DWINGELOO, The Netherlands (25 June 2014) - 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. 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. The observations were done with the European VLBI Network (EVN) and the data were correlated at the Joint Institute for VLBI in Europe (JIVE) in Dwingeloo, the Netherlands.
"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 —especially in active galactic nuclei (AGN)— 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 time, very little is known about black hole systems that are so close to one another that they emit detectable gravitational waves. Co-author Zsolt Paragi from JIVE adds: "VLBI is widely recognized as one of the best ways to confirm close-pair black hole systems, but the main difficulty has always been pre-selecting the most promising candidates. Our research shows that close-pair black holes may be much more common than previously thought, although their detection require extremely sensitive and high-resolution observations. We have always argued that next generation radio telescopes such as the SKA should operate in VLBI mode as well, jointly with existing radio telescope arrays. This will allow to broaden our understanding of how black holes grew and evolved together with their host galaxies."
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-to-find pointers to them, much like using a flare to mark your location at sea. This may provide sensitive future telescopes an additional way to find binary black holes with much greater efficiency.
About VLBI and e-VLBI
VLBI is an astronomical method by which multiple radio telescopes distributed across great distances observe the same region of sky simultaneously. Data from each telescope is sent to a central "correlator" to produce images with higher resolution than the most powerful optical telescopes. Typically this data is recorded onto hard disks which are shipped to the correlator, but data can also be streamed and correlated in real-time, a technique known as e-VLBI. The authors used the e-VLBI technique for the initial selection of their target from a sample of six dual supermassive black hole candidates.
The Joint Institute for VLBI in Europe (JIVE, www.jive.nl) is a scientific foundation with a mandate to support the operations of the European VLBI Network (EVN, www.evlbi.org). For this purpose it maintains, operates and develops the EVN data correlator, a powerful supercomputer that combines the signals from radio telescopes located across the planet.
Dr. Zsolt Paragi, JIVE
E-mail: zparagi [at] jive [dot] nl
Tel: +31 (0)521-596536
Dr. Roger Deane, University of Cape Town, South Africa
E-mail: roger [dot] deane [at] ast [dot] uct [dot] za
Tel: +27 78 582-2308
A close-pair binary in a distant triple supermassive black hole system,
R. P. Deane, Z. Paragi, M. J. Jarvis, M. Coriat, G. Bernardi, R. P. Fender,
S. Frey, I. Heywood, H.-R. Klöckner, K. Grainge & C. Rumsey,
Nature, 3 July 2014
Figure 1: The inner pair of black holes of the triple system as seen by the European VLBI Network (EVN). Contours show radio emission at 1.7 GHz, the colour scale show radio emission at 5 GHz frequency.
Credit: R.P. Deane et al.
Figure 2:"Helical jets from one supermassive black hole caused by a very closely orbiting companion (see blue dots). The third black hole is part of the system, but farther away and therefore emits relatively straight jets."
Credit: Roger Deane (large image); NASA Goddard (inset bottom left; modified from original)
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