The hole story: A simulated image of what you might see as you descended into a black hole, with the Milky Way in the background.
Credit: Wikimedia
SYDNEY: The gravitational waves created when supermassive black holes plunge together can kick one right out of the centre of its galaxy, says a Japanese study.
The research, which mathematically models what happens when supermassive black holes collide, could help experts to spot merged black holes in distant galaxies and is slated for publication in the Astrophysical Journal Letters.
Event horizon
Supermassive black holes sit in the centre of most galaxies, including our own. They are 100,000 to 10 billion times the size of the Sun and their gravitational pull is so strong that they absorb everything within their event horizon (an imaginary boundary representing the gravitational point of no return), including light.
Theoretically, as galaxies merge and grow, their black holes must also merge, forming a common event horizon. As the black holes join together, they emit a 'last gasp' in the form of an enormous burst of gravitational radiation. The idea is that if merging black holes are of unequal mass, or have an opposing spin, the final remnant black hole recoils or is kicked, sending it outside the centre of the galaxy.
Despite the theory, astronomers have found no direct evidence of merged black holes or those that have been booted out of place.
In this new study, Yutaka Fujita of the University of Osaka, tested one candidate for booted black holes; extremely bright objects found in many disk galaxies called ultra-luminous X-ray sources or ULXs. ULXs are of interest to astronomers because they are brighter than neutron stars and stellar-mass black holes, and are not found near the centre of their galaxies.
Different kind of evidence
Despite Fujita's hunch, his mathematical modelling study of colliding black holes revealed that though the X-ray luminosities and off-centre position of kicked black holes matched those of ULXs, the estimated speed didn't fit. ULXs are therefore unlikely to be the missing black holes, he said.
Nevertheless, the study has suggested that a different kind of evidence may yet be found, he said. "Recoiled supermassive black holes could be found through future extensive surveys."
Unlike smaller black holes, a recoiled supermassive black hole can shine even in the ordinary region of a galactic disk because of its huge mass, he said. "Although they would be bright just after the kick because of the emission from the accretion disk carried by the black holes, they would soon get dim as the disk is consumed by the black holes."
Interesting line of enquiry
Astrophysicist Geoff Bicknell from the Research School of Astronomy and Astrophysics at the Australian National University in Canberra, said the research followed an "interesting line" of enquiry despite the negative result.
"Black holes should get kicked when they merge [but]… if we don't see evidence of this, then there may be something wrong with our idea about what happens in this process," he said.
"One difficulty with observing the kick from a black hole merger is that, except under very favourable circumstances the kick will be small — about 100 to 200 km/s," said Bicknell. "Another potential problem is that most supermassive black holes appear to have formed at early epochs of the universe so the effect of that kick may have dissipated."
