A runaway black hole has been found leaving a trail of newborn stars in its wake

A trail found in the gas surrounding a distant galaxy could be the smoking gun pointing at a supermassive black hole on the run.

Based on an analysis of light that traveled over 7.5 billion years to reach us, a team of astronomers has presented evidence of a colossal object ejected from its host galaxy 39 million years ago, which is now accelerating through space. intergalactic at 1,600 kilometers. (994 miles) per second.

While the black hole itself is invisible, its wake is not: shocks left in the tenuous intergalactic medium leave behind a trail of star formation in the compressed gas. The team’s work shows a way in which we could identify quiescent supermassive black holes ejected from their galaxies to zoom, unseen and untethered, through intergalactic space.

The research, led by astrophysicist Pieter van Dokkum of Yale University, was accepted in The letters from the astrophysical journal and is available on the arXiv prepress server.

The idea that a supermassive black hole could be ejected from its galaxy isn’t all that outlandish. In fact, astronomers have already identified what they think could be multiple supermassive black holes ejected from the centers of their galaxies (although none have yet traversed intergalactic space), and even one galaxy that appears to lack its supermassive black hole.

But these supermassive black holes all have one thing in common: they’re active, which means they’re surrounded by a cloud of material that’s falling into their gaping maws of destruction. This process generates insane amounts of heat and light, which makes them much easier to detect.

But not all black holes are active. And those that are silently going about their business between snacks, just doing their thing, don’t emit light that we can detect and are therefore essentially invisible to our technology.

However, something as heavy as a supermassive black hole — millions to billions of times the mass of the Sun — can still leave traces that we can detect. That’s what van Dokkum and his colleagues proposed: that the wake of an ejected supermassive black hole can be detected in the gas surrounding a galaxy, known as the circumgalactic medium.

The discovery was made in the course of other investigations. The researchers were using Hubble to study a much closer dwarf galaxy called RCP 28. It was in this image that they discovered something that could just be the trail of a supermassive black hole on the run.

The image revealed a bright streak pointing directly towards the center of an irregular galaxy. Initially, the researchers thought it was a cosmic ray, but it showed up in both filters used to process the images. Then, in October 2022, they took follow-up images using the Keck Observatory, to calculate the galaxy’s and band’s redshift. That gave them a size: the band measures over 200,000 light-years across.

The analysis showed that the galaxy and the band have the same redshift, which means they are likely associated with each other, and the band and the galaxy have the same color. The team had never seen anything like it.

Looking closer, they found that the band was not uniform in color or brightness. It also shows signs of strong ionization and shock regions. Part of the ionization can be explained by the presence of very young, hot and massive stars; this is consistent with astrophysical shocks, which tend to compress gas and cause clumps to collapse under gravity, forming baby stars.

Bands of light emerging from the centers of galaxies are not uncommon; these are usually astrophysical jets, narrow and powerful streams of plasma traveling at speeds close to the speed of light, launched from the polar regions of active supermassive black holes. The streak the team found shows none of the characteristics of an astrophysical jet.

It’s possible, the team speculated, that a passing jet could have left a trail of star formation in its wake; but the scratch in the images does not match any observed or simulated instances of jet-induced star formation on record.

In fact, the observed trace is exactly the opposite of what astronomers would expect from a jet of gas; strongest at the farthest point of the galaxy, where there is less material, and narrowest over greater distances, rather than spreading out like a jet.

The team believes the best explanation is a supermassive black hole running amok, disturbing and compressing the circumgalactic medium as it travels, leaving star formation behind.

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You’re probably wondering what might eject a supermassive black hole from your galaxy, and the answer is: another supermassive black hole. Or two. In the researchers’ scenario, two galaxies merged once; the supermassive black holes in the cores of these galaxies coalesced into a binary supermassive black hole and stayed that way for a while.

Then came a third galaxy, and the supermassive black hole sank into the center of the newly merged trio of galaxies, resulting in a three-body interaction known as the Hills mechanism that flung one of the black holes away at high speed.

diagram showing six images of merging galaxies, with three black holes forced to meet and orbit each other
Diagram (left) showing in six steps a scenario in which three black holes (in red) can interact to produce the observed trail of stars. (van Dokkum et al., arXiv, 2023)

Future observations at various wavelengths could help astronomers discover whether this is indeed the case. In the meantime, because the feature is so distinctive, other examples should be relatively easy to find, particularly with more powerful instruments like the upcoming infrared Nancy Roman Grace Space Telescope.

“We argue that the feature is the hallmark of a runaway SMBH, relying on the small number of papers that have been written on this topic over the last fifty years,” the researchers write in their paper.

“This area could benefit from more theoretical work, mainly because these papers propose a variety of formation mechanisms for wakes. Hydrodynamic simulations that model shocks and also take gravitational effects into account can bring these initial studies together into a self-consistent framework. “

The research was accepted for publication in the journal The letters from the astrophysical journaland is available on arXiv.

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