An optical (Hubble) and X-ray (Chandra, blue) image of the galaxy cluster SpARC 1049 shows the galaxies in the cluster center. Stars are forming at a furious rate right in the center of this image. Credit: X-ray: NASA/CXO/Univ. of Montreal/J. Hlavacek-Lar
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An optical (Hubble) and X-ray (Chandra, blue) image of the galaxy cluster SpARC 1049 shows the galaxies in the cluster center. Stars are forming at a furious rate right in the center of this image. Credit: X-ray: NASA/CXO/Univ. of Montreal/J. Hlavacek-Larrondo et al; Optical/IR: NASA/STScI)

When the supermassive black hole's away, the stars will play

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Aug 10, 2020, 10:40 AM EDT (Updated)

It probably wouldn’t surprise you to know that if you put a supermassive black hole in the middle of a party, it’ll have a pretty big impact.

We know this is true if the party is actually a galaxy cluster. These are like cities of galaxies, collections of hundreds and sometimes thousands of galaxies all orbiting one another. They make up some of the largest structures in the Universe (called superclusters) and are themselves some of the dominant structures in the Universe.

Galaxy clusters tend to have a lot of hydrogen gas floating around between the galaxies, and it can even be the most massive component of the cluster, outweighing the galaxies themselves. That gas tends to be hot, like really hot, tens of millions of degrees. For a long time this was a mystery, because gas like that should cool pretty rapidly (in a few hundred million years, which is short compared to the age of the Universe).

Artwork depicting the wind of gas blown out by the extremely hot disk of material around a supermassive black hole. Credit: NASA/JPL-Caltech

But now we understand that there’s usually a really massive galaxy in the center of the cluster, and it hosts a supermassive black hole. As material falls toward the black hole it piles up into a disk, which gets extremely hot… so hot that it heats the gas around it violently, literally blowing it away. This stuff then moves out into the cluster, heating the gas there, and preventing it from cooling.

That happens nearly every time, but astronomers have looked for years to see if there’s an exception out there.

An optical (Hubble) and X-ray (Chandra, blue) image of the galaxy cluster SpARC 1049 shows the galaxies in the cluster center. Stars are forming at a furious rate right in the center of this image. Credit: X-ray: NASA/CXO/Univ. of Montreal/J. Hlavacek-Larrondo et al; Optical/IR: NASA/STScI)

It looks like now they’ve found one. The cluster SpARCS104922.6+564032.5 (yeah, let’s just call it SpARCS 1049) is located about 10 billion light years away, so we see it as it was 10 billion years ago. It’s fairly beefy, with a total mass of well over 100 trillion times that of the Sun. That’s enough material to make a lot of galaxies like our own.

Like other clusters, it’s loaded with hot gas. But observations in the infrared using Spitzer Space Telescope revealed something weird: The existence of much cooler material called polycyclic aromatic hydrocarbons (or PAHs; they are literally organic molecules very similar to soot). This is the kind of thing you expect to see where lots of stars are being born.

And that’s weird. The gas in a cluster is so hot that it can’t form stars. When you heat up a gas it expands, and gravity has a hard time collecting it. It’s only when gas cools that it can form stars, and it would have to cool a lot to do so.

It gets weirder: The Spitzer observations show that a huge number of stars are being born, over 850 times the Sun’s mass worth of stars per year. That’s hundreds of times the rate they’re born in our galaxy, by comparison. Hundreds. That’s a staggering rate.

Even more weirder, the star formation appears to be happening in region about 80,000 light years from the center of the cluster, where no galaxy is seen. These are orphan stars, born outside a galaxy, just out there in the cluster.

The same image as before, but labeled. The star formation is occurring between the blob of hot gas and the central galaxy (which hosts a quiet supermassive black hole). Credit: X-ray: NASA/CXO/Univ. of Montreal/J. Hlavacek-Larrondo et al; Optical/IR: NASA/STScI)

So what’s happening here? The astronomers think that the supermassive black hole in SpARCS 1049’s central galaxy is… lazy. Or at least its pantry has run dry. It’s not actively feeding on material, so there’s no wind blowing from it that heats the cluster gas up. This allows the gas to cool, which in turn means it can form stars. Nothing quite like this has ever been seen before.

Why isn’t the gas falling into the black hole’s maw? It’s not clear, but the astronomers note that there may have been what’s called a minor merger, a collision with a much smaller galaxy cluster. This could have stirred things up a bit, moving the gas away from the core of SpARCS 1049, away from the supermassive black hole lurking there. If it had been a big merger, with another massive cluster, that could have released a torrent of gas toward the black hole (and then we’d see it, since that central galaxy host would be blazingly bright from all the activity), but with a much smaller one the effect is more subtle.

So this is pretty neat. It provides a counterexample to the more usual process in clusters of an active black hole suppressing star formation, in a way proving the consensus view of how it works by showing a quiet black hole allows star formation. The exception to the rule proving the rule.

When I was in grad school the question of what happens to the gas in a cluster was a big one. It was reasoned it should fall to the cluster center as it cooled, forming stars there. This even had a name: a cooling flow. Now we know why that doesn’t happen… usually. It’s actually pretty cool and even satisfying to see that it actually can happen. It’s just rare. But if you look hard enough, even the rare can be found.

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