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 that’s 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 moves within its own galaxy. He has ants in his pants and witches in his pants, and while it’s unclear why, the possible explanations are really exciting.
“We don’t 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 so heavy that it is difficult to get them going. Consider how much more difficult it is to kick a moving bowling ball than to kick a soccer ball, realizing that in this case, the ‘bowling ball’ is several millions of times the mass of our Sun. That will take a pretty powerful kick. “
Catching a roaming supermassive black hole in the act is no easy feat. They can only be found across vast gulfs of space, millions to billions of light years; at these distances, isolating the motion of an object, even if that object is a supermassive black hole, across an entire galaxy is challenging.
Pesce and his team thought they might be lucky with a type of galactic core called a megamaser. This is a type of active supermassive black hole with a disk of accumulation of gas and dust that is sucked into it, generating large amounts of heat and light.
With a megamaser, there is an additional ingredient in this formula: molecules like hydroxyl, water, formaldehyde, and methine that amplify the luminosity of the nucleus at microwave wavelengths.
Using a technique called very long baseline interferometry, which combines observations from a radio telescope antenna array to effectively create a huge observation dish, the speeds of these megamasers can be measured very precisely.
By studying water megamasers in particular, Pesce and his colleagues hoped to identify any supermassive black holes moving at a different speed than the galaxy around them.
“We ask: Are the speeds of black holes the same as the speeds of the galaxies in which they reside?” he said. “We hope they have the same speed. If not, that implies that the black hole has been disturbed.”
The team closely examined the 10 megamasers, comparing the black hole’s velocity data with observations from across the galaxy. Sure enough, nine of them were consistent with our expectations of supermassive black holes lurking in the galactic center, like a spider in a web.
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 the mass of the Sun, 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 around 4,810 kilometers per second (2,990 miles per second). The galaxy’s neutral hydrogen, on the other hand, appears to be receding at a speed of 4,910 kilometers per second. Based on observations of the movements of stars and gases, the velocity of the inner region of the galaxy is 3,860 kilometers per second.
Because all these measurements differ from each other quite significantly, and the entire velocity structure of the galaxy seems quite complicated, it is difficult to say exactly why everything is wobbling there.
there are several possible explanations. The galaxy could be experiencing a continuous encounter with another massive object, such as another galaxy. The supermassive black hole could have collided with another supermassive black hole, generating a recoil kick that pushed the black hole out of position; the wobble could be the galaxy and the black hole settling again.
Or the black hole could have an invisible binary companion, the two objects orbiting a mutual center of gravity within the galactic nucleus.
“Despite all the expectations that they really should exist in abundance, scientists have struggled to identify clear examples of binary supermassive black holes,” said Pesce.
“What we could be seeing in the galaxy J0437 + 2456 is one of the black holes in that pair, and the other remains hidden from our radio observations due to its lack of maser emission.”
If it’s a kickback kick or a binary partner, that would be extraordinary news for astrophysicists. There are many unanswered questions about supermassive black holes, such as how they get so large and if supermassive black hole binaries can close the last parsec away between them. Evidence from mergers and supermassive black hole binaries could help us answer these questions.
It’s also great news for us here in the Milky Way: as we are a few billion years away from a galactic merger, there is very little chance that our supermassive black hole, Sagittarius A *, will develop wanderlust in the short term.
The team hopes to make more observations of the galaxy and its peculiar nucleus to try to pinpoint the cause of its strange behavior.
The research has been published in The Astrophysical Journal.