Supermassive black holes, which lurk at the centers of most galaxies including the Milky Way, can be spotted by their gravitational effects and the brilliant radiation of ill-fated stars and gas that they consume.
But a distant galaxy called A2261-BCG, located 2.6 billion light years away, lacks clear signs of a black hole, even though it should contain a real whopper with a mass between three and 100 billion Suns.
The mystery of the missing black hole inside A2261-BCG persists even after a dedicated search effort with the powerful Chandra X-ray Observatory, according to a forthcoming study. In addition to looking for the object at the center of the galaxy, scientists investigated the possibility that it was forcefully ejected from the galactic core, which would make it a never-before-seen “recoiling black hole.”
“The maximum reasonable kick, or recoil speed, that [supermassive black holes] can get, is 5,000 kilometers per second, which is enough to kick it out of anything,” said Kayhan Gultekin, an astrophysicist at the University of Michigan who led the new research, in a call.
“That would send it pretty quickly out of the galaxy and into intergalactic space, at which point it would be impossible to find just because there's not enough stuff to make it glow,” he added.
Gultekin and his colleagues used Chandra observations captured in 2018 to examine A2261-BCG. After finding no clear signs of the black hole at the galactic core, the team focused on four “stellar knots,” which are dense concentrations of stars and gas located thousands of light years from the galactic center. Those knots might be telltale signs of a recoiling black hole absconding with stellar material into the outskirts of A2261-BCG.
“One of those four could be what's called a cloak—the remaining bound stars to the black hole that’s been kicked away,” Gultekin explained.
“I was very skeptical and thought we would see something at the very center,” he continued. “But that turned out not to be the case. It turned out to be not in any of these locations.”
It’s possible that there is a supermassive black hole lurking inside A2261-BCG in an unexpected place, or that the object is “being a very stealthy black hole,” Gultekin said, meaning it is relatively dim compared to other objects of its class.
Regardless, finding the answer to this riddle is important because it could provide a glimpse of the fallout from the merger of two supermassive black holes. These volatile unions are thought to happen during galactic collisions, when supermassive black holes from different galaxies eventually coalesce into one object.
Mergers between supermassive black holes have never been observed, and scientists aren’t totally sure they happen. Capturing direct evidence of supermassive black hole mergers is a major project for astronomy that would shed light on a host of open cosmic questions.
We know that mergers occur between smaller black holes because scientists have detected gravitational waves, or ripples in spacetime, created by these events. The combination of light-based and gravitational wave astronomy, and any other streams of data we may unlock in the future, is called “multi-messenger astronomy,” and it is key to building a comprehensive model of the universe and its bizarre processes.
“What excites me the most is learning about supermassive black holes through gravitational waves,” Gultekin said. “We need to know for certain that they are merging and this would be one way of showing that that's happening.”
“There are all sorts of things you can learn with gravitational waves about supermassive black holes, as a population or individual sources, that you are either really hard or impossible to learn with traditional electromagnetic astronomy,” he noted.
The European Space Agency’s Laser Interferometer Space Antenna (LISA), which is due for launch in the 2030s, might be able to pick up gravitational waves from supermassive black hole mergers. But the best bet for finding A2261-BCG’s hidden hole is NASA’s James Webb Space Telescope (JWST), currently due for launch in October 2021.
If JSWT were to train its sights on A2261-BCG for a few days, it could produce a dynamical model of the galaxy that would likely pinpoint the warping of stars and gas around the supermassive black hole—assuming the galaxy still has one.
“We really need these dynamical models,” Gultekin said. “I think our last chance of finding out where it is.”