Colliding galaxy clusters stir up hot, 'sloshing' gas

The universe is in a constant state of motion, shaped by gravity, collisions, and the unseen force of dark matter. Galaxy clusters – the largest gravitationally bound structures in the cosmos – contain vast amounts of hot gas, providing an ideal laboratory to study these interactions.

Scientists have long theorized that cluster mergers drive cosmic evolution, but finding direct evidence has remained a challenge.

A recent breakthrough by the XRISM collaboration has changed this. By examining X-ray data from the XRISM satellite, astronomers have uncovered large-scale flows of hot gas in the core of the Centaurus Cluster.

The findings confirm that galaxy cluster collisions trigger a process known as “sloshing,” where hot gas moves in vast waves within the cluster.

This not only explains how clusters evolve but also provides a long-sought answer to a puzzling question – why does the gas in these clusters remain hot when it should have cooled long ago?

Immense gravitational forces

Galaxy clusters are massive cosmic structures held together by dark matter.

Astronomers have theorized that the immense gravitational forces between galaxies and galaxy clusters drive their growth through mergers and collisions. However, direct evidence for this has been lacking.

The discovery of sloshing gas in the Centaurus Cluster provides the strongest confirmation yet of these dynamic interactions.

Clusters do not exist in isolation. They move, collide, and merge over billions of years, shaping the structure of the universe.

When two clusters interact, their hot gas – known as the intracluster medium (ICM) – does not remain static. Instead, it is disturbed by these colossal encounters, triggering massive flows that propagate throughout the cluster.

This movement plays a crucial role in maintaining the high temperatures observed in these systems.

Hot gas movement in a galaxy cluster

The international XRISM (X-ray Imaging and Spectroscopy Mission) collaboration has observed the Centaurus galaxy cluster with the XRISM satellite, launched in 2023 by the Japan Aerospace Exploration Agency (JAXA).

The onboard spectrometer, called Resolve, features advanced precision spectroscopy, allowing for the accurate identification of gas velocities.

Looking at the core of the Centaurus Cluster, including the central galaxy NGC 4696, the experts 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.

First direct evidence of sloshing gas

A task team led by Professor Yutaka Fujita from Tokyo Metropolitan University and Professor Kosuke Sato from the High Energy Accelerator Research Organization analyzed the data.

The team compared their findings with simulations and found that the observed gas movement matched a process known as “sloshing.”

This phenomenon occurs when hot gas, also called the intracluster medium (ICM), moves due to collisions with other galactic clusters. Their research provides the first direct evidence of this kind of “sloshing.”

It confirms a long-standing hypothesis about how galaxy clusters evolve over time.

Sloshing gas keeps clusters hot

This breakthrough does not just confirm a theoretical concept. It also provides a missing piece in understanding how galaxy clusters maintain their extreme temperatures.

The discovery that gas flows inside clusters act as a natural heat transport mechanism solves a major astrophysical mystery.

The study 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.

The future of astrophysics

These remarkably 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.

The ability to measure gas velocities with such high precision opens new doors for studying not just galaxy clusters but also other cosmic phenomena. Understanding how energy moves through space and how massive structures interact over time can help refine models of the universe’s evolution.

Future missions may build upon XRISM’s discoveries, using even more advanced technology to observe larger and more distant clusters.

By continuing to study the motion of hot gas, astronomers hope to deepen their understanding of how the universe’s largest structures are shaped by time, gravity, and unseen forces.

The study is published in the journal Nature.

Image Credit: JAXA

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