Black hole flies between galaxies, leaving stars in its wake – Ars Technica

Enlarge / Artist’s impression of a supermassive black hole dragging galaxies behind it.

If you saw a similar streak in one of your photos, you’d probably take a few moments to clean the lens. But the footage, 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-bound hardware. Thus, a team of researchers decided to understand what the long and thin smear could represent.

They’re still not sure, but the best explanation seems to be the wake left by a supermassive black hole that was released from the galaxy that hosted it. Its release likely resulted from two additional supermassive black holes, all brought together by a galaxy merger. If true, this will be the first example of this behavior we have ever seen.

What is this?

Back in the days of film cameras, when it was sometimes possible to go months or even years between taking a photo and developing it, it was not uncommon to take your newly developed shots and wonder what you took. a picture of. You can almost hear echoes of those days in the astronomers’ description of seeing the smear in one of the Hubble images: “a thin, nearly straight streak was readily apparent in a visual assessment of data quality.”

Close examination revealed that the sequence extended into a somewhat strange galaxy. “Having not encountered anything similar before in our own images or in the literature, we decided to include the feature in the observation plan of a programmed Keck. [telescope] run.”

The contrail (center in both images) appears to be coming from a galaxy in the upper right at two different wavelengths.

The contrail (center in both images) appears to be coming from a galaxy in the upper right at two different wavelengths.

van Dokkum, et. Al.

Based on the object’s light redshift, it and the galaxy are about the same distance from Earth, suggesting the two are related. The researchers estimate that the contrail is about 200,000 light-years across. While supermassive black holes at the center of galaxies can emit just as long (and even longer) jets of matter, these jets tend to spread out as they travel away from the galaxy. In this case, the drag remained thin along its entire length.

A look at the emission from the stars present in the sequence suggests that in general the stars get younger as you move away from the galaxy. Putting it all together, it looks like the contrail started forming around 40 million years ago, and its tip has been steadily moving away from the galaxy at around 1000 miles per second since then.

Old theories

A possible explanation for this movement is that the galaxy ejected a supermassive black hole. This is unavoidable 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, supermassive black holes from pre-merger galaxies will eventually collide. This can lead to ejection in two ways. The first is that if two of these supermassive black holes undergo a merger where the production of gravitational energy is unequal, it can give a directional kick to the post-merger product.

An alternative pathway to ejection occurs if one or more galaxy mergers occur in relatively rapid mergers (in astronomical terms), it is possible that not all of their central black holes have yet merged. 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 modeled these kinds of interactions a lot, so we have a good understanding of the ejection process. What we don’t have is a very good understanding of what might happen once the black hole leaves the galaxy. Turns out we started modeling this in the 1970s for the wrong reasons. People have suggested that ejected central black holes could be a way to explain the huge quasar jets, which would mean that the quasars weren’t that far away or that bright. But it turns out quasars were really brilliant, so the whole line of thinking turned out to be wrong, and the idea was quickly scrapped.

The models suggest that almost anything involving this process would be quite interesting. For one thing, the ejected black hole would retain a shell of companion stars that was at the core of the galaxy before the merger. After ejection, this shell would be similar in size to a large globular cluster or an extremely small dwarf galaxy. But the stars inside would move incredibly fast because they were orbiting a supermassive black hole.

If the ejected cluster – called the hypercompact star system – encountered gas after leaving the galaxy, it would create a shock wave in the gas, potentially explaining why the tip of the streak is its brightest point. In its wake, the gas would collapse into the vacuum left by the shock waves and trigger a round of star formation. This perfectly explains the progression of older stars moving up towards the galaxy.

On another side

So, at least on a rough level, the general outlines of this sequence resemble a rogue supermassive black hole moving away from its former home. But there are a few problems beyond the fact that the models haven’t really been updated in decades, and our understanding of cosmology and computing power has increased dramatically since then.

Probably the biggest problem is that there seems to be something on the other side of the galaxy from the location of the footage. It’s not as far from the galaxy as the tip of the sequence, 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’s a sparse trail of ionized matter leading out into the galaxy.

So if it’s 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 both objects were ejected in same time). And it has to travel through different materials because it doesn’t trigger the same type of star formation.

More problematic still, there is no obvious way to eject a second black hole simultaneously but in the opposite direction. The simplest ejection mechanism involves three black holes, one of which is ejected and the other two remain at the core of the galaxy. It’s possible that these two objects will merge and get a gravitational kick from the merger, but that kick probably won’t send it off in any particular direction, but this object is heading straight away from the ejected black hole. It is possible to eject all three black holes, but this requires that they all have similar mass, which is not particularly likely.

So for now, the researchers tentatively suggest that this third object is another hypercompact star system with two supermassive black holes; this is clearly something that needs more observation.

But this applies to the whole region. There is yet another object that is to the side of the footage which appears to be a random alignment, but this has not been confirmed. While astronomers have managed to get the telescope time to look at the sequence, they haven’t had much, and there’s a lot more that can potentially be done with deeper exposures and more advanced spectroscopy. So hopefully a longer look will give us a better idea of ​​what we’re looking at.

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

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