Moons orbit planets, planets orbit stars, and stars in galaxies orbit supermassive black holes. And that’s more or less where the chain ends, because the super masses involved tend to anchor those black holes in place. But now, researchers have found evidence of one that’s on the move within its galaxy.
As the name suggests, supermassive black holes pack an incredible amount of mass, usually on the order of millions or even billions of times the mass of the Sun. As such, they have an incredible influence on the universe, trapping hundreds of millions of stars in orbit around them to form galaxies. Most of the time they’re a pretty stationary anchor, but it’s been speculated that perhaps some supermassive black holes do get around.
"We don't expect the majority of supermassive black holes to be moving; they're usually content to just sit around," says Dominic Pesce, lead author of the study. "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.”
So for the new study, researchers at the Center for Astrophysics/ Harvard & Smithsonian investigated whether they could find one of these supermassive monsters on the move. They studied 10 distant galaxies, which all had black holes with water in their accretion disks. This makes it relatively easy to measure the velocity of the black hole, since the water creates a radio light signal called a maser, which can then be studied using radio antennas.
It’s important to note that nothing is ever completely stationary – at the very least, distant objects will appear to be moving away from us, thanks to the expansion of the universe. So for this study, the astronomers were looking for black holes that were “moving” relative to the stars in their galaxies.
"We asked: Are the velocities of the black holes the same as the velocities of the galaxies they reside in?" says Pesce. "We expect them to have the same velocity. If they don't, that implies the black hole has been disturbed.”
Using a technique called very long baseline interferometry (VLBI) to study masers from these black holes, the team found that nine of the 10 supermassive black holes were relatively “at rest,” as expected. But one of them seemed to be moving.
With the mass of three million Suns, this wanderer sits at the center of a galaxy designated J0437+2456, about 230 million light-years away. Follow-up observations using Arecibo and the Gemini Observatory helped the team clock its movement at around 110,000 mph (177,000 km/h).
So what could have given this proverbial bowling ball such a huge push? The researchers say they might be witnessing the recoil after a collision between two supermassive black holes. Or they may have found a binary pair.
"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," says Pesce. "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.”
Further observations may help shed light on the mystery.
The research was published in the Astrophysical Journal.
Source: Center for Astrophysics