Galaxy Clusters Should Be Cold – XRISM Just Found Out Why They’re Not

Astronomers may have finally cracked one of the universe’s long-standing puzzles: why galaxy clusters remain mysteriously warm despite constantly losing energy through X-rays.

Using Japan’s XRISM satellite, scientists observed a fast, swirling flow of superheated gas in the Centaurus cluster. This gentle but powerful motion appears to redistribute heat throughout the cluster, acting like a cosmic stirrer and preventing it from cooling.

Galaxy Clusters Stay Hot Despite Cooling X-rays

Scientists using the XRISM satellite have uncovered a major clue to one of astronomy’s longstanding mysteries: why galaxy clusters stay hot, even though they constantly lose energy through X-ray emissions. These X-rays should cool the clusters over time, but that’s not what astronomers observe.

By studying the Centaurus cluster, located about 150 million light-years from Earth, the XRISM science team, including members of Nagoya University, discovered a surprising phenomenon at its core. They found a fast-moving stream of superheated gas flowing through the center of the cluster. This energetic motion appears to be delivering heat and preventing the cluster from cooling down. Their results, published in Nature, offer a promising solution to what’s known as the “cooling flow problem.”

A Mismatch Between Cooling and Star Formation

Galaxy clusters are colossal structures made up of hundreds of galaxies. They are the largest known objects in the universe and play a key role in helping scientists understand how galaxies form, evolve, and interact on a cosmic scale.

These clusters are wrapped in massive halos of dark matter, which pull in scorching-hot gas from the surrounding intergalactic space. As this gas falls inward, it gives off X-rays, much like steam rising from hot water. Naturally, this should lead to cooling, and as the gas cools, it’s expected to condense and form stars at the cluster’s center.

But here’s the puzzle: not enough stars are forming, and the cores of these clusters are still hot. Something is offsetting the cooling, but until now, scientists weren’t sure what it was.

XRISM’s Breakthrough Discovery

To investigate, the team focused on the Centaurus cluster using XRISM’s cutting-edge instrument called Resolve, which can precisely measure the motion and temperature of X-ray-emitting gas. What they found was a clear flow of high-temperature gas moving rapidly through the cluster’s center. This “sloshing” motion acts like a cosmic stirrer, distributing energy and helping maintain the cluster’s overall heat.

Energy Transfer Through Gentle Stirring

“We found little turbulence of the high-temperature gas in the galaxy cluster,” Professor Nakazawa said. “The mechanism, which stops the cooling of the hot gas of this cluster, is a general ‘stirring’ of the hot gas that supplies energy to the center from outside regions, thus maintaining the high temperature.”

Computer simulations of the movement of hot gas after the merging of clusters of galaxies that happened during their growth process were used to explain these motions, which is called ‘gas sloshing.’

Unlocking Cosmic Evolution

“High-precision spectroscopy will also help us to better understand how the massive structure of galaxy clusters evolves,” Nakazawa said. “It deepens our understanding not only of galaxy clusters, but also of the formation and evolution of large-scale structures in the universe as a whole.”

Reference: “The bulk motion of gas in the core of the Centaurus galaxy cluster” by XRISM collaboration, 12 February 2025, Nature.
DOI: 10.1038/s41586-024-08561-z

Never miss a breakthrough: Join the SciTechDaily newsletter.
Follow us on Google and Google News.