Have astronomers found a runaway monster black hole?

Have astronomers found a runaway monster black hole or just a very strange galaxy?

By Phil Plait edited by Lee Billings

There is something inherently terrifying about a supermassive black hole hurtling through space beyond three million kilometers per hour.

Normally these giants squat in the center of galaxies and for good reason; they are usually the most massive single objects in their host galaxy, and so are not easily shaken.

But then there is the RBH-1. As a hint, the acronym “RBH” stands for “running supermassive black hole,” and this object may be just that: a monster tens of millions of times the mass of the Sun hurtling astronomically through intergalactic space at mind-numbing speeds.

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Or it could just be a strange galaxy. This uncertainty may be the strangest part of the whole story: not that a running giant black hole might exist, but that 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 investigating routine Hubble Space Telescope observations of a nearby dwarf galaxy when they spotted something peculiar: a long, linear beam of light aligned with a distant galaxy. Follow-up observations that obtained and analyzed this structure’s spectrum—its brightness versus color, which can reveal a range of information about the emitting object—revealed that it was about 7.7 billion light-years away from Earth. This means that it is quite large, about 200,000 light-years long – about twice the width of our Milky Way. The spectrum also shows that the structure is a mixture of gas and stars and suggests that the outer end is a bright knot of gas that shines as brightly as nearly 50 million suns.

The discovery team came up with several interpretations of the structure, including debris from a galactic collision, gas stripped from a high-velocity galaxy moving through the thin intergalactic medium, and more. But the researchers concluded that the best explanation was that the object is a runaway supermassive black hole that was ejected from the core of a galaxy and has followed a wake of material as it plows through space.

This may seem far-fetched, given that giant black holes are not known to go on walks. Amazingly, however, there are several ways to eject a black hole, even one this gigantic. For example, when two galaxies collide, their black holes may fall towards 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 in the observable universe combined.

If that energy isn’t released symmetrically—for example, if the spins of the colliding black holes aren’t aligned with the plane of their mutual orbits—it can give 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 during a particularly complex three-way galaxy collision, all three black holes interact gravitationally, resulting in two forming a close binary system while the third is thrown away.

So this idea is not as goofy as it initially seems. The astronomers also presented evidence to support their conclusion.

Almost immediately, however, this finding 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 old stars. Such galaxies are rare, but not unknown. Viewed edge-on, such a galaxy will appear as a thin line, and it may have the same mixture of gas and stars suggested by the spectrum.

This team pointed out that a supermassive black hole hurtling through space would have a hard time raising a sufficiently strong wake to collapse gas into stars. Furthermore, the researchers said the timeline was too short; given the speed of the putative black hole and the distance from the putative host galaxy, the ejection event occurred about 39 million years ago—a relatively short time, cosmically speaking, to form so many stars.

Back and forth between different astronomers about these observations is still ongoing, with some falling in the “black hole” camp and others siding with the team’s “flat galaxy.”

To be clear, this is good science! Everyone involved uses solid, if limited, data and knowledge built up over decades to try to explain them. Supporting some positions while rejecting others is how we learn; Scientists will of course be right. But by and large, they want to know what is right.

So which one is it?

More recent observations have put a new spin on this. Members of the original discovery team used the James Webb Space Telescope (JWST) to obtain infrared spectra from the object to see if the tip of the structure was consistent with a massive shock wave from a black hole slamming into intergalactic material. In an article published in Astrophysical Journal Letters, they concluded that the observations actually supported this conclusion. Looking along the entire structure, for example, there is a huge change in the velocity of the gas: it drops at about 600 kilometers per second at the tip, which is about what you’d expect for a hypersonic black hole sending shock waves through surrounding gas.

But the original dissenting team of astronomers also analyzed the same JWST spectrum and came to a different conclusion. In an article published in Research notes from AAS, these researchers found that the data were more consistent with light emitted from fairly standard star-forming galactic gas clouds than they were with gas that had been strongly shocked. Again, that points to the structure being an edge-on-disc galaxy, not the wake of a stomping black hole.

So what can we do about all this? Despite the confidence on both sides, I think it is still too early to declare this case closed. I’d like to be able to say that this object is a steamrolling colossus creating newborn stars in its wake because that would be exciting. On the other hand, such an extremely elongated flat galaxy would also be quite strange and, although less flashy, still of considerable interest to astronomers. At this point, however, we still don’t know.

But again, this is good science! Controversies like this are grist to astronomy’s mill and provide a chance to push consensus one way or the other through further smart observation and analysis. This is how we learn what the cosmos tells us.

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