We鈥檝e caught another glimpse of a shy cosmic monster. Middle-weight black holes are elusive, but our best evidence of one so far comes from inside one of the Milky Way鈥檚 brightest star clusters 鈥� and its existence suggests similar objects may have seeded the supermassive black holes occupying the centres of giant galaxies.
Black holes come in two main weight classes. Stellar-mass black holes form when a large star collapses, and are roughly ten times the mass of the sun. In contrast, supermassive black holes, such as Sagittarius A* at the Milky Way鈥檚 centre, typically have millions or billions of solar masses.
And black holes in between? Good luck finding one: most claims for their existence have diminished in the light of new evidence.
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Now, of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and his colleagues have used a new technique to uncover an intermediate-mass black hole. It鈥檚 at the centre of the bright globular cluster 47 Tucanae, which is 15,000 light years from Earth. The cluster鈥檚 many stars radiate a combined luminosity 500,000 times more than the sun鈥檚 and have a total mass 700,000 times greater.
A good bet
The astronomers tracked the motion of 23 of the cluster鈥檚 pulsars 鈥� stars that emit precisely timed radio signals. 鈥淭o our surprise, we found that the existence of a black hole is required in order to explain the observations,鈥� K谋z谋ltan says. The pulsars鈥� positions and accelerations imply the black hole has roughly 2200 solar masses.
Unlike most of its better-known brethren, this black hole has no surrounding gas to emit X-rays or radio waves. That means we can only observe it based on its effect on the stars that orbit it.
It鈥檚 the best evidence yet for an intermediate-mass black hole in a globular cluster, says of the University of Michigan in Ann Arbor, who was not part of the discovery team.
at the California Institute of Technology in Pasadena has bet $100 against some of the other black holes claimed to inhabit globular clusters. 鈥淚鈥檓 not taking a bet on this one,鈥� he says. Still, he calls the result 鈥渢antalising鈥� rather than definitive.
Intermediate-mass black holes may grow into supermassive ones, K谋z谋ltan says. When the Milky Way was young, a globular cluster may have passed near the galactic centre, spilling stars and an intermediate-mass black hole there. That black hole could then have grown into Sagittarius A*. Finding more examples of these black holes in globular clusters could help explore how that happens.
Nature
