Astronomers have discovered three closely orbiting supermassive black holes in a galaxy more than 4 billion light years away. This is the tightest trio of black holes known to date and is remarkable since most galaxies have just one at their centre (usually with a mass between 1 million to 10 billion times that of the Sun). The discovery suggests that these closely packed supermassive black holes are far more common than previously thought and has been published in the highly esteemed scientific journal Nature.
The team, led by South African Dr Roger Deane from the University of Cape Town, used a technique called Very Long Baseline Interferometry (VLBI) to discover the inner two black holes of the triple system. This technique combines the signals from large radio antennas separated by up to 10 000 kilometres to see detail 50 times finer than that possible with the Hubble Space Telescope.
"What remains extraordinary to me is that these black holes, which are at the very extreme of Einstein's Theory of General Relativity, are orbiting one another at 300 times the speed of sound on Earth", says Deane. "Not only that, but using the combined signals from radio telescopes on four continents we are able to observe this exotic system one third of the way across the Universe. It gives me great excitement as this is just scratching the surface of a long list of discoveries that will be made possible with the Square Kilometre Array (SKA)."
Such systems are important to understand for several reasons; in terms of galaxy evolution it is known that black holes influence how galaxies evolve, and understanding how often black holes themselves merge is key to this work. Furthermore, closely orbiting systems such as this are sources of gravitational waves in the Universe, if General Relativity is correct. Future radio telescopes such as the SKA will be able to measure the gravitational waves from such systems as their orbits decrease.
At this point, very little is actually known about black hole systems that are so close to one another that they emit detectable gravitational waves. According to Prof Matt Jarvis from the Universities of Oxford and the Western Cape, "This discovery not only suggests that close-pair black hole systems are much more common than previously expected, but also predicts that radio telescopes such as MeerKAT and the African VLBI Network (AVN, a network of antennas across the continent) will directly assist in the detection and understanding of the gravitational wave signal. Further in the future the SKA will allow us to find and study these systems in exquisite detail, and really allow us gain a much better understanding of how black holes shape galaxies over the history of the Universe."
While the VLBI technique was essential to discover the inner two black holes (which are in fact the second closest pair of supermassive black holes known), Deane and co-authors have also shown that the binary black hole presence can be revealed by much larger scale features. The orbital motion of the black hole is imprinted onto its large jets, twisting them into a helical or corkscrew-like shape. So even though black holes may be so close together that our telescopes can't tell them apart, their twisted jets may provide easy-tofind pointers to them, much like using a flare to mark your location at sea. This may provide sensitive future telescopes like MeerKAT and the SKA a way to find binary black holes with much greater efficiency.
Deane, an SKA postdoctoral fellow, adds "The South African Government has made a major investment in astronomy, in funding what will be the most sensitive radio telescope in the Southern Hemisphere, but also in the significant Human Capital Development programme. As someone who may not have become a professional astronomer without the support of these initiatives, it is gratifying to be able to produce internationally recognised research following the strong support I have received from SKA SA and the SA Government. Four South African institutions (the Universities of Cape Town and the Western Cape, as well as Rhodes University and SKA South Africa) are represented by the research team which is in itself a very positive signal for radio astronomy here. This demonstrates that South Africa has the scientific and technical expertise to be a world leader in this research area and contribute directly to gravitational wave experiments that will provide fundamental insights not only to astronomy, but also more broadly to physics."