You think our galaxy is special? Ha. Our boring pinwheel of gas and dark matter might be a nice hangout for humans. But 750 or so million light years away, there's an elliptical galaxy, Galaxy 0402+379, whose two supermassive black holes are orbiting each other from a distance of only 24 or so light years. Their combined mass is around 15 billion times that of our Sun.
If you're not convinced, a team lead by scientists from the University of New Mexico have actually spotted the black holes moving relative to one another using the Very Long Baseline Array, a system of ten radio telescopes across the US, with locations in Hawaii and the Caribbean. Not only is it the first direct measurement of galactic centres orbiting each other like this, but it was a challenging calculation to actually do.
One of the study's authors, Roger Romani from Stanford University, told Gizmodo "I think, to me it's a technical tour de force to measure such small motion in the sky," he said. "It took a dozen years of measurement with the largest telescopic array on earth."
These black holes are big -- probably a few thousand times more massive than the Milky Way's central monster, Sagittarius A*. They're likely the result of a whole cluster of galaxies merging together, according to Romani. And while they're 24 light years apart, that's actually an incredibly close distance when it comes to orbiting black holes. "Two black holes that close together is pretty exceptional," he said. Still, it would take around 30,000 years for the behemoths to complete one full orbit. The researchers published these results in The Astrophysical Journal.
Capturing such motion from such distant sources in the sky requires the use of the VLBA, a network of radio telescopes set up to do Very Long Baseline Interferometry or VLBI. Essentially, the Earth itself is turned into a telescope, but instead of focusing light in a mirror as a single telescope does, the light gets captured in individual radio telescopes and focused by comparing the information in supercomputers. These kinds of observations let researchers resolve very distant but (relatively) small things. A network of telescopes performing VLBI are trying to resolve the point-of-no-return around Sagittarius A*, for example.
Black holes may be one of the universe's most bizarre phenomena. They're literally divide-by-zeros in the sky, places where the mathematics of Albert Einstein's theory of general relativity falls apart. These dense behemoths have such strong gravitational fields that time stops, and all futures point directly at the centre, and light crossing the boundary, or event horizon, can't escape. But no one's ever taken a picture of a black hole, and scientists want to change that.
Other researchers were excited about the observation. "It's an absolutely beautiful result. This is highly compelling evidence for what would be the first known supermassive black hole visual binary ever observed," meaning a pair of super massive black holes that can be resolved separately with a light telescope, Grant Tremblay, astrophysicist at Yale and Harvard told Gizmodo. "It will be a hugely compelling follow-up target for years to come." Romani pointed out that studying black hole binaries like these could help us understand how black holes coalesce in the center of large galaxies.
Another astronomer, Sarah Spolaor at West Virginia University, called out how difficult these supermassive black hole binaries are to find, and that none of the candidates are completely confirmed just yet (even this one). She still thought this observation was exciting.
"Their target source 0402+379 is... one of the most credible candidates for a number of reasons; perhaps one of the most prominent is that it has actually been 'photographed' and we can see material separately marking both black holes," she said. "Thus, discovering orbital motion in 0402+379 would be a dream come true for many," and maybe one day scientists might be able to make a radio wave video of orbiting black holes.
She did point out that "orbital motion isn't the only possible explanation for what they have seen," something that Romani agreed with. He'll feel more confident once they have taken more data for more time. Or maybe they can find another orbiting pair.
"I'd be happier if we found a close pair that's faster," he said, "So instead of waiting 30,000 years to go around once you can wait only 100 years."