The Phoenix cluster is a group of galaxies bound together by gravity 5.8 billion lightyears away, that shouldn't be forming stars at such a stupendous rate, and the James Webb Space Telescope has finally discovered why it is.
At the centre of the galaxy cluster is a supermassive black hole about 10 billion times the mass of our Sun.
In other galaxy clusters, central supermassive black holes generate energetic particles and radiation that blast the surrounding area, preventing gas from cooling enough to form stars.
So how can the Phoenix galaxy cluster be producing stars at such a high rate?
Astronomers have been using Webb to re-examine the flow of gas within the cluster to find out.
The problem with the Phoenix cluster
"We can compare our previous studies of the Phoenix cluster, which found differing cooling rates at different temperatures, to a ski slope," says Michael McDonald of the Massachusetts Institute of Technology, principal investigator of the program.
"The Phoenix cluster has the largest reservoir of hot, cooling gas of any galaxy cluster — analogous to having the busiest chair lift, bringing the most skiers to the top of the mountain.
"However, not all of those skiers were making it down the mountain, meaning not all the gas was cooling to low temperatures.
"If you had a ski slope where there were significantly more people getting off the ski lift at the top than were arriving at the bottom, that would be a problem!"
The team say they've found the answer using the James Webb Space Telescope and building on previous studies using the Chandra X-ray Observatory and Hubble Space Telescope, as well as ground-based observatories.
In other words, Webb has found the proverbial skiers at the middle of the mountain!
How Webb solved the problem
The James Webb Space Telescope has tracked and mapped the missing cooling gas feeding star formation in the Phoenix cluster.
This gas was found within cavities tracing the hot gas – a scorching 18 million degrees Fahrenheit (10 million Celsius) – and the cooled gas of around 18,000 degrees Fahrenheit (10,000 Celsius).
The team used the Medium-Resolution Spectrometer on Webb’s Mid-Infrared Instrument (MIRI) to take 2D spectroscopic data.
"Previous studies only measured gas at the extreme cold and hot ends of the temperature distribution throughout the center of the cluster," says McDonald.
"We were limited — it was not possible to detect the ‘warm’ gas that we were looking for. With Webb, we could do this for the first time."
Webb's instruments enabled the team to detect the specific temperature of cooling gas, around 540,000 degrees Fahrenheit (300,000 degrees Celsius).
The science behind the detection involves neon and oxygen atoms.
At these temperatures, the emission from oxygen is 100 times brighter but only visible in ultraviolet.
Neon is fainter but glows in infrared, which is the wavelength in which Webb is primed to observe.
"In the mid-infrared wavelengths detected by Webb, the neon VI signature was absolutely booming," says Michael Reefe, also of the Massachusetts Institute of Technology, lead author on the paper published in Nature.
"Even though this emission is usually more difficult to detect, Webb’s sensitivity in the mid-infrared cuts through all of the noise."
The team say they'll now use the same method to explore star formation in other galaxy clusters.
Read the full paper in Nature at arxiv.org/abs/2502.08619
