Scientists solve a fundamental mystery of the evolution of the Universe — why gas in galaxies does not cool over time

An international team of researchers has used the X-ray spectrograph XRISM to answer the question of why gas in galaxy clusters never cools.

This discovery answers the long-standing question of how how galaxies evolve and interact with each other and why the gas inside them remains hot despite the fact that it constantly emits powerful X-rays into space. Astronomers have long been inclined to believe that galaxy clusters grow through constant mergers and collisions, but have not been able to obtain direct evidence of these processes.

In the new study, the XRISM spectrograph provided definitive evidence of these processes in the heart of a galaxy cluster. The central regions of galaxy clusters remain one of the brightest sources of X-rays. Theoretically, due to the constant radiation, the gas should gradually cool down through a process known as radiative cooling.

However, observations indicate that the gas remains extremely hot, challenging scientists’ expectations and forcing them to look for an answer to this question.

One hypothesis is that the constant movement of the gas plays a key role in maintaining the high temperatures, but until now, astronomers have not had the right tools to confirm this. Using XRISM, an international group of astronomers (XRISM Collaboration) was able to accurately measure the velocity of hot gas in the core of a galaxy cluster.

The findings show that the gas is constantly moving, «splashing» back and forth in response to past collisions and mergers. These fluctuations keep the gas in a state of agitation, preventing it from cooling and maintaining high temperatures in the clusters.

This discovery was a major breakthrough in the understanding of processes of galaxy formation and cluster evolution. XRISM demonstrated the dynamics of gas movement in unprecedented detail and provided scientists with information on how the the largest space structures in the Universe.

What role do dark matter and gravity play in the evolution of galaxies

All large cosmic structures, such as our solar system and galaxies, especially their clusters, were formed gradually, under the influence of gravity. The most well-known structures in the Universe that were formed by mergers and collisions are galaxy clusters. These huge masses are held together by gravitational attraction of mysterious dark matter, which makes up most of the mass of the Universe.

However, the largest mass in galaxy clusters is gas, which consists of hydrogen and helium left over from the Big Bang. When this primordial gas enters a galaxy cluster, the high energy of gravitational interaction turns it into superheated gas with a temperature of tens of millions of degrees.

At such extreme temperatures, the gas begins to emit X-rays, which are very important for astronomers studying the evolution and dynamics of clusters. The mass of this gas is much greater than the mass of the galaxies themselves.

From December 2023 to January 2024, the research team used XRISM to observe the nearest galaxy cluster — the Centauri cluster, located about 100 million light-years from Earth. The goal was to study the motion of hot gas in the core of the galaxy cluster.

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The article was published in the journal Nature

Source: SciTech Daily