June 9, 2023

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A black hole loops between galaxies, leaving behind stars – Ars Technica

Zoom in / Artist’s impression of a supermassive black hole with galaxies behind it.

If you see a similar line on one of your photos, you should probably take a few minutes to clean the lens. But the streak, in this case, was in an image taken by the Hubble Space Telescope, which is unaffected by the schmutz that everyday life leaves on Earth-connected devices. Therefore, a team of researchers decided to find out what a long, thin smear might represent.

They’re still not sure, but the best explanation seems to be an awakening left over from a supermassive black hole that was blasted away from the galaxy that used to host it. Its liberation is likely caused by two additional supermassive black holes brought together by a galaxy merger. If true, it would be the first example of this behavior we’ve ever seen.

What is this?

Back in the days of film cameras, when it was sometimes possible to spend months or even years between taking an image and developing it, it wasn’t unusual to take newly developed footage and find yourself wondering what you’d captured a photo of. You can almost hear an echo of those days in the astronomers’ description of seeing the smear via one of the Hubble images: “The thin, almost straight line was readily apparent in the visual assessment of the quality of the data.”

A closer look revealed that the line extended toward a somewhat strange-looking galaxy. “We had never encountered anything like this before in our own images or in the literature, so we decided to include the feature in our Keck Table monitoring plan.” [telescope] Being.”

The line (center in both images) appears to have originated in the galaxy at upper right at two different wavelengths.

The line (center in both images) appears to have originated in the galaxy at upper right at two different wavelengths.

Van Dokkum and others. the.

Based on the redshift of the light from the object, both of them and the galaxy are about the same distance from Earth, which indicates that the two are linked. Researchers estimate that the line is about 200,000 light-years long. While the supermassive black holes at the center of galaxies can emit jets of material for a long time (and even longer), these jets tend to spread out as they get farther from the galaxy. In this case, the line was kept thin throughout its length.

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A look at the emitting stars in the streak indicates that stars in general get younger as they move down the streak and farther from the galaxy. Putting everything together, it appears that the line began forming about 40 million years ago, and its tip has been gradually moving away from the galaxy at a speed of about 1,600 kilometers per second since then.

old theories

One possible explanation for this motion is that the galaxy ejected a supermassive black hole. This is inevitable because of two observations: almost all galaxies seem to have a supermassive black hole at their core, and most galaxies are built up through multiple mergers. As a result, the supermassive black holes of pre-merger galaxies will eventually collide with each other. There are two ways this can lead to expulsion. One is that if two of these supermassive black holes undergo a merger where the gravitational energy production is unequal, that could impart a directional impulse to the post-merger product.

An alternative route to eviction occurs if one or more galactic mergers occur in relatively fast (in astronomical terms) mergers, then it is possible that not all of the central black holes have merged yet. In these cases, you could have three or more of these giants orbiting each other, allowing gravitational interactions to pull off one of them.

We’ve done a lot of modeling of these kinds of interactions, so we have a pretty good understanding of the expulsion process. What we don’t have is a really good understanding of what might happen once a black hole leaves the galaxy. It turns out we started designing this in the ’70s for the wrong reasons. People have suggested that ejected central black holes might be a way to explain the massive outpourings of quasars, implying that the quasars were not far away or bright. But it turned out to be quasars He was Really bright, so the whole line of thinking turned out to be wrong, and the idea was quickly dropped.

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Models indicate that almost everything in the process will be interesting. First, the ejected black hole would retain a shell of companion stars that were at the core of the pre-merger galaxy. After ejection, this shell would be comparable in size to either a large globular cluster or a very small dwarf galaxy. But the stars inside were moving incredibly fast because they were orbiting a supermassive black hole.

If the ejected cluster—called a high-pressure star system—encounters some of the gas after leaving the galaxy, it will create a shock wave within the gas, which may explain why the tip of the line is its brightest point. In its wake, the gas will collapse into the void left by the shock waves and unleash a round of star formation. This accurately explains the progression of older stars in the galactic direction.

On the other side

So, at least on a rough level, the general outline of this line looks like a massive supermassive black hole heading away from its former home. But there are two problems other than the fact that the models haven’t really been updated in decades, and our understanding of cosmology and computational power has grown exponentially since then.

Perhaps the biggest problem is that something appears to be on the other side of the galaxy from the location of the line. It is not as far from the galaxy as the tip of the line, and there is no line of stars connecting it to the galaxy. But at the same time, it appears to have an ionized shock front near it, and there is a sparse trail of ionized material flowing back into the galaxy.

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Therefore, if it is also a supermassive black hole, it must be heavier even than the one ejected along the line because it appears to be moving slowly by comparison (this assumes that both objects were ejected at the same time). And they have to travel through different materials because they don’t lead to the same kind of star formation.

Most problematic is that there is no obvious way to eject a second black hole at the same time but in the opposite direction. The simplest ejection mechanism involves three black holes, one ejected and the other two residual at the galactic core. It’s possible for these two to merge and get a gravitational kick from the merger, but it’s unlikely that it would send that kick in any particular direction – yet that object would travel directly away from the ejected black hole. It is possible to eject all three black holes, but this would require that they all be similar in mass – which is not particularly likely.

So, for now, the researchers tentatively suggest that this third object is another highly compressed star system with two supermassive black holes; Obviously something that requires more notes.

But this applies to the entire region. There is another object off the side of the line that seems to just align by chance, but this has not been confirmed. While astronomers managed to get a telescope time to look at the line, they didn’t get much out of it, and there’s a lot that can be done with deeper exposures and more spectral analysis. So, hopefully, a longer look will give us a better idea of ​​what we’re looking at.

Astrophysical Journal Letters, 2023. DOI: 2041-8213/acba86 (about DOIs).