A galaxy cluster once thought to be “dead” is unexpectedly forming stars at an astonishing rate, challenging established theories about star formation in aging galaxies. The Phoenix Cluster, located 5.8 billion light-years away, was believed to have exhausted its star-forming potential.
A new observations using the James Webb Space Telescope (JWST) have revealed an active process of star birth, raising fundamental questions about how cold gas can still exist in such an environment.
The Mystery Of The Phoenix Cluster
Galaxy clusters are the largest gravitationally bound structures in the universe. They consist of hundreds to thousands of galaxies, as well as hot gas and dark matter. Over time, these clusters tend to run out of the cold, dense gas needed for star formation. When this happens, their central galaxies are considered “quenched“, meaning they have entered a phase where new stars are no longer forming.
The Phoenix Cluster was expected to follow this pattern. Yet, astronomers have detected a ferocious burst of star formation, producing up to 1,000 stars per year—a staggering number compared to the Milky Way, which forms fewer than 10 stars per year.
The Cooling Flow Problem
In theory, hot gas in galaxy clusters should cool over time, condensing into cold clouds that form new stars. In most clusters, this process doesn’t seem to happen. Instead, a supermassive black hole (SMBH) at the center of the cluster emits powerful jets, heating the surrounding gas and preventing it from cooling. This leads to a scenario where star formation remains low, despite the presence of hot gas.
Yet, in the Phoenix Cluster, the usual rules don’t seem to apply. Not only has astronomers’ assumption about the absence of cold gas been proven wrong, but there is also clear evidence of a large-scale process cooling the gas and fueling star formation.
JWST Provides New Clues
To solve the mystery, a research team led by Michael Reefe, a physics graduate student at MIT’s Kavli Institute for Astrophysics and Space Research, used the James Webb Space Telescope to observe the Phoenix Cluster’s core.
Unlike previous telescopes, JWST’s infrared capabilities allowed it to detect warm gas, which acts as an intermediate stage between hot and cold gas.
This intermediate-temperature gas, detected through neon emissions, provides the missing link between the hot intracluster medium and the cold gas necessary for star formation.
A Black Hole Fueling Star Birth?
One of the most surprising findings is that the Phoenix Cluster’s supermassive black hole might be playing a role in cooling the gas rather than heating it. Typically, black hole jets prevent star formation by releasing energy that keeps the surrounding gas too hot. In the Phoenix Cluster, the opposite seems to be happening:
- The black hole’s activity may be creating conditions that allow gas to cool.
- Large-scale cooling is leading to 20,000 solar masses of cold gas per year.
- This cold gas is directly fueling the formation of new stars.
“I think we understand pretty completely what is going on, in terms of what is generating all these stars. We don’t understand why. But this new work has opened a new way to observe these systems and understand them better,” explained MIT astrophysicist Michael McDonald, a co-author of the study.
If confirmed, this discovery could challenge existing models of black hole feedback and its effect on galaxy evolution.
