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When I was in grad school, one of the hottest (ironically) topics of research was a phenomenon called “cooling flows.” When we look at clusters of galaxies — literally, collections of dozens or hundreds of galaxies like the Milky Way all orbiting one another, bound by their mutual gravity — we see lots of hot gas surrounding them. Over time, that gas should cool, flow down to the center, collect there, and form lots of stars.
The problem was, nobody could find those stars. Many of these clusters have galaxies right at their center, grown huge due to collisions with other galaxies that fell to the center. According to the numbers, though, so much cooling gas flows into them that they should be bright blue due to star formation — if you form enough stars, some percentage of them will be massive, hot, and blue, and those are so bright they overwhelm the light from redder stars. That’s why the Milky Way’s spiral arms look blue; it’s where star formation is going on.
But in cluster after cluster the central dominant galaxy wasn’t as blue as it should be. It was a big mystery. Were cooling flows real?
It turns, out, yes, they’re there. But the situation is complicated. The key to it is self-regulation. The process has been seen now for a while, but new observations of the galaxy cluster Abell 2597 — a beehive of galaxies about a billion light years from Earth — show it in stark detail.
Bonus: The observations are really, really pretty. See for yourself:
As you can see, there’s a lot going on there. This image shows the center of the cluster. What’s shown in red is warm hydrogen gas in observations by the Very Large Telescope, and yellow is colder gas (called molecular gas, since it’s cool enough to form molecules of various types) observed using the Atacama Millimeter/submillimeter Array (ALMA). Violet is extremely hot and thin gas emitting X-rays, observed by the Chandra X-Ray Observatory.
This almost looks like some sort of internal imaging of a human chest, with a spine, lungs, and heart. And hey, that analogy isn’t that bad. There is a circulation system here, and key to it is its heart: a black hole.
In the central part of the central galaxy there is a supermassive black hole — every big galaxy has one, including our own. Matter falling toward it tends to form a flat disk called an accretion disk. This can be very large, with the inner edge right over the black hole and the outer edge billions of kilometers farther out. Material closer to the black hole is orbiting near the speed of light (!), whereas material farther out is moving much more slowly. This means there’s a lot of friction and faster stuff rubs past slower stuff, and the disk gets ridiculously hot. Literally millions of degrees toward the center.
Also, this material can have an embedded magnetic field, which gets wound up near the center. The physics is pretty complicated, but the forces are so strong that the magnetic field can focus and launch hot gas in streams up and down, away from the disk, and this material moves. Packed with energy, this superheated gas screams away at high speed. They’re like the beams from a lighthouse, and astronomers called them jets.
Now zoom back a little. There’s gas in the central galaxy, and it’s heated by the jets. You can see that as the reddish glow in the image above. Now zoom even farther back, so we see the whole cluster. The material between the galaxies in the cluster isn’t empty. It’s filled with gas. There’s a big halo of extremely hot but extremely tenuous gas that emits X-rays. That’s the purple stuff in that image.
As the jets plow into gas it heats it. This gas inflates (after all, hot air expands) and rises inside the surrounding gas. We can actually see these as gigantic buoyant bubbles in the gas, cavities in the halo.
But as they rise, they cool, and cold gas tends to sink. This gas isn’t initially ejected from the black hole accretion disk rapidly enough to escape the cluster, so it slows, stops, and falls back down toward the center, cooling so much that molecules like carbon monoxide can form. These clouds make their way back to the center, where they fall toward the black hole, feeding back into the accretion disk.
See? Circulation. And the black hole is the heart that takes that cold stuff, heats it up, and flings it back up into intergalactic space. Another analogy is that it’s like a garden fountain, where the black hole is the pump and the gas is water shot up into the sky, which then falls back down into the pond and feeds the pump once again.
The sheer scale of all this is mind-numbing. The team of astronomers measures the mass of cold gas in the inner parts of the galaxy at about three billion times the mass of the Sun! Yet the power of the black hole and its accretion disk is sufficient to shoot this material out, where it can travel for hundreds of thousands of light years before falling back down. For comparison, I get tired and my fingers hurt carrying an eight-kilogram bucket of water out to my goats. I will never have a future as a supermassive black hole.
That’s the key to why we don’t see lots of star formation, either. The jets disrupt the gas near the black hole, preventing stars from forming efficiently. Some do form, but not in the numbers you’d expect just from the amount of gas falling to the center. So the central galaxy isn’t nearly as blue as you’d naively expect with all that gas flowing in, which is why we were scratching our heads over cooling flows back in the '90s.
Mind, you, there’s a vast amount more going on here, and I’m simplifying the situation hugely. But the point here is that we now have the tools to probe these fountains of material far better than we could when I was in grad school, and there’s been a lot of progress in the theory behind it all as well.
Besides the amazing science here, what’s fun personally for me when I read these papers on Abell 2597 is seeing the names of some of my friends from grad school who studied this phenomenon, like Brian McNamara, Megan Donahue, and Mark Voit. The reason we understand these systems of galaxies that are so far, far away is due to the dedication and hard work they’ve put into these studies. Science is done by people, and as brain-crushing as some of this stuff is, it’s just as soul-soothing to know that there are smart people out there figuring it out. That makes me pretty happy.