First black hole found in globular star cluster

 作者:达婺钌     |      日期:2019-03-01 03:19:04
By David Shiga (Image: NASA/ESA/H Richer/UBC) A black hole has been found slowly devouring a companion star at the heart of a dense star cluster – providing the first clear sign that black holes inhabit the dense stellar cities known as globular clusters. Strong evidence for colossal black holes weighing millions or billions of times the Sun’s mass has been found at the centres of galaxies. And smaller black holes have been discovered in a range of environments, including within the spiral arms of the Milky Way. But there has previously been no clear-cut evidence for black holes of any size within globular clusters, spherical groupings of millions of stars. That is of interest because there are competing theories about what would happen to such black holes. Some theorists believe any black holes formed there – through the deaths of massive stars – would tend to be thrown out of the clusters after gravitational interactions with other stars. Others argue that the star-sized black holes would eventually glom together, forming ever larger black holes that might one day become the supermassive black holes at the centres of galaxies. Discriminating between the two scenarios is difficult because previously no black holes had been clearly observed in globular clusters. Hints of their existence came from either X-ray emissions – thought to arise as the black holes consumed their companion stars – or the motion of stars near the centre of some clusters (see Middleweight black holes are ‘missing link’). But some scientists have suggested the fast-moving stars near the cluster centres could instead result from the gravity of many dim, dead stars such as white dwarfs or neutron stars. Likewise, they say the strong X-ray emissions could come from matter piling onto several neutron stars that are too close together to distinguish, rather than from a single black hole. Now, astronomers led by Thomas Maccarone of the University of Southampton, UK, have found the best evidence yet for a black hole in a globular cluster. Using data from the European Space Agency’s XMM-Newton satellite, Maccarone and his colleagues found a bright source of X-rays coming from a globular cluster around a galaxy called NGC 4472, which lies about 60 million light-years away. The source is too bright to be explained by neutron stars sucking matter from their companions, unless several of them are contributing to the X-ray emission. And that seems unlikely, since the source was seen to vary in brightness by a factor of seven in just a few hours. Multiple objects would have to vary in sync with one another to achieve this. That leaves just one plausible explanation – that the source is a black hole siphoning matter from a companion star such as a red giant, Maccarone says. “This is much stronger evidence than there has been previously,” he told New Scientist. “All the other claims there has been quite a viable alternative explanation for, and I don’t think there is for this case.” The likely black hole is the only one found among the 6000 globular clusters that XMM-Newton observed in the vicinity of NGC 4472. Vicky Kalogera, an astronomer at Northwestern University in Evanston, Illinois, US, has done theoretical studies of how black holes should behave in globular clusters. She agrees that this is the best evidence so far of a black hole in one of these objects. She says it is possible that every globular cluster hosts a black hole. Her studies suggest, however, that black holes would only rarely have companion stars so nearby that they could steal matter from them and thus shine in X-rays. This would explain why it has been so difficult to find black holes in globular clusters, she says. “[Maccarone’s team] found one, but they found it in a galaxy that has lots of globular clusters – many more than our own galaxy,” she told New Scientist. Journal reference: Nature (doi: