We tend to regard supermassive black holes as relatively stationary things – sitting there in the middle of a galaxy while everyone else is spinning over it.
But this is not always the case, and now astronomers have the best evidence yet for a supermassive black hole that not only moves through the universe, but also moves within its own galaxy. It has ants in its pants and witches in its brackets, and while it’s not clear why, the possible explanations are really exciting.
“We do not expect most supermassive black holes to move; they are usually content to just sit,” explained astronomer Dominic Pesce of the Harvard & Smithsonian Center for Astrophysics.
“They are just so heavy that it’s hard to get them going. Consider how much harder it is to kick a bowling ball than to kick a soccer ball – and realize that the ‘bowling ball’ in this “a few million times the mass of our sun. It’s going to take a pretty big kick.”
It’s no easy thing to catch a peripatetic, supermassive black hole. They can only be found over large gorges of space, millions to billions of light-years; at these distances, it is a challenge to isolate the motion of one object – even if it is a supermassive black hole – in an entire galaxy.
Pesce and his team thought they would have a bit of luck with some kind of galactic nucleus called a megamaser. It is a kind of active supermassive black hole with a disk of gas and dust wrapped around it, which generates large amounts of heat and light.
With a megamaser, this formula contains an additional ingredient: molecules such as hydroxyl, water, formaldehyde and methine that enhance the brightness of the core in microwave wavelengths.
Using a technique, very long base interferometry, which combines observations from a network of radio telescope antennas to effectively create one large observation scale, the velocities of these megamasers can be measured very accurately.
By studying aquaters in particular, Pesce and his colleagues hoped to identify any supermassive black holes that move at a different velocity than the galaxy that surrounds them.
“We asked: are the velocities of the black holes the same as the velocities of the galaxies in which they live?” he said. “We expect them to have the same speed. If they do not, it means the black hole is disturbed.”
The team looked at 10 megamasers and compared the data of the black hole velocity with the observations of the entire galaxy. Sure enough, nine of them were in line with our expectations of supermassive black holes lurking in the galactic center, like a spider in a web.
However, one of them showed different behaviors. The spiral galaxy J0437 + 2456, about 228 million light-years away, has a supermassive black hole about 3 million times the mass of the Sun, which apparently moves at a significantly different velocity than the rest of the galaxy.
According to the team’s analysis, the velocity of the supermassive black hole is about 4,810 kilometers per second (2,990 miles per second). The neutral hydrogen of the galaxy, on the other hand, appears at a speed of 4,910 kilometers per second. According to observations of star and gas motions, the velocity of the inner region of the galaxy is 4,860 kilometers per second.
Since all these measurements differ quite significantly from each other, and the highway structure of the galaxy seems quite complicated, it is difficult to say exactly why everything is moving around there.
There are several possible explanations. The galaxy may experience a constant encounter with another massive object, such as another galaxy. The supermassive black hole could have collided with another supermassive black hole, creating a kickback kick that pushed the black hole out of position; the wobbly can be the galaxy and black hole that sinks again.
Or the black hole may have an unseen companion, and the two objects revolve around a mutual center of gravity in the galactic core.
“Despite every expectation that they really should be there to a large extent, scientists have had a hard time identifying clear examples of binary supermassive black holes,” Pesce said.
“What we can see in the J0437 + 2456 galaxy is one of the black holes in such a pair, while the other remains hidden from our radio observations due to the lack of emission of machine.”
If it’s a kickback kick or a binary companion, this is extraordinary news for astrophysics. There are many unanswered questions about supermassive black holes, such as how they get so big, and whether supermassive black hole binaries can close the final distance between them. Evidence for extraordinary black hole binaries and mergers can help us answer these questions.
This is also good news for us here in the Milky Way: since we are a few billion years away from a merger, there is very little chance that our supermassive black hole, Sagittarius A *, will soon develop roaming loop.
The team hopes to take more observations of the galaxy and its peculiar core to try to limit the cause of its strange behavior.
The research was published in The Astrophysical Journal.