The galaxies we see in the present day Universe were built through the merger of smaller ones. Almost all of the galaxies contain supermassive black holes at their cores. Basic logic would suggest that the mergers would also have placed supermassive black holes in close proximity at the cores of galaxies. What’s less certain is what happened to them once they were brought together.
Ultimately, the fate of these black holes will be to merge. But if the process is slow enough, we should see a large number of binary supermassive black holes lingering in the cores of galaxies, producing gravity waves as they interact. We’d love to detect those gravity waves but it’s hard to justify building the appropriate detector until we know they’re out there which means we need to determine whether supermassive black hole binaries are common.
On Wednesday, an international team of researchers has announced that they’ve found 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 center (usually with a mass between 1 million to 10 billion times that of the Sun).
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 kilometers to see detail 50 times finer than that possible with the Hubble Space Telescope.
Deane and his group originally became interested in this particular galaxy known by the unwieldy name SDSS J150243.091111557.3, because it had been flagged by the Sloan Digital Sky Survey (thus the “SDSS” in the name) as having what looked like two sources of bright light in its core.
That indicated the possibility of two black holes there with the light coming not from the invisible objects themselves but from the whirlpools of gas heated to incandescence as they spiral in under the black holes’ intense gravity. Jets emitted by the black holes pinpointed their location.
Astronomers have learned over the past decade or two that virtually every full size galaxy such as our own Milky Way has a giant black hole lurking in its core. These monsters weigh in with a mass equal to millions or even billions of stars. The new observations, however, described in the journal Nature, suggest that many galaxies have not one but two or more giant black holes in their centers, orbiting each other in a tight gravitational dance that will ultimately lead the objects to merge together into something even more gigantic. Watching these mergers will offer insight into how gravity behaves when stretched to its limits, astronomers predict, with clues revealed by monster black hole mash-ups such as the just-discovered triplet.
The two are close enough together that orbital interactions have twisted the jets of one black hole into an S-shaped curve. The authors estimate that the separation between the two is only 140 parsecs, making them the second-closest pair of supermassive black holes we’ve ever spotted. This is also only the fourth triple black hole system that’s been detected.
The authors think that the S-shaped jets might be a diagnostic indication of these closely spaced black hole systems. They looked at a total of only six galaxies before spotting this pair which they take as an indication that similar systems may be common, a contrast to previous searches which had found very few.
“We were quite surprised to find it,” says Roger Deane, of the University of Cape Town in South Africa, lead author of the report.
Dr Roger Deane added, “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. 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 Kilometer Array (SKA).”
The researchers conclude by arguing that it’s worth continuing the search for more systems with multiple black holes. Identifying them will help us determine how common gravity waves of different frequencies are, which will help us design the next generation of detectors.