SA science team find super rare triple black hole galaxy

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Humanity’s best estimates put the total number of galaxies in the known universe at somewhere between 100 billion and 200 billion. We also know that most, if not all, of them have a “supermassive” black hole at the centre, which provides the huge gravitational force required to suck stars into galactic formation.

But how many galaxies do we know have three separate supermassive black holes at their heart, each orbiting the other on what will eventually be a collision course? Yesterday, it was four. Today, thanks to pioneering research from a team of astronomers led by the University of Cape Town, it’s five.

South African Dr Roger Deane (and a group of co-authors) have just published a paper in the journal Nature describing a triple black hole system some four billion light years away, in which three supermassive black holes are orbiting each other at 300 times the speed of sound on Earth. While the finding itself is described as “remarkable”, Deane says that it has big implications for our understanding of how galaxies are formed and therefore the underlying physics of the universe itself. It also, says Deane, bodes well for scientists working with the Square Kilometre Array (SKA) radio telescope currently under construction in the Karoo.

Currently astronomers believe that large, galaxy-forming supermassive black holes are sometimes formed when two or more smaller black holes collide and combine. Right now, it’s believed that systems with three black holes in are very rare – but Deane’s research challenges that consensus.

“It’s very rare to see more than two black holes together,” Deane explains, “We only know of five systems that have separations of less than a galaxy width, so could be considered a single galaxy. You expect there to be multiple binary supermassive black hole systems but we just haven’t observed them. Because we searched very few systems [for this paper] we either got very lucky or this means that they are more common than we thought.”

Deane's image shows two black holes tightly orbiting each other, with twisting jets, and a third moving around them some distance away.
Deane’s image shows two black holes tightly orbiting each other, with twisting jets, and a third moving around them some distance away. Credit: Roger Deane (large image); NASA Goddard (inset)

Astronomers have predicted the existence of these kinds of systems for some time now, says Deane, but its only recently that telescopes have been developed with imaging resolutions high enough to see them. Data for his research came from the European Very Long Baseline Interferometry (VLBI) network of antennae, which uses large telescopes of up to 100m diameter widely spaced out. The South African telescope at Hartebeesthoek and the Arecibo observatory in Puerto Rico – best known as evil Sean Bean’s hideout in the Bond movie GoldenEye – were both used to filter data and improve resolution.

When complete, SKA’s ability to search for finely separated gravitational waves will become a key part of future research into the subject area of how galaxies form. Black holes are the primary source of this form of radiation, and by measuring the speed at which they form astronomers can deduce whether or not two black holes have combined to produce one supermassive one. In the meantime, the MeerKAT observatory in Carnarvon and the existing African VLBI network will remain leading places for understanding gravitational waves.

The middle pair of supermassive black holes in today’s paper are the closest together of any pair so far observed, and Deane says that his research will enable scientists using SKA to better filter data for signs of multiple black holes.

“The other big result out of our paper is that we were directly imaging the inner two black holes and found that the jets they produced were distorted,” says Deane, “that’s the first time this has been seen, and suggests that we can use this signature pattern to search for other black holes that are even closer together, closer than we can currently image.”

Arecibo Observatory, as seen in GoldenEye.
Arecibo Observatory, as seen in GoldenEye.

[Main image – Hartebeesthoek telescope, M Gaylard/HartRAO]



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