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Supermassive black holes are usually the centerpiece of a galaxy. They sit there at the core, weighing millions (if not billions) of times the mass of our Sun. So it came as a big surprise when astronomers caught one moving at high speed through its host galaxy. Such a setup was thought possible, but actually finding a speeding black hole is far from easy.
Reporting in The Astrophysical Journal, the team studied supermassive black holes in 10 galaxies. Nine of those showed nothing amiss, but observation showed galaxy J0437+2456 has a black hole moving at about 180,000 kilometers (110,000 miles) per hour.
"We don't expect the majority of supermassive black holes to be moving; they're usually content to just sit around," lead author Dr Dominic Pesce, from the Harvard Smithsonian Center for Astrophysics, said in a statement. "They're just so heavy that it's tough to get them going. Consider how much more difficult it is to kick a bowling ball into motion than it is to kick a soccer ball — realizing that in this case, the 'bowling ball' is several million times the mass of our Sun. That’s going to require a pretty mighty kick."
The motion of the black hole was estimated using water. Water molecules swirling around black holes can sometimes produce incredibly bright radio-signals similar to a laser but usually in the microwave portion of the electromagnetic spectrum, hence their name, masers.
The maser of J0437+2456 shows a different velocity compared to what has been measured for the galaxy as a whole. It has also a different velocity from the gas and star measurements from the core of the galaxy. These pieces of evidence support the idea that the whole system experienced some major disruptions at some point. The researchers have put forward two hypotheses to explain what they saw.
"We may be observing the aftermath of two supermassive black holes merging," co-author Jim Condon, a radio astronomer at the National Radio Astronomy Observatory, explained. "The result of such a merger can cause the newborn black hole to recoil, and we may be watching it in the act of recoiling or as it settles down again."
The alternative is that another supermassive black hole was indeed involved but not via a merger. The two supermassive black holes may be orbiting each other, forming a binary pair. Such systems are not easy to find.
"Despite every expectation that they really ought to be out there in some abundance, scientists have had a hard time identifying clear examples of binary supermassive black holes," Pesce added. "What we could be seeing in the galaxy J0437+2456 is one of the black holes in such a pair, with the other remaining hidden to our radio observations because of its lack of maser emission."
More observations of this galaxy, located 230 million light-years from Earth, will be necessary to work out what sent this supermassive black hole on its merry high-speed trek.
