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).
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