Distinctive black gap is trailed by 200,000 light-year-long tail of stars

Black holes might have a reputation as terrifying monsters, devouring all they come into contact with — but they can be a force of creation too, feeding the formation of new stars. Researchers using data from the Hubble Space Telescope recently spotted an unexpectedly huge trail of stars forming in the wake of a rogue black hole.

While most very large black holes, called supermassive black holes, sit at the center of galaxies, occasionally these enormous beasts can be found wandering alone in the depths of space. That’s the case with the recently discovered black hole with the mass of 20 million suns, which is streaking through the sky at tremendous speed. This likely began with two galaxies merging, each with its own supermassive black hole, which formed a binary system. Then a third galaxy got too close, and in the chaos of a three-way merger one of the black holes was kicked out and sent zipping off into space — so fast that if it were in our solar system, it would travel from the Earth to the moon in 14 minutes.

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This is an artist's impression of a runaway supermassive black hole that was ejected from its host galaxy as a result of a tussle between it and two other black holes.
This is an artist’s impression of a runaway supermassive black hole that was ejected from its host galaxy as a result of a tussle between it and two other black holes. As the black hole plows through intergalactic space it compresses tenuous gas in front of it. This precipitates the birth of hot blue stars. This illustration is based on Hubble Space Telescope observations of a 200,000-light-year-long contrail of stars behind an escaping black hole. ARTWORK: NASA, ESA, Leah Hustak (STScI)

The lonely black hole has been traveling space ever since, and it is now followed by a tail of stars an astonishing 200,000 light-years long. That’s twice as long as the Milky Way is wide. The effect seems to have occurred because the black hole is ramming into clouds of gas and leaving a trail of warm gas in its wake, creating a cozy environment for stars to form.

“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,” explained lead author Pieter van Dokkum of Yale University in a statement. “What we’re seeing is the aftermath. Like the wake behind a ship, we’re seeing the wake behind the black hole.”

This is the first time such a phenomenon has been observed, and it was spotted by chance when the researchers were poring over Hubble images and investigating what looked like scratches.

“This is pure serendipity that we stumbled across it,” van Dokkum said. “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 research is published in The Astrophysical Journal Letters.

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