In 2012, McDonald and his team discovered a “bonafide cooling flow” in the Phoenix cluster. To this day, “it remains the only such system that we’ve discovered, which makes it a valuable laboratory to study a variety of rare physical phenomena” related to the nature and origins of the universe, he says.

To explain why the hot gas within galaxy clusters is expected to cool, why most clusters don’t show evidence of cooling, and why the Phoenix cluster is different, McDonald was quoted in 2019 using an analogy, an explanatory technique at which he excels.

Comparing a galaxy cluster to a cup of coffee, he told MIT News that, “It’s a 10-million-degree cup of coffee, but even a 10-million-degree cup of coffee is going to cool.” Most galaxy clusters, though, behave more like a cup of coffee on a warming plate. “The warmer is the black hole in the center of the galaxy, and every cluster observed has a warm interior.”

Except for the Phoenix cluster.

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