We’ve heard of runaway galaxies. But black holes? That’s a new one. Astronomers have seen suspected runaway black holes before, but this is the first confirmation of one. 8 billion light-years away sits a galaxy designated 3C186.
Using the Hubble Space Telescope, astronomers noticed something wasn’t right with the galaxy. A bright quasar wasn’t shining from its usual spot. Quasars are the bright, energetic signs of an active black hole. Instead of nestled at the galactic core as it should be, it was located far from it.
The supermassive black hole has already traveled 35,000 light-years from the center of 3C186. And is moving at a blistering 4.6 million miles per hour. The Hubble website explains it another way. It could close the distance between the Earth and Moon in just three minutes.
You might be wondering how astronomers can figure this out since we can’t directly observe a black hole. Astronomers conducted a spectroscopic analysis of its surrounding gas to estimate speed and mass of the black hole.
What caused this?
Stefano Bianchi, co-author of the study, describes what kind of energy is required. “We estimate that it took the equivalent energy of 100 million supernovae exploding simultaneously to jettison the black hole.”
There are several scenarios that could explain what happened, but one theory proposed by the team is believed to be the most plausible.
About 1 to 2 billion years ago, two galaxies – each armed with a massive black hole – merged. These two supermassive black holes circled each other giving off gravitational waves “that were flung out like water from a sprinkler.”
But the two black holes weren’t the same. One was bigger than the other. Because of this, gravitational waves were emitted more strongly in one direction. When the two finally merged, the gravitational waves heading in the same direction created a kick that tossed the merged black hole away from the galaxy’s center.
“If our theory is correct, the observations provide strong evidence that supermassive black holes can actually merge,” says Bianchi. “There is already evidence of black hole collisions for stellar-mass black holes, but the process regulating supermassive black holes is more complex and not yet completely understood.”
Like most discoveries, catching a glimpse of this runaway black hole required some luck. Plenty of black hole mergers come and go without fanfare. Not only is this event unique, but the astronomers were able to spot it with the Hubble Space Telescope.
The team is hoping to get another good look at this runaway black hole. They’re looking into securing more time with Hubble along with the Atacama Large Millimeter/submillimeter Array (ALMA). Additional observations should give the astronomers a better handle on how fast the black hole and its surrounding gas disc is moving, and maybe reveal other mysteries about the leftovers of two supermassive black holes merging.