The first solid evidence of a new class of medium-sized black holes has been discovered in a distant galaxy by an international team of astronomers.
The black hole is more than 500 times the mass of the Sun, the researchers report in the journal Nature.
Until now, identified black holes have been either super-massive (several million to several billion times the mass of the Sun) in the centre of galaxies, or about the size of a typical star (between three and 20 Solar masses).
The team, led by astrophysicists at the Centre d'Etude Spatiale des Rayonnements in France, detected the new black hole approximately 290 million light years from Earth with the European Space Agency's XMM-Newton X-ray space telescope.
It had been long believed by astrophysicists that there might be a third, intermediate class of black holes, with masses between a hundred and several hundred thousand times that of the Sun. However, such black holes had not been reliably detected until now.
"While it is widely accepted that stellar mass black holes are created during the death throes of massive stars, it is still unknown how super-massive black holes are formed," says a member of the team, Dr Sean Farrell, who recently completed his PhD studies at the Campus of the University of New South Wales at the Australian Defence Force Academy and now works at the University of Leicester, UK.
"One theory is that super-massive black holes may be formed by the merger of a number of intermediate mass black holes. To ratify such a theory, however, you must first prove the existence of intermediate black holes.
"The identification of HLX-1 is therefore an important step towards a better understanding of the formation of the super-massive black holes that exist at the centre of the Milky Way and other galaxies."
This new source, identified as HLX-1 (Hyper-Luminous X-ray source 1), lies towards the edge of the galaxy ESO 243-49. It is ultra-luminous in X-rays, with a maximum X-ray brightness of approximately 260 million times that of the Sun.
The X-ray signature of HLX-1 and the lack of a counterpart in optical images confirm that it is neither a foreground star nor a background galaxy, and its position indicates that it is not the central engine of that galaxy.
Using XMM-Newton observations carried out on the 23rd November 2004 and the 28th November 2008, the team showed that HLX-1 displayed a variation in its X-ray signature. This indicated that it must be a single object and not a group of many fainter sources. The huge radiance observed can only be explained if HLX-1 contains a black hole more than 500 times the mass of the Sun. No other physical explanation can account for the data.