Astronomers Solve Mystery behind Galaxy Clusters’ Endless Heat

Using the cutting-edge XRISM satellite, they proved these flows were created by dramatic collisions between enormous galaxy clusters, causing gas within to slosh around energetically. The team’s breakthroughs has been published in the scientific journal Nature.

This discovery finally solves a decades-old cosmic mystery: how the gas at the heart of galaxy clusters stays incredibly hot despite constant energy loss. These insights offer a thrilling glimpse into the dramatic interactions shaping our ever-evolving universe.

The XRISM collaboration has discovered flows of hot gas within the core of the Centaurus Cluster. Using advanced X-ray measurements from the XRISM satellite and comparing them with detailed numerical simulations, researchers confirmed that these gas movements are caused by collisions between galaxy clusters, resulting in gas “sloshing” within the clusters. This finding solves the longstanding mystery of how galaxy cluster cores remain hot and provides new insights into the ongoing evolution of our universe.

Astronomers have long theorized that powerful gravitational forces between galaxies and galaxy clusters — massive structures bound together primarily by dark matter — drive their growth through mergers and collisions. However, direct observational evidence has previously been lacking.

Now, the international XRISM (X-ray Imaging and Spectroscopy Mission) collaboration has provided this evidence. The team used the XRISM satellite, launched in 2023 by the Japan Aerospace Exploration Agency (JAXA), to study the Centaurus galaxy cluster. The satellite’s onboard instrument, called Resolve, offers unprecedented precision in X-ray spectroscopy, allowing astronomers to measure gas velocities accurately and confirm these groundbreaking results.

Looking at the core of the Centaurus Cluster, including the central galaxy NGC 4696, they discovered for the first time a bulk flow of hot gas traveling around 130 to 310 kilometers per second in the line-of-sight from Earth. They were also able to create a map of how the velocity varies at locations away from the center.

Making comparisons with simulations, a task team led by Professor Yutaka Fujita from Tokyo Metropolitan University and Associate Professor Kosuke Sato from the High Energy Accelerator Research Organization found that this is consistent with the “sloshing” of the hot gas, also known as the intracluster medium (ICM), caused by collisions with other galactic clusters. This is the first direct evidence for this kind of “sloshing,” validating a long-hypothesized picture of the evolution of the universe.

It also solves a long-standing unsolved mystery for astronomers of how such bright X-ray emitting gas stays hot. Theoretically, such intense radiation should entail a loss of energy, leading to cooling of the gas; this is known as radiative cooling. The time scale over which this cooling should occur is shorter than the age of the cluster, but observations so far suggest that, somehow, the gas manages to stay hot. These new findings present an elegant solution to this problem. If the gas in the cluster core can “slosh,” involving vast bulk flows of gas to-and-fro around the center, energy can be transported to the core through a mixing process, keeping the gas hot and the emissions bright. These unprecedentedly precise measurements are a significant leap forward in our understanding of the formation and evolution of galactic clusters. With years still left in the XRISM mission, the world of astrophysics eagerly awaits more insights into the changing nature of the universe.

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