50,000 solar masses – and that’s just a midsize black hole
Astronomers have been looking for medium-sized black holes for a long time. You’ve probably heard about the giant black holes at the center of galaxies and those that start with the mass of one star as a result of a supernova. But as small black holes, like from a supernova, gradually grown into giants, they must pass through intermediate stages sometime. The only problem is that these midsize black holes are not very easy to find.
The Hubble Space Telescope has now delivered some important evidence that such black holes actually exist. In 2006, the Chandra and XMM-Newton X-ray observatories detected radiation outbursts in the X-ray range, which were designated X-ray source 3XMM J215022.4−055108. Researchers conjectured that they might have been produced when a star was torn apart by a compact, massive object – something like a black hole. What was really interesting, however, was that the X-ray radiation wasn’t coming from the center of our galaxy, giving the researchers hope that they might have found a medium-sized black hole.
But first they had to rule out alternative explanations – like a neutron star cooling down. To do that, they pointed the Hubble Telescope toward the source. What they found wasn’t exactly what they had expected: 3XMM J215022.4−055108 isn’t located in the Milky Way, but instead in a dense star cluster that belongs to the outer regions of a different galaxy. But those findings also point precisely to a medium-sized black hole: such formations are expected in the core of dwarf galaxies. The dense star cluster could be the remnants of a dwarf galaxy that lost most of its stars when it merged with a larger galaxy. The black hole would have to be about 50,000 solar masses, as the astronomers have written in a paper.