An international team of astronomers have uncovered the first reported mid-sized black hole through Australia’s CSIRO radio telescope. Dubbed Hyper-Luminous X-ray Source 1 (or HLX-1 for short), the black hole was spotted in ESO 243-29, a galaxy 300 million light-years away. The chance discovery occurred when researchers noticed the black hole emitting vast amounts of X-rays.
Prior to this only “stellar mass” and supermassive black holes were observed, the latter possessing mass up to a billion times that of the Sun. Dr. Sean Farrel, ARC Postdoctoral Fellow at the University of Sydney, estimates the likely mass of the black hole to be around 20,000 solar masses, although researchers have placed an upper limit of up to 90,000 solar masses.
“A number of other bright X-ray sources have been put forward as possibly being middleweight black holes. But all of those sources could be explained as resulting from lower mass black holes,” Dr Farrell said. “Only this one can’t. It is ten times brighter than any of those other candidates. We are sure this is an intermediate-mass black hole — the very first.”
When a star orbits too close to the black hole, it strips gasses from the star. As the gasses are sucked into the black hole they are heated to extreme temperatures that release an abundance of X-rays. This “feeding” allows astronomers to spot the black hole through a radio telescope.
With black holes known to be a hot topic for many years in astronomy, some are wondering why it took so long to discover this mid-sized class. “There maybe lots of others out there that are not currently feeding, and so are not detectable, or are feeding at a very low rate, so they don’t stand out as intermediate-mass black holes,” Dr. Ron Ekers said.
The discovery of HLX-1 is exciting not only because it is a new classification of black hole, but also because it might explain the nature of larger black holes. CSIRO’s Dr. Ron Ekers said, “We don’t know for sure how supermassive black holes form, but they might come from medium-size ones merging. So finding evidence of these intermediate-mass black holes is exciting.”
Along with contributing to the understanding of other black holes, astronomers are speculating to the nature of the mid-sized black hole. The team is now looking for signs of disturbance around the site of HLX-1 that would support the idea that the black hole was at the center of a dwarf galaxy that has since been engulfed by ESO 243-49.