If you saw a similar streak in one of your photos, you'd probably take a few moments to clean off the lens. But the streak, in this case, was in an image taken by the Hubble Space Telescope, which is not affected by the schmutz that daily life leaves on Earth-bound hardware. So, a team of researchers decided to figure out what the long, thin smear might represent.
They're still not certain, but the best explanation appears to be the wake left behind by a supermassive black hole that has been shot free of the galaxy that used to host it. Its liberation likely resulted from two additional supermassive black holes, all brought together by a merger of galaxies. If this is right, it will be the first instance of this behavior we've ever seen.
What is that?
Back in the days of film cameras, when it was sometimes possible to go months or even years between taking a photo and getting it developed, it wasn't unusual to pick up your newly developed snapshots and find yourself wondering what it was you had taken a picture of. You can almost hear echoes of those days in astronomers' description of seeing the smear across one of Hubble's images: "an almost-straight, thin streak was readily apparent in a visual assessment of the data quality."
A close look revealed that the streak extended toward a somewhat odd-looking galaxy. "Not having encountered something quite like this before in our own images or in the literature, we decided to include the feature in the observing plan for a scheduled Keck [telescope] run."
Based on the redshift of light from the object, both it and the galaxy are roughly the same distance from Earth, suggesting that the two are related. The researchers estimate that the streak is about 200,000 light-years long. While the supermassive black holes at the center of galaxies can emit jets of material that long (and even longer), those jets tend to spread out as they get farther from the galaxy. In this case, the streak remained thin throughout its entire length.
A look at the emission of the stars present in the streak suggests that, in general, the stars get younger as you move down the streak away from the galaxy. Putting everything together, it appears that the streak started forming about 40 million years ago, and its tip has been progressively moving away from the galaxy at about 1,600 kilometers a second since.
Old theories
One possible explanation for that movement is that the galaxy ejected a supermassive black hole. This is inevitable due to two observations: Almost all galaxies seem to have a supermassive black hole at their core, and most galaxies are built by multiple mergers. As a result, the supermassive black holes of the pre-merger galaxies will eventually run into each other. There are two ways this can lead to an ejection. In one scenario, if two of these supermassive black holes undergo a merger where the production of gravitational energy is uneven, this can impart a directional kick to the post-merger product.
An alternative road to ejection occurs if one or more galaxy mergers happen in relatively quick (in astronomical terms) succession. When this happens, it's possible that not all of their central black holes will have merged yet. In these cases, you can potentially have three or more of these giants looping around each other, allowing gravitational interactions to throw one out.
We've done a lot of modeling of these sorts of interactions, so we have a good understanding of the ejection process. What we don't have is a really good understanding of what might happen once the black hole leaves the galaxy. It turns out that we had started modeling this back in the 1970s for the wrong reasons. People suggested ejected central black holes might be a way to explain the immense jets of quasars, which would mean quasars weren't that distant or that bright. But it turns out quasars were really bright, so the whole line of thinking turned out to be wrong, and the idea was quickly dropped.
The models suggest that nearly everything about this process would be pretty interesting. For one, the ejected black hole would retain a shell of companion stars that used to be at the core of the pre-merger galaxy. After ejection, this shell would be similar in size to either a large globular cluster or an extremely small dwarf galaxy. But the stars within it would be moving incredibly rapidly because they were orbiting a supermassive black hole.
If the ejected cluster—termed a hypercompact stellar system—encountered some gas after it left the galaxy, it would create a shockwave within the gas, potentially explaining why the tip of the streak is its brightest point. In its wake, the gas would collapse into the void left by the shockwaves and set off a round of star formation. This neatly explains the progression of older stars trailing back toward the galaxy.
On the other side
So, at least on a rough level, the general outlines of this streak look like a rogue supermassive black hole heading away from its former home. But there are a couple of problems beyond the fact that the models haven't really been updated for decades, and our understanding of cosmology and computational power have both grown greatly since.
The biggest issue is probably that there seems to be something on the other side of the galaxy from the location of the streak. It's not as far from the galaxy as the tip of the streak, and there's no line of stars connecting it to the galaxy. But at the same time, it does seem to have an ionized shock front near it, and there's a sparse trail of ionized material leading back to the galaxy.
So, if it is also a supermassive black hole, it must be even heavier than the one ejected along the streak since it appears to be moving slowly in comparison (this assumes that both objects were ejected at the same time). And it must be traveling through different materials since it's not triggering the same sort of star formation.
More problematic still, there's no obvious way to eject a second black hole simultaneously but in the opposite direction. The simplest ejection mechanism involves three black holes, with one of them ejected and the other two remaining at the galaxy's core. It's possible that these two could merge and get a gravitational kick from the merger, but this kick isn't likely to send it in any particular direction—yet this object is traveling directly away from the ejected black hole. It is possible to eject all three black holes, but this requires that all of them be similar in mass—something that's not especially likely.
So, for now, the researchers are tentatively suggesting this third object is another hypercompact stellar system with two supermassive black holes; it's clearly something that requires more observations.
But that applies to the whole area. There's yet another object that's off to the side of the streak that just seems to be a chance alignment, but that hasn't been confirmed. While the astronomers managed to get telescope time to look at the streak, they didn't get a lot of it, and there's a lot more that can potentially be done with deeper exposures and further spectroscopy. So, hopefully, a longer look will give us a better sense of what we're looking at.
The Astrophysical Journal Letters, 2023. DOI: 2041-8213/acba86 (About DOIs).