BLACK holes really do come in all sizes: last week, astronomers announced strong evidence that certain bright X-ray sources in the sky are indeed “intermediate-mass” black holes, hundreds of times the mass of our Sun. But their origins remain a puzzle.
We know of several small black holes in our Galaxy, similar in mass to our Sun, as well as monster black holes of up to a billion solar masses that live in the centres of galaxies. But until now we did not know whether it was possible to have black holes of any size in between.
Although we cannot see black holes directly, we can see the X-rays emitted by gas as it is sucked into a hole. The bigger the hole, the brighter the X-rays. When astronomers saw bright X-ray sources, or ULXs, the obvious inference was that they were medium-sized black holes. But others suggested that ULXs could simply be sending narrow beams of X-rays in our direction, from small holes after all.
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The latest results, given at a meeting of the American Astronomical Society at Mount Tremblant, Quebec, appear to rule that out. Tod Strohmayer and Richard Mushotzky of the University of Maryland used the European Space agency’s XMM-Newton satellite and NASA’s Rossi X-ray Timing Explorer to look at a ULX in the galaxy M82. The results imply that the X-rays do come from a disc of matter spiralling into the black hole, rather than a concentrated beam.
The strength of the emission oscillates in a roughly periodic way, as if coming from a rotating disc, and the object’s spectrum is characteristic of a spiralling gas. “By a process of elimination, we can say it is massive,” says Mushotzky.
Meanwhile, a group led by Jon Miller of the Centre for Astrophysics at Harvard University used XMM-Newton to measure the temperature of two ULXs in galaxy NGC 1313, about 10 million light years away. The gas around these objects is at a cool million degrees, compared with about 10 million for solar-mass black holes. Such a cool disc must be large to be such a bright X-ray source. The group calculates that each source probably holds a black hole of 200 to 500 solar masses. But it is hard to explain how these black holes formed.
Holes of up to 20 solar masses can be created when the cores of massive stars collapse. But there are no known stars big enough to make a black hole of 50, let alone 500, solar masses. One theory is that giant stars existed in the early Universe, and gave birth to these bigger black holes. Or they might form in dense star clusters, by eating stars, or each other.
Our Milky Way probably holds some middleweight black holes that are not visible to us. “They could be wandering through the Galaxy, starving for billions of years, until they find a supply of gas,” says Mushotzky.