The nucleus of a substantial conglomerate of galaxies seems to be producing considerably more stars than expected. Researchers at MIT and other institutions have now uncovered a crucial component within the cluster that elucidates the nucleus’s abundant starburst.

In a recent study published in Nature, the scientists detail using NASA’s James Webb Space Telescope (JWST) to examine the Phoenix cluster — a vast assembly of gravitationally linked galaxies that orbit a central colossal galaxy approximately 5.8 billion light years from Earth. This cluster is the largest of its kind observed by scientists to date. In consideration of its dimensions and estimated age, the Phoenix should be what astronomers categorize as “red and dead” — signifying it has long ceased any star formation typical of younger galaxies.

However, astronomers had previously found that the core of the Phoenix cluster registered an unexpectedly bright appearance, and the central galaxy appeared to be generating stars at an exceedingly vigorous pace. These observations sparked a conundrum: How was the Phoenix sustaining such swift star formation?

In younger galaxies, the “fuel” for producing stars exists in the form of extremely chilly and dense clouds of interstellar gas. For the significantly older Phoenix cluster, it was ambiguous whether the central galaxy could experience the dramatic cooling of gas necessary to account for its stellar production or if cooler gas was migrating from other, younger galaxies.

Now, the MIT research team has acquired a much clearer perspective on the cluster’s nucleus by leveraging JWST’s expansive, infrared-measuring capabilities. For the first time, they have mapped areas within the nucleus where there are pockets of “warm” gas. Astronomers had previously observed signs of both extremely hot gas and extremely cold gas, but nothing in the interim range.

The detection of warm gas validates that the Phoenix cluster is actively cooling and has the capacity to produce a significant amount of stellar fuel independently.

“For the first time, we have a comprehensive view of the hot-to-warm-to-cold transition in star formation, which has really never been recorded in any galaxy,” says study lead author Michael Reefe, a physics graduate student at MIT’s Kavli Institute for Astrophysics and Space Research. “There exists a halo of this intermediate gas all around that we can observe.”

“The current question is, why this system?” adds co-author Michael McDonald, an associate professor of physics at MIT. “This massive starburst could potentially be a phase every cluster experiences at some point, but we’re only currently witnessing it in one cluster. Another possibility is that there’s something unique about this system, and the Phoenix has followed a path that other systems do not. That would be worth investigating.”

Hot and cold

The Phoenix cluster was first identified in 2010 by astronomers utilizing the South Pole Telescope in Antarctica. This cluster consists of approximately 1,000 galaxies and is located in the constellation Phoenix, after which it is named. Two years later, McDonald spearheaded an initiative to focus on the Phoenix using multiple telescopes, eventually discovering that the central galaxy of the cluster was exceptionally luminous. This unexpected brightness was attributed to an intense rate of star formation. He and his team estimated that this central galaxy was producing stars at an astounding rate of about 1,000 per year.

“Prior to the Phoenix, the galaxy cluster known for the highest star formation in the universe produced around 100 stars per year, and even that was considered an anomaly. The usual figure is closer to one,” McDonald notes. “The Phoenix is significantly different from the rest of the population.”

Since that initial discovery, scientists have periodically revisited the cluster seeking insights to clarify the unusually elevated level of stellar production. They have noted the presence of both ultra-hot gas, around 1 million degrees Fahrenheit, and regions of extremely cold gas, approximately 10 kelvins, or just above absolute zero.

The presence of very hot gas is not surprising: Most massive galaxies, regardless of their age, host black holes at their centers that emit jets of highly energetic particles capable of continuously heating the gas and dust in the galaxy over its lifespan. Only in the early phases of a galaxy’s development does some of this million-degree gas cool significantly to ultracold temperatures, which can then lead to star formation. For the central galaxy of the Phoenix cluster, which should be well beyond the phase of significant cooling, the existence of ultracold gas posed a challenge.

“The question has been: Where did this cold gas originate?” McDonald states. “It’s not guaranteed that hot gas will ever cool, as feedback from black holes or supernovae could prevent it. Therefore, several plausible scenarios exist, the simplest being that this cold gas was driven into the center from nearby galaxies. Alternatively, this gas could be directly cooling from the hot gas present in the nucleus.”

Neon signs

In their new study, the researchers operated under a crucial presumption: If the cold, star-forming gas in the Phoenix cluster originates from within the central galaxy rather than from surrounding galaxies, the central galaxy should contain not just pockets of hot and cold gas, but gas that exists in an intermediate “warm” state. Detecting such intermediate gas would be akin to finding evidence of gas in the process of extreme cooling, thereby confirming that the cluster’s core was indeed the source of the cold stellar fuel.

Following this logic, the team aimed to locate any warm gas within the Phoenix nucleus. They searched for gas temperatures ranging between 10 kelvins and 1 million kelvins. To identify this “Goldilocks gas” in a system that is 5.8 billion light years away, the researchers turned to JWST, which has the capability to observe farther and with greater clarity than any astronomical observatory to date.

The team utilized the Medium-Resolution Spectrometer on JWST’s Mid-Infrared Instrument (MIRI), which allows scientists to map light in the infrared spectrum. In July 2023, the team concentrated the instrument on the Phoenix nucleus and amassed 12 hours of infrared imagery. They searched for a specific wavelength emitted when gas — particularly neon gas — experiences a certain loss of ions. This transition occurs at approximately 300,000 kelvins, or 540,000 degrees Fahrenheit — a temperature that aligns with the “warm” range the researchers sought to identify and map. The team analyzed the images and pinpointed the areas where warm gas was detected within the central galaxy.

“This 300,000-degree gas is akin to a neon sign glowing in a specific wavelength of light, and we could observe clusters and filaments of it throughout our entire field of view,” Reefe explains. “It was visible everywhere.”

Based on the extent of warm gas in the nucleus, the team estimates that the central galaxy is going through a substantial degree of extreme cooling and generating an amount of ultracold gas annually equivalent to the mass of about 20,000 suns. With such a robust supply of stellar fuel, the team suggests it’s highly likely the central galaxy is indeed producing its own starburst, rather than relying on fuel from neighboring galaxies.

“I believe we have a comprehensive understanding of the mechanisms generating all these stars,” McDonald asserts. “While we don’t fully grasp the reasons behind it. However, this new research has opened a fresh avenue for observing these systems and enhancing our understanding of them.”

This research was funded, in part, through NASA.