The Phoenix galaxy cluster is among the largest, brightest, and most active known galaxy clusters in the universe, containing around 1,000 galaxies and producing roughly 1,000 stars per year. Its activity, however, is unexpected for older clusters like Phoenix, due to their inability to rapidly cool ambient gas to fuel star formation — a phenomenon known as the cooling flow problem. This inconsistency drove scientists to understand why the Phoenix cluster is an outlier.

Galaxy clusters are exactly what their name suggests: clusters of multiple galaxies. In between those galaxies exists a hot, ambient gas called intracluster medium (ICM). Traditionally, astrophysicists believed that the ICM in clusters would rapidly cool and condense by releasing X-ray light, fueling the formation of new stars in a process called the cooling effect. However, observation showed that this wasn’t the case. In fact, most galaxy clusters were not undergoing this rapid cooling, and thus were not forming many stars, which introduced the cooling flow problem. Though there are different possible explanations for this phenomenon, the one most widely accepted amongst astrophysicists and astronomers is that many older galaxy centers have supermassive black holes that emit energy through jets and radiation to heat ambient gas, preventing the natural cooling flow process. 

Given the cooling flow problem, this finding is puzzling: why is this particular galaxy cluster able to produce so many stars if it shouldn’t be able to efficiently cool the ICM?

The Phoenix galaxy cluster shines brighter than all other observed galaxy clusters in telescope images and produces stars at extremely high rates. Given the cooling flow problem, this finding is puzzling: why is this particular galaxy cluster able to produce so many stars if it shouldn’t be able to efficiently cool the ICM?

This is the question that guided Michael McDonald and his team at MIT in their recent research on the Phoenix cluster’s star production. Previous studies had only revealed the presence of either extremely hot or cold pockets of gas in the Phoenix cluster. Because of the cooling flow problem, extremely cold gas is typically only found in younger galaxies, leading McDonald and his team to wonder whether the cold gas was from the Phoenix cluster itself or from another, younger galaxy. 

To determine the source of the cold gas in the Phoenix cluster, McDonald and his team assumed that if the cold gas was indeed being produced from within the cluster, then they should not only be able to find hot and cold gas, but also warm gas that is in the process of being cooled. Using the James Webb Space Telescope to analyze the Phoenix cluster, they identified various pockets of warm gas at the core of the cluster, and were able to map its temperature and location in space. 

The discovery of the warm gas led to the conclusion that the Phoenix cluster has the ability to generate so many stars because despite supermassive black holes that heat ambient gas, another (still unknown) force causes rapid cooling of that gas, which provides the necessary fuel to create new stars. The Phoenix cluster is a rare case and is the only known galaxy cluster that behaves in this way. Though the mechanism of the high star production has now been uncovered, researchers are still unsure why the Phoenix cluster defies the cooling flow phenomenon while other galaxy clusters don’t. Future research will likely delve into what sets the Phoenix cluster apart, and search for others of its kind across the universe.