- Maggie McKee
Is a second black hole lurking in the heart of the Milky Way? The evidence to date is inconclusive, but astronomers say a relatively simple test could settle the matter: look for a pair of stars fleeing the galaxy at break-neck speed.
Astronomers believe there is a colossal black hole – weighing about 3.6 million times the Sun's mass – at the centre of the Milky Way. But some contend there is also a second black hole there that weighs 1000 to 10,000 Suns.
The evidence comes from observations of a cluster of young stars located just a fraction of a light year from the monstrous black hole – where gravitational forces should prevent any stars from forming. The cluster could have formed farther away and migrated there, however, if it contained a middleweight black hole that was gravitationally drawn towards the galactic centre (see Second black hole may lurk at Milky Way's heart).
But it has been impossible to prove the suggestion. Now, Lu and colleagues say the case could be clinched if astronomers find a pair of so-called "hypervelocity" stars speeding away from the dangerous region.
Three's a crowd
Ten of these cosmic speed demons have turned up since December 2004, when the first was clocked streaking through space at 850 kilometres per second - fast enough to eventually escape the galaxy altogether.
Astronomers believe that something very massive must be accelerating the stars and that the acceleration arises in an interaction between three objects – but which three objects is up for debate.
In one scenario, a pair of stars wanders too close to a single, supermassive black hole, and one star gets captured while the other gets flung outwards at up to 4000 km/sec. In the other scenario, a single star approaches a pair of black holes and is ejected at high speed.
Now, Youjun Lu, an astrophysicist at the University of California in Santa Cruz, US, and colleagues say there is an observational test to distinguish between the two mechanisms. Their calculations suggest that finding a pair of hypervelocity stars hurtling through space at 1000 km/sec or faster would be "definitive evidence" of the existence of two large black holes in the Milky Way.
Sphere of influence
Due to their orbit around each other, two black holes would boast a wider sphere of influence on their surroundings than a single black hole. So if they were approached by a pair of stars that were quite close to each other (less than about a third the distance between the Sun and Earth), they would treat that stellar pair as a single star and shoot the pair outwards at hypervelocities.
That could not happen for a single black hole, says Lu. It would treat the stars as distinct individuals rather than a single unit, capturing one star and kicking out the other. "For a single black hole, the probability to eject a hypervelocity binary star is negligible," he told New Scientist.
Lu says about 10% of the stars in the Sun's neighbourhood have a close partner. So if that proportion is the same near the galactic centre, one of the 10 hypervelocity stars that have already been found may actually be a binary.
The stars are all too distant and dim to see if they are single or double. But their spectra should reveal if they "wobble" due to gravitational tugs from a close companion. The wobble would vary with the period of the stars' orbit around each other, which is expected to last from days to weeks.
Easy to test
Warren Brown, an astronomer at the Harvard-Smithsonian Center for Astrophysics, led a team that has found eight of the 10 known hypervelocity stars. He hopes to re-observe all of them in the fall and winter and aims to search for a signal on such timescales. "I think it's a fairly easy prediction to test observationally," he told New Scientist.
Lu says finding a second black hole in the Milky Way's centre would not only explain the puzzlingly young star cluster found there but would also mesh with standard theories of how galaxies grow.
Galaxies are thought to bulk up through mergers between small galaxies or star clusters, each of which may contain its own massive central black hole. Over time, the black holes are thought to draw closer to each other before they eventually merge, as well. "So we may have more than one black hole in the galaxy's centre," Lu says.
Brown agrees that the Milky Way's supermassive black hole may have grown by swallowing other black holes in the past, but he believes the evidence points to just one black hole in the galactic centre today. "A single black hole seems to be the most likely scenario in my mind," he says. "But there's no evidence that [a second black hole] wasn't there 100 million years ago and it's merged and we don't see it now."
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