A near-record breaking supermassive black hole has been spotted lurking in almost complete isolation around four million light-years from Earth.
In a surprise finding, Jens Thomas from the Max Planck Institute for Extraterrestrial Physics in Germany and colleagues used the Gemini North telescope in Hawaii to pinpoint the mass of a dormant black hole, 17 billion times as massive as the Sun, in an out-of-the-way galaxy called NGC 1600.
The work, which was published in Nature, could mean such behemoths are more common than once thought.
Supermassive black holes sit in the centre of galaxies. They grow by guzzling gas fuel from their own and neighbouring galaxies. In the early Universe, when it was smaller with more fuel, black holes could feed voraciously, spew out vast blasts of radiation and grow to massive proportions in a relatively short period.
Should two galaxies approach, they can merge into one, with the black holes' cataclysmic crash warping spacetime in the form of gravitational waves.
The biggest supermassive black hole found so far tips the scales at 21 billion times the mass of the Sun, and was discovered in 2011 in the Coma Cluster around 300 million light-years away. The Coma Cluster is a richly populated section of the Universe, with thousands of galaxies each containing millions or billions of stars.
But the galaxy NGC 1600 that houses this latest supermassive black hole discovery is an outlier: it's one of only around 20 galaxies in its group.
“It’s a bit like finding a skyscraper in a Kansas wheat field, rather than in Manhattan,” says Chung-Pei Ma of the University of California Berkeley and study co-author.
So how did the hermit black hole grow so large?
One big clue is the galaxy's isolation. The researchers believe it simply cannibalised nearly all the galaxies and their supermassive black holes around it.
It also seemed to thin out stars in its galaxy's centre – but not by guzzling them. When two black holes spiral around each other as they're slowly but inexorably are drawn to an explosive merger, they can slingshot stars out of the way.
Given its loneliness, "the black hole is a sleeping giant" at the moment, Ma says. As it's not feeding, and therefore not emitting radiation, the team had to look for it in indirect ways.
It was the slingshotted stars that gave the game away. When they focused on NGC 1600, they saw some stars were orbiting the central black hole, but others were travelling in the straight line, indicating they'd been booted out.
And given there are far more small or average-sized galaxy groups than super-crowded regions such as the Coma Cluster, it follows that there may well be more lonely monster black holes out there too.
"So the question now is, ‘Is this the tip of an iceberg?’" Ma says. "Maybe there are a lot more monster black holes out there that don’t live in a skyscraper in Manhattan, but in a tall building somewhere in the Midwestern plains."