We tend to think of supermassive black holes as relatively stationary things – sitting there in the middle of a galaxy, while everything else revolves around it.
But this is not always the case, and now astronomers have the best evidence so far for a supermassive black hole that not only moves through the Universe, but moves inside its own galaxy. He has ants in his pants and witches in rifles and, although it is not clear why, the possible explanations are really exciting.
“We don’t expect most supermassive black holes to move; they’re usually just sitting around,” said astronomer Dominic Pesce of the Harvard & Smithsonian Center for Astrophysics.
“They’re so heavy that it’s hard to let them go. Consider how much harder it is to move a bowling ball than to throw a soccer ball – realizing that in this case, ‘bowling ball’ it’s a few million times the mass of our Sun. It’s going to take a pretty big hit. “
Catching a peripatetic supermassive black hole in the act is not an easy task. They can only be found in vast bays of space, from millions to billions of light years; at these distances, isolating the motion of an object — even if that object is a supermassive black hole — in an entire galaxy is a challenge.
Pesce and his team thought they might have some luck with a type of galactic nucleus called a megamaser. This is a type of supermassive active black hole, with a disk of gas and dust accumulation that is left on it, generating large amounts of heat and light.
With a megamaser, there is an additional ingredient to this formula: molecules such as hydroxyl, water, formaldehyde and methine that amplify the brightness of the nucleus in the wavelengths of microwaves.
Using a technique called very long basic interferometry, which combines observations from a network of radio telescope antennas to effectively create a huge observation antenna, the speed of these megamassers can be measured very accurately.
By studying water megamassers in particular, Pesce and his colleagues hoped to be able to identify any supermassive black holes that move at a different speed from the surrounding galaxy.
“I asked, ‘Is the speed of black holes the same as the speed of the galaxies I live in?’ he said. “We expect them to have the same speed. Otherwise, that means the black hole has been disturbed.”
The team looked closely at 10 megamassers, comparing data on the speed of the black hole with observations of the entire galaxy. Sure enough, nine of them were in line with our expectations of supermassive black holes hiding in the galactic center, like a spider in a canvas.
One of them, however, showed a different behavior. The spiral galaxy J0437 + 2456, located about 228 million light-years away, has a supermassive black hole about 3 million times larger than the Sun’s mass, which appeared to be moving at a significantly different speed from the rest of the galaxy.
According to the team’s analysis, the speed of the supermassive black hole is about 4,810 kilometers per second (2,990 miles per second). On the other hand, the galaxy’s neutral hydrogen appears to be retreating at a speed of 4,910 kilometers per second. According to observations of star and gas movements, the speed of the inner region of the galaxy is 4,860 kilometers per second.
Because all of these measurements differ significantly from each other, and the entire structure of the galaxy’s speed seems quite complicated, it’s hard to say exactly why everything is shaking there.
There are several possible explanations. The galaxy could experience a continuous encounter with another massive object, like another galaxy. The supermassive black hole could have collided with another supermassive black hole, generating a kickback that pushed the black hole out of position; the oscillation could be the galaxy and the black hole sitting back.
Or the black hole could have an unseen binary companion, the two objects orbiting each other’s center of gravity inside the galactic nucleus.
“Despite any expectation that it should really be in abundance, scientists have been able to identify clear examples of binary supermassive black holes,” Pesce said.
“What we could see in the galaxy J0437 + 2456 is one of the black holes in such a pair, the other remaining hidden from our radio observations due to the lack of maser emission.”
If it’s a kickback or a binary companion, this would be great news for astrophysics. There are many unanswered questions about supermassive black holes, such as how they become so large and whether the supermassive black hole binaries can close the final parsec of the distance between them. Evidence for binary and supermassive black hole fusions could help us answer these questions.
This is also great news for us here in the Milky Way: since we are a few billion years away from a galactic fusion, there is very little chance that our supermassive black hole, Sagittarius A *, will develop lust anytime soon.
The team hopes to take more observations of the galaxy and its particular core to try to narrow down the cause of its strange behavior.
The research was published in The astrophysics journal.