Think four-leaf clovers and double rainbows are rare? Try wrapping your head around this: A galaxy with three supermassive black holes chasing each other at its core more than 4 billion light years from Earth.
In a paper published online Wednesday in the journal Nature, an international team of astronomers announced the discovery of the fifth known triple supermassive black hole system in the universe.
"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," Roger Deane, lead study author and University of Cape Town astrophysicist, said in a statement.
The system bears the very unpoetic name of SDSS J150243.09+111557.3 — or just J1502+1115 for short — and was detected by an array of distantly spaced radiotelescopes using a technique known as very long baseline interferometry, or VLBI.
Every galaxy the size of our own and larger is believed to contain a supermassive black hole at its center. (Supermassive in this case would be 1 million to 10 billion times more massive than our sun.)
However, some have more than one central black hole — each orbiting the other in relatively close proximity — and scientists say this is probably the result of two or more smaller galaxies merging.
Though the detection of such tight binary systems is a rare event, increasing use of VLBI is changing some long-held assumptions.
Study authors note that they made their discovery after searching just six likely candidate galaxies.
"Our research shows that close-pair black holes may be much more common than previously thought, although their detection requires extremely sensitive and high-resolution observations," said study co-author Zsolt Paragi, an astronomer at the Joint Institute for VLBI in Europe, or JIVE, in Dwingeloo, the Netherlands.
Scientists say the gravitational effects of binary systems may provide new insights into the formation and evolution of galaxies.
"As they coalesce, tight binary supermassive black hole systems emit powerful gravitational waves — ripples in the fabric of space-time that propagate at the speed of light," said Greg Taylor, a University of New Mexico astrophysicist who was not involved in the discovery, but wrote an accompanying News and Views article in Nature.
"The detection of these waves would provide additional confirmation of Einstein's general theory of relativity, and would give astrophysicists a new way to explore the cosmos," Taylor wrote.
In the case of J1502+1115, the two closest black holes are separated by a distance of 140 parsces (one parsec equals about 19 trillion miles). The third supermassive black hole is much farther away.
Like most other supermassive black holes, the three eject enormous streams of matter in so-called jets.
The jets of the distant black hole are straight, while those of the two closest black holes are twisted into a helical, or corckscrew pattern, because of their orbital motion.
Resarchers say that similar jets spotted elsewhere bear closer scrutiny, as they may also reveal binary supermassive black holes.
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