‘Runaway’ black hole tearing through the universe leaving ‘trail of stars’ like nothing ever seen

Unique object appeared to be scratches on Hubble images

A “runaway” black hole shooting through the universe is like nothing ever seen, scientists have said.

The object first appeared as “scratches” on images from Nasa’s Hubble Space Telescope.

But scientists now believe that it is a black hole, thrown out of its home galaxy and tearing through the cosmos, leaving a trail of stars in its wake.

“We think we’re seeing a wake behind the black hole where the gas cools and is able to form stars. So, we’re looking at star formation trailing the black hole,” said Pieter van Dokkum of Yale University.

“What we’re seeing is the aftermath. Like the wake behind a ship we’re seeing the wake behind the black hole.” 

Professor Van Dokkum spotted the accidental discovery in images from the Hubble telescope and documented them in a new paper, ‘A Candidate Runaway Supermassive Black Hole Identified by Shocks and Star Formation in its Wake’, published in The Astrophysical Journal Letters.

The black hole weighs as much as 20 million Suns and is travelling so fast that if it were in our solar system it would travel between the Earth and the Moon in 14 minutes. Behind it is a trail of stars that stretches 200,000 light years across, twice as wide as our Milky Way galaxy.

Those stars are thought to have formed when the black hole shot through space, compressing gas which then condenses and makes stars. Scientists think that happens because the black hole is moving so quickly through that gas that it shocks it – but still don’t know exactly how that works.

The whole unique phenomenon was spotted by accident when Professor Van Dokkum was looking for globular star clusters in a nearby dwarf galaxy. Instead, he saw what appeared to be artifacts on the images, and said it was “pure serendipity that we stumbled across it”.

“I was just scanning through the Hubble image and then I noticed that we have a little streak. I immediately thought, ‘oh, a cosmic ray hitting the camera detector and causing a linear imaging artifact.’

“When we eliminated cosmic rays we realized it was still there. It didn’t look like anything we’ve seen before.”

The runaway black hole was probably formed by a group of black holes that collided together over time. The first two were probably formed when two galaxies merged about 50 million years ago, with two supermassive black holes at their centre, which then whirled around each other.

Another galaxy then joined, with its own supermassive black hole. That destabilised the partnership and one of them was probably thrown out into space, though scientists do not know which.

Scientists hope to confirm that story using follow-up observations, checking whether the object really is a black hole and how it might have formed.

If that can be confirmed, it will be the first time that a black hole has been confirmed to have been kicked out of its home galaxy. Such a phenomenon has been predicted for decades.

“Something like this has never been seen anywhere in the universe,” said Professor Van Dokkum.

“We’ve known for a long time that supermassive black holes exist and it had been predicted for about 50 years that they could sometimes be ejected from galaxies. If confirmed, this would be the first evidence of a runaway supermassive black hole, proving this prediction.”