Scientists have found a "cosmic killer" black hole that could be a long-anticipated missing link and solve one of the mysteries of the cosmos.
The black hole is thought to be an example of a "intermediate-mass" object, which scientists have long thought to exist but which has not been decisively discovered.
New research is the best evidence yet of such mid-sized black holes, which have until now proved mysterious to scientists. The black holes are about 50,000 times the mass of our Sun, smaller than a supermassive black hole that would inhabit the core of large galaxies, but bigger than the ones formed by the collapse of massive stars.
Scientists have seen evidence of such black holes before, but the new discovery is the most convincing example yet of such a phenomenon.
Astronomers captured the new evidence when the black hole was responsible for what Nasa referred to as a "cosmic homicide". It was spotted when it tore apart a star that passed too close, alerting scientists to the existence of the usually elusive object.
To spot it, astronomers had to use two X-ray observatories – one from Nasa and the other from ESA – as well as the Hubble Space Telescope. By combining the observations of all three satellites, they were able to trace back the powerful flare of X-rays to its likely source.
"Intermediate-mass black holes are very elusive objects, and so it is critical to carefully consider and rule out alternative explanations for each candidate. That is what Hubble has allowed us to do for our candidate," said Dacheng Lin of the University of New Hampshire, principal investigator of the study.
Hints of the possible black hole were first found when the X-ray observatories – Nasa's Chandra and ESA's XMM-Newton – picked up a powerful flare of X-rays coming from somewhere in the universe, in 2006.
The data was not enough to decide whether the flash had come from inside or outside of our galaxy. Astronomers were only able to say that it was probably caused by a star that was torn apart when it came too close to a powerful object like a black hole.
Scientists were surprised to find that the flash had not come from the centre of a galaxy, where such a massive black hole would be expected to be found. That was the first suggestion that the star might have been torn apart by an intermediate-mass black hole, or IMBH.
The other explanation was that the flare might have been sent out from inside our own galaxy, the Milky Way. The data might have been the result of a neutron star – the crushed remnants of a star that has exploded – that was cooling off after being heated up.
When the Hubble Space Telescope was pointed towards the source of the X-rays, however, it showed that it appeared to be coming from a star cluster on the edge of another galaxy. That indicated not only that the black hole was outside of our Milky Way, but also that it was coming from exactly the sort of place that astronomers had speculated would be the likely home of an IMBH.
The discovery of such an IMBH would answer a host of questions about the cosmos, including the mystery of where they have been lurking all this time. But the discovery by Lin and his team also makes way for other mysteries, which they hope to solve with more detective work, such as whether an IMBH can go on to grow into a supermassive black hole.