The Phoenix cluster, located in the constellation Phoenix, was first discovered in 2010 using the South Pole Telescope. It contains about 1,000 galaxies.
The cluster’s core appeared very bright, and the central galaxy was producing stars at an unusually high rate, leading to the question: How is the Phoenix cluster fueling such rapid star formation?
In a new study published in Nature, scientists used NASA’s James Webb Space Telescope (JWST) to observe the Phoenix cluster. This massive group of galaxies is bound together by gravity, with a central massive galaxy about 5.8 billion light-years from Earth.
The Phoenix cluster is the largest of its kind observed so far. Given its size and estimated age, it should be “red and dead,” meaning it should no longer form new stars like younger galaxies.
In younger galaxies, stars form from very cold and dense interstellar gas clouds. For the older Phoenix cluster, scientists weren’t sure if the central galaxy could cool gas enough to produce stars or if the gas came from younger galaxies.
Using the James Webb Space Telescope, the MIT team observed the cluster’s core and found pockets of “warm” gas for the first time.
Previously, only very hot or very cold gas was seen. This warm gas confirms that the Phoenix cluster is actively cooling and can produce much stellar fuel.
Study lead author Michael Reefe, a physics graduate student at MIT’s Kavli Institute for Astrophysics and Space Research, said, “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. There is a halo of this intermediate gas everywhere that we can see.”
Astronomers discovered a mysterious stream of ancient stars
Why this system?
The starburst happening in the Phoenix cluster might be a phase every galaxy cluster experiences, but we’re only currently observing it in this one. Alternatively, the Phoenix cluster might be unique, having taken a different evolutionary path than other clusters. Exploring this could reveal fascinating insights.
Before the Phoenix cluster, the most active galaxy cluster formed around 100 stars per year, which was already unusual compared to the typical rate of about one star per year. The Phoenix cluster’s high star formation rate stood out even more.
Scientists have periodically studied the cluster to understand its high star production. They found regions of both ultrahot gas (around 1 million degrees Fahrenheit) and extremely cold gas (10 kelvins, or 10 degrees above absolute zero).
Most massive galaxies have black holes at their cores that emit energetic particles, heating the galaxy’s gas and dust. Typically, this hot gas cools enough to form stars only in a galaxy’s early stages. However, the Phoenix cluster’s central galaxy has ultracold gas despite being past this stage, which puzzles scientists.
They wondered: Where did this cold gas come from? It isn’t certain that hot gas will ever cool due to potential feedback from black holes or supernovae. One possibility is that cold gas was flung into the center from nearby galaxies. Another is that the gas directly cools from the hot gas in the core.
For their new study, researchers assumed that if the Phoenix cluster’s cold, star-forming gas came from its central galaxy, this galaxy should have hot, cold, and “warm” gas. Finding this warm gas would prove the core is the source of the cold gas needed for star formation.
The team used the James Webb Space Telescope to observe farther and more clearly than any other observatory, so they looked for warm gas (between 10 kelvins and 1 million kelvins) in the Phoenix core.
The team observed the Phoenix core using the Medium-Resolution Spectrometer on JWST’s Mid-Infrared Instrument (MIRI). In July 2023, they collected 12 hours of infrared images, looking for a specific wavelength emitted by neon gas at around 300,000 kelvins (540,000 degrees Fahrenheit).
This temperature falls within the “warm” range they aimed to detect and map. They found clumps and filaments of warm gas throughout the central galaxy, indicating active cooling and the presence of stellar fuel.
The team estimates that the central galaxy is cooling rapidly and producing ultracold gas equal to the mass of about 20,000 suns each year. This suggests that the galaxy is generating its starburst rather than using gas from nearby galaxies.
Co-author Michael McDonald, associate professor of physics at MIT, says they understand what is causing the star formation but not why it’s happening. The new study has provided a new way to observe and better understand these systems.
Journal Reference:
- Reefe, M., McDonald, M., Chatzikos, M. et al. Directly imaging the cooling flow in the Phoenix cluster. Nature 638, 360–364 (2025). DOI: 10.1038/s41586-024-08369-x
Source: Tech Explorist
