A team of astronomers has made a groundbreaking discovery, revealing that the Phoenix Cluster, a star system that was thought to be nearing its “death,” is actually undergoing a process of “rebirth.” The core material of this cluster is experiencing intense gas cooling, leading to the birth of a large number of new stars.
The research team from universities including the Massachusetts Institute of Technology in the United States used NASA’s James Webb Space Telescope’s infrared instrument (MIRI) to observe an unprecedented phenomenon of “cold-hot exchange” at the center of the Phoenix Cluster, where a significant amount of stars are being born. This latest finding was published in the scientific journal “Nature” on February 5 and ranks in the top 5% of all research outcomes evaluated by Altmetric scoring system.
Fifteen years ago, astronomers using telescopes in Antarctica discovered an exceptionally bright cluster of galaxies in the direction of the Phoenix constellation, which was named the Phoenix Cluster. Located approximately 5.8 to 8.6 billion light-years away from Earth, this massive cluster consists of 1,000 galaxies and surpasses the size of typical star clusters by far.
What has puzzled the research team is the fact that for an ancient and massive star cluster like Phoenix, the core galaxy should have long exhausted the cold gas “fuel” essential for star formation, entering a state of “red and dead.” However, the central galaxy remains extremely bright with a high rate of star formation, estimated to produce around 1,000 stars per year.
This high star formation rate is exceptionally rare compared to other galaxies, as scientists have previously observed that most galaxy clusters generate around 100 stars per year, while ordinary star clusters only produce one star annually.
The Phoenix Cluster is challenging scientists’ previous understanding of galaxy cluster evolution. Additionally, researchers are contemplating how the central galaxy of the Phoenix Cluster maintains such a high star formation rate. Speculations include the extreme gas cooling process occurring in the central galaxy or the possibility that the cold gas originates from younger galaxies.
To unravel this mystery, the team used the James Webb Space Telescope’s infrared instrument to observe Ne-Vi emission lines emitted from the core of the Phoenix Cluster and constructed a map of the coronal gas. Ne-Vi emission lines are distinctive lines that only appear when neon gas reaches temperatures close to 300,000°C, serving as an ideal “probe” to explore the state of gas in galaxy cluster cores.
The results revealed that the core of the Phoenix Cluster is very bright, with surrounding gas undergoing intense “cold-hot alternation,” presenting an overall “warm” condition. The temperature in the hottest region reaches up to 378,000°C, while the coldest area reaches -263.15°C (theoretical absolute zero limit is -273.15°C).
This discovery astonished the research team as it is the first time that the simultaneous existence of extremely hot and cold gases in a “warm” state has been observed in the scientific community. Previously, extreme hot or cold gases were only observed in other galaxy cluster cores, with no identification of gases existing between the two extremes.
Furthermore, the research team discovered that the central galaxy is undergoing extreme cooling, producing super-cold gas mass equivalent to 5,000 to 23,000 times the mass of the Sun annually. These “cold substances” are believed to provide enough “fuel” for star formation, enabling the central galaxy to undergo a starburst (an overall high rate of star formation) without relying on fuel from surrounding galaxies.
The lead author of this study, Michael Reefe, a graduate student in astrophysics and space research at the Kavli Institute for Astrophysics and Space Research at the Massachusetts Institute of Technology, stated, “We are witnessing the complete stages of star formation for the first time, from high temperature to moderate temperature to low temperature, a process never observed in any other galaxy. And we see this gas of intermediate temperatures everywhere, forming a ring-like structure.”
Co-author Michael McDonald, an associate professor of physics at the Massachusetts Institute of Technology, added, “This massive starburst may be a phase that every galaxy cluster goes through at some point. However, we currently only see this in the Phoenix Cluster, suggesting it may be taking a different path from other galaxies. This will be a very intriguing exploration.”
“We speculated that these cold gases may have been ejected from nearby galaxies and reached the core, or that this gas somehow caused the central hot gas to cool rapidly. This observation now allows us to understand what is happening and the reasons for the birth of these bright stars. This new study also provides a new pathway for future research on the universe,” he said.
This work was partially funded by NASA.