Study reveals the Phoenix galaxy cluster in the act of extreme cooling

The core of a massive cluster of galaxies appears to be pumping out far more stars than it should. Now researchers at MIT and elsewhere have discovered a key ingredient within the cluster that explains the core’s prolific starburst.

In a new study published in Nature, the scientists report using NASA’s James Webb Space Telescope (JWST) to observe the Phoenix cluster — a sprawling collection of gravitationally bound galaxies that circle a central massive galaxy some 5.8 billion light years from Earth. The cluster is the largest of its kind that scientists have so far observed. For its size and estimated age, the Phoenix should be what astronomers call “red and dead” — long done with any star formation that is characteristic of younger galaxies.

But astronomers previously discovered that the core of the Phoenix cluster appeared surprisingly bright, and the central galaxy seemed to be churning out stars at an extremely vigorous rate. The observations raised a mystery: How was the Phoenix fueling such rapid star formation?

In younger galaxies, the “fuel” for forging stars is in the form of extremely cold and dense clouds of interstellar gas. For the much older Phoenix cluster, it was unclear whether the central galaxy could undergo the extreme cooling of gas that would be required to explain its stellar production, or whether cold gas migrated in from other, younger galaxies.

Now, the MIT team has gained a much clearer view of the cluster’s core, using JWST’s far-reaching, infrared-measuring capabilities. For the first time, they have been able to map regions within the core where there are pockets of “warm” gas. Astronomers have previously seen hints of both very hot gas, and very cold gas, but nothing in between.

The detection of warm gas confirms that the Phoenix cluster is actively cooling and able to generate a huge amount of stellar fuel on its own.

“For the first time we have a complete picture of the hot-to-warm-to-cold phase in star formation, which has really never been observed in any galaxy,” says study lead author Michael Reefe, a physics graduate student in MIT’s Kavli Institute for Astrophysics and Space Research. “There is a halo of this intermediate gas everywhere that we can see.”

“The question now is, why this system?” adds co-author Michael McDonald, associate professor of physics at MIT. “This huge starburst could be something every cluster goes through at some point, but we’re only seeing it happen currently in one cluster. The other possibility is that there’s something divergent about this system, and the Phoenix went down a path that other systems don’t go. That would be interesting to explore.”

Hot and cold

The Phoenix cluster was first spotted in 2010 by astronomers using the South Pole Telescope in Antarctica. The cluster comprises about 1,000 galaxies and lies in the constellation Phoenix, after which it is named. Two years later, McDonald led an effort to focus in on Phoenix using multiple telescopes, and discovered that the cluster’s central galaxy was extremely bright. The unexpected luminosity was due to a firehose of star formation. He and his colleagues estimated that this central galaxy was turning out stars at a staggering rate of about 1,000 per year.

“Previous to the Phoenix, the most star-forming galaxy cluster in the universe had about 100 stars per year, and even that was an outlier. The typical number is one-ish,” McDonald says. “The Phoenix is really offset from the rest of the population.”

Since that discovery, scientists have checked in on the cluster from time to time for clues to explain the abnormally high stellar production. They have observed pockets of both ultrahot gas, of about 1 million degrees Fahrenheit, and regions of extremely cold gas, of 10 kelvins, or 10 degrees above absolute zero.

The presence of very hot gas is no surprise: Most massive galaxies, young and old, host black holes at their cores that emit jets of extremely energetic particles that can continually heat up the galaxy’s gas and dust throughout a galaxy’s lifetime. Only in a galaxy’s early stages does some of this million-degree gas cool dramatically to ultracold temperatures that can then form stars. For the Phoenix cluster’s central galaxy, which should be well past the stage of extreme cooling, the presence of ultracold gas presented a puzzle.

“The question has been: Where did this cold gas come from?” McDonald says. “It’s not a given that hot gas will ever cool, because there could be black hole or supernova feedback. So, there are a few viable options, the simplest being that this cold gas was flung into the center from other nearby galaxies. The other is that this gas somehow is directly cooling from the hot gas in the core.”

Neon signs

For their new study, the researchers worked under a key assumption: If the Phoenix cluster’s cold, star-forming gas is coming from within the central galaxy, rather than from the surrounding galaxies, the central galaxy should have not only pockets of hot and cold gas, but also gas that’s in a “warm” in-between phase. Detecting such intermediate gas would be like catching the gas in the midst of extreme cooling, serving as proof that the core of the cluster was indeed the source of the cold stellar fuel.

Following this reasoning, the team sought to detect any warm gas within the Phoenix core. They looked for gas that was somewhere between 10 kelvins and 1 million kelvins. To search for this Goldilocks gas in a system that is 5.8 billion light years away, the researchers looked to JWST, which is capable of observing farther and more clearly than any observatory to date.

The team used the Medium-Resolution Spectrometer on JWST’s Mid-Infrared Instrument (MIRI), which enables scientists to map light in the infrared spectrum. In July of 2023, the team focused the instrument on the Phoenix core and collected 12 hours’ worth of infrared images. They looked for a specific wavelength that is emitted when gas — specifically neon gas — undergoes a certain loss of ions. This transition occurs at around 300,000 kelvins, or 540,000 degrees Fahrenheit — a temperature that happens to be within the “warm” range that the researchers looked to detect and map. The team analyzed the images and mapped the locations where warm gas was observed within the central galaxy.

“This 300,000-degree gas is like a neon sign that’s glowing in a specific wavelength of light, and we could see clumps and filaments of it throughout our entire field of view,” Reefe says. “You could see it everywhere.”

Based on the extent of warm gas in the core, the team estimates that the central galaxy is undergoing a huge degree of extreme cooling and is generating an amount of ultracold gas each year that is equal to the mass of about 20,000 suns. With that kind of stellar fuel supply, the team says it’s very likely that the central galaxy is indeed generating its own starburst, rather than using fuel from surrounding galaxies.

“I think we understand pretty completely what is going on, in terms of what is generating all these stars,” McDonald says. “We don’t understand why. But this new work has opened a new way to observe these systems and understand them better.”

This work was funded, in part, by NASA.