We tend to think of supermassive black holes as relatively stationary things – we’re right there smack bang in the middle of a galaxy with everything else swirling around it.
But that’s not always the case, and now astronomers have the best evidence yet of a supermassive black hole moving not only through the Universe, but also in its own galaxy. It has ants in its pants and witches in its pants, and while it’s not clear why, the possible explanations are really exciting.
“We don’t expect the majority of supermassive black holes to move; they are usually content to just stay put,” explains astronomer Dominic Pesce of the Harvard & Smithsonian Center for Astrophysics.
“They’re just so heavy it’s hard to get them going. Think how much harder it is to get a bowling ball moving than kicking a soccer ball – you realize that in this case the ‘ bowling ball “is millions of times the mass of our sun. It takes quite a powerful kick.”
Capturing a peripatetic supermassive black hole in the act is not easy. They can only be found in vast fissures of space, millions to billions of light years; at these distances, it is challenging to isolate the motion of one object – even if that object is a supermassive black hole – in an entire galaxy.
Pesce and his team thought they might be lucky with some sort of galactic core called a megamaser. This is a type of active supermassive black hole with an accretion disk of gas and dust that is slurped on it, generating massive 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 enhance the clarity of the core in microwave lengths.
Using a technique called very long baseline interferometry, which combines observations from a network of radio telescope antennas to effectively create one huge observation dish, the speeds of these megamasers can be measured very accurately.
By studying water megamasers in particular, Pesce and his colleagues hoped to identify supermassive black holes that move at a different speed than the surrounding galaxy.
“We asked, are the speeds of the black holes the same as the speeds of the galaxies they are in?” he said. ‘We expect them to have the same speed. If they don’t, it means the black hole has been disrupted. ‘
The team looked closely at 10 megamasers and compared the velocity data from the black hole with observations from the entire galaxy. Sure enough, nine of them matched our expectations of supermassive black holes lurking in the galactic center, like a spider in a web.
However, one of them showed different behavior. Spiral galaxy J0437 + 2456, located about 228 million light-years away, has a supermassive black hole about 3 million times the mass of the Sun, which appeared to move at a significantly different speed than the rest of the galaxy.
According to the team’s analysis, the supermassive black hole’s speed is about 4,810 kilometers per second (2,990 miles per second). On the other hand, the galaxy’s neutral hydrogen appears to be withdrawing at a speed of 4,910 kilometers per second. According to observations of star and gas movements, the speed of the inner part of the galaxy is 4,860 kilometers per second.
Since all of these measurements are quite different from each other, and the entire velocity structure of the galaxy seems rather complicated, it’s hard to say exactly why everything swings around in there.
There are several possible explanations. The galaxy could experience a constant encounter with another huge object, such as another galaxy. The supermassive black hole could have collided with another supermassive black hole, causing a recoil that pushed the black hole out of position; the wobble could be the galaxy and the black hole settling back down.
Or the black hole could have an invisible binary companion, with the two objects revolving around a common center of gravity in the galactic core.
“Despite all the expectations that they really should be plentiful, scientists have had a hard time identifying clear examples of binary supermassive black holes,” Pesce said.
“What we might see in the galaxy J0437 + 2456 is one of the black holes in such a pair, while the other remains hidden from our radio observations due to the lack of maser emission.”
If it is a backlash or a binary companion, that would be extraordinary news to astrophysicists. There are many unanswered questions about supermassive black holes, such as how they get so big, and whether supermassive black hole binaries can narrow the last parsec of distance between them. Evidence for supermassive black holes and mergers can help us answer these questions.
It’s also great news for us here in the Milky Way: Since we are a few billion years away from galactic fusion, our supermassive black hole, Sagittarius A *, is unlikely to develop wanderlust any time soon.
The team hopes to make more observations of the galaxy and its peculiar core to uncover the cause of its strange behavior.
The research is published in The Astrophysical Journal