Have astronomers found a runaway monster black hole?

Have astronomers discovered a monstrous runaway black hole or just a very strange galaxy?

By Phil Plait edited by Lee Billings

There is something fundamentally terrifying about a supermassive black hole hurtling through space at a speed greater than the speed of space. three million kilometers per hour.

Normally, these giants crouch at the center of galaxies and for good reason; they are generally the most massive objects in their host galaxy and therefore are not easy to move.

But then there is the RBH-1. For reference, the acronym “RBH” stands for “runaway supermassive black hole,” and this object may be just that: a monster with a mass tens of millions of times that of the sun moving astronomically through intergalactic space at breathtaking speed.

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Or maybe it’s just a strange galaxy. This uncertainty is perhaps the strangest part of the whole story: not that a giant black hole could exist, but the data is so ambiguous that we can’t be sure what we’re actually seeing.

Even funnier, astronomers discovered RBH-1 by accident! They were reviewing routine Hubble Space Telescope observations of a nearby dwarf galaxy when they spotted something peculiar: a long, linear streamer of light aligned with a distant galaxy. Follow-up observations that obtained and analyzed this structure’s spectrum (its brightness relative to its color, which can reveal a wealth of information about the emitting object) revealed that it was approximately 7.7 billion light-years from Earth. That means it’s quite large, measuring around 200,000 light years, or about twice the width of our galaxy, the Milky Way. The spectrum also shows that the structure is a mixture of gas and stars and suggests that the far end is a bright knot of gas as bright as nearly 50 million suns.

The discovery team proposed several interpretations of the structure, including debris from a galaxy collision, gas extracted from a high-speed galaxy moving through the tenuous intergalactic medium, and much more. But the researchers concluded that the best explanation was that the object was a runaway supermassive black hole that had been ejected from the core of a galaxy and was trailing a trail of material as it raced through space.

This may seem far-fetched, given that giant black holes are not known to wander around. Surprisingly, however, there are several ways to eject a black hole, even a gargantuan one. For example, when two galaxies collide, their black holes may fall toward each other and eventually merge. When this happens, a truly staggering amount of energy is released as gravitational waves in a pulse so powerful that it can briefly be thousands of times more energetic than all the stars of the observable universe combined.

If this energy is not released symmetrically – for example, if the rotations of the colliding black holes are not aligned with the plane of their mutual orbit – it can deliver a ridiculously strong kick to the resulting merged black hole, which is then ejected from the galaxy at high speed. It is also possible that in a particularly complex three-way galaxy collision, the three black holes interact gravitationally, resulting in two black holes forming a tight binary system while the third is thrown out.

This idea is therefore not as far-fetched as it seems at first glance. The astronomers also presented evidence to support their conclusion.

But almost immediately, this conclusion was called into question. Another team of astronomers published a different conclusion: the structure is actually an exceptionally flat “bulgeless” galaxy; that is, a disk galaxy similar to our Milky Way but lacking a central bulge of ancient stars. Such galaxies are rare but not unknown. Viewed from above, such a galaxy would appear as a thin line and could have the same mixture of gas and stars that the spectrum implies.

This team pointed out that a supermassive black hole passing through space would struggle to cause a wake powerful enough to collapse the gas into stars. Additionally, the researchers said the time frame was too short; Given the speed of the supposed black hole and the distance between it and the supposed host galaxy, the ejection event occurred about 39 million years ago – a relatively short time, cosmically speaking, to form so many stars.

Discussions between different astronomers about these observations continue, with some falling into the “black hole” camp and others siding with the “flat galaxy” team.

To be clear, this is good science! Everyone involved uses solid, albeit limited, data and knowledge accumulated over decades to try to explain them. Supporting certain positions while separating others is how we learn; Scientists of course want to be right. But overall they want to know what is this RIGHT.

So what is it?

Recent observations have brought a new twist to this phenomenon. Members of the original discovery team used the James Webb Space Telescope (JWST) to obtain infrared spectra of the object to see if the tip of the structure matched a massive shock wave from a black hole striking intergalactic material. In an article published in the Letters from astrophysical journals, they concluded that the observations did indeed support this conclusion. For example, looking along the structure, there is a huge change in the speed of the gas: it drops about 600 kilometers per second at the tip, which is about what you would expect for a hypersonic black hole sending shock waves through the surrounding gas.

But the original breakaway team of astronomers also analyzed this same JWST spectrum and came to a different conclusion. In an article published in AAS Research Notes, These researchers found that the data was more consistent with light emitted by fairly standard star-forming clouds of galactic gas than with gas that had been strongly shocked. Again, this indicates that the structure is a peripheral disk galaxy, not the wake of a runaway black hole.

So what can we make of all this? Despite the trust shown by both parties, I believe it is still premature to declare this matter closed. I would love to be able to say that this object is a rolling colossus creating newborn stars in its wake, because that would be exciting. On the other hand, such an elongated flat galaxy would also be quite strange and, although less striking, would still be of considerable interest to astronomers. But at this point, we still don’t know.

But again, it’s good science! Controversy like this is grist for astronomy’s mill and offers a chance to move the consensus one way or the other through more intelligent observations and analysis. This is how we learn what the cosmos tells us.

We don’t know what RBH-1 is, but we can also add my favorite word in all of science to the end of that statement: “yet.”