Astronomers have found black holes and supermassive black holes. But the discovery of a mid-size black hole could support the idea that supermassive black holes grow by eating others.
S. Farrell/ Sydney Institute For Astronomy, University of Sydney / NASA, ESA
In a galaxy 290 million light-years away, a cosmic fugitive has emerged from hiding, bearing clues about how galaxies and the supermassive black holes at their centers evolve.
The fugitive: the never-seen-before intermediate-sized black hole.
For three years, an international team of astronomers has had the elusive black hole under surveillance. The researchers now say they've found the strongest candidate yet for the mid-sized black hole, which may have been at the core of a dwarf galaxy that collided with a larger galaxy less than 200 million years ago.
If the analysis holds up, it would strengthen the notion that the black holes at the center of galaxies grow through collisions and mergers involving their host galaxies, just as the galaxies themselves are thought to grow. In the case of the fugitive, dubbed ESO 243-49 HLX-1, it's expected to eventually merge with the supermassive black hole at the center of its host galaxy.
A black hole is an object with gravity so strong that nothing can escape its grip, even when traveling at the speed of light. Astronomers have found convincing evidence that black holes, which form from the collapse of individual stars, exist. They've also found convincing evidence that supermassive black holes exist in the hearts of galaxies.
But questions remain about whether intermediate-mass black holes exist, let alone about how they formed, notes Mathieu Servillat, an astrophysicist and member of the team reporting the results in the journal Astrophysical Review Letters.
Although the team has been studying HLX-1 as if it were an intermediate-mass black hole, astronomers have yet to agree on any candidate as the real deal. In the past eight years, two other groups have each identified a candidate. But the evidence for each failed to convince other researchers.
“If intermediate black holes exist, they would have an impact at different stages of the evolution of the universe,” says Dr. Servillat, a researcher at France's National Center for Space Studies. “At the very beginning, you have to form intermediate-mass black holes. Is this a giant star that is collapsing? Is this a number of stellar-mass black holes accumulating to become an intermediate black hole?”