirals in towards the black hole.
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鈥淭he ring of stars saps the momentum of incoming gas, so it spirals in towards the black hole鈥�
Their simulations show that when there is enough gas present to prompt significant amounts of star formation, the newly formed stars orbiting a black hole naturally align to create an elliptical disc that can stretch out dozens of light years from the centre of the galaxy. This oval structure tugs unevenly on incoming gas, causing different streams to collide. The gas loses momentum and eventually gets close enough to the black hole to be swallowed up. In this way, black holes could consume as much as 10 solar masses of gas each year, Hopkins says. That鈥檚 enough to feed galactic black holes at the peak of their gluttony, some 10 billion years ago ().
Evidence may be in our neighbouring galaxy. Andromeda sports a 鈥渄ouble nucleus鈥� 鈥� two bright spots at its heart 鈥� thought to be a sign of an oval disc of stars and gas.
One test will be to see whether other galaxies have this stellar feature. 鈥淸Andromeda] is not unique. What we see there is likely to be commonplace,鈥� says of the National Optical Astronomy Observatory in Tucson, Arizona, who has identified several such galaxies.
鈥淭he attractive feature of this model is it seems to be a single calculation that can get the gas all the way in,鈥� says Scott Tremaine of the Institute for Advanced Study in Princeton, New Jersey.
