XRISM X-Ray Imaging and Spectroscopy Mission │ JAXA

Science Results

Science Results The Bulk Motion of Gas in the Core of the Centaurus Galaxy Cluster

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Key Points

  • Superior capabilities of the X-ray spectrometer onboard XRISM enabled the detection of the presence of oscillating hot gas motion, for the first time in the world, in the center of the Centaurus Cluster (figure 1).
  • The observed gas motion provides direct evidence of collisions and mergers between galaxy clusters.
  • Direct measurement of the velocity structure of the gas helps elucidate the heating mechanism in the cluster core, which has been a mystery for the past few decades.

Abstract

The Universe evolves under the influence of gravity. Galaxies, vast collections of stars and interstellar medium, form larger structures known as galaxy clusters—colossal cosmic assemblies bound together by the gravitational pull of dark matter. Within these clusters, gas trapped by intense gravitational forces is heated to tens of millions of degrees Kelvin, emitting powerful X-rays. Astronomers have long believed that galaxy clusters continue to grow through repeated collisions and mergers, but direct observational evidence has been elusive until now. In a groundbreaking study, XRISM has, for the first time, captured definitive proof of this dynamic process at the core of a galaxy cluster.

The central regions of galaxy clusters are among the brightest X-ray sources in the Universe. Theoretically, the loss of energy from their intense radiation should cause the surrounding gas to cool over time —a phenomenon known as radiative cooling. However, observations have revealed that the gas remains unexpectedly hot, leaving scientists puzzled. One leading hypothesis suggested that gas motion could play a key role in maintaining these high temperatures, yet previous instruments lacked the precision to confirm this.

An international research team (XRISM Collaboration, hereinafter referred to as the “research team”) has precisely measured the velocity structure of hot gas in a galaxy cluster’s core. Their findings reveal that gas motion aligns with oscillations (“sloshing”) triggered by collisions and mergers between clusters. These oscillations stir the hot gas, preventing excessive cooling and maintaining its high temperature.

This discovery marks a significant leap forward in our understanding of galaxy formation, cluster evolution, and the large-scale structure of the Universe by uncovering new details about gas dynamics with unprecedented precision.

Figure 1: Illustration of the center of the Centaurus Cluster drawn based on XRISM observations and other observations. The bluish color indicates high-temperature gas. The white indicates galaxies, and the reddish brown indicates low-temperature gas. (JAXA)

Main Text

How did the Universe evolve into its current structure after the Big Bang? This fundamental question has driven decades of astronomical research. The Universe is filled with vast cosmic structures. The Solar System is a collection of planets and small Solar System bodies orbiting the Sun, while a galaxy is a vast assembly of stars bound by gravity. However, these structures did not exist from the Universe’s beginning; they gradually formed and grew under the influence of gravity acting on matter. Violent cosmic events, such as collisions and mergers between celestial bodies, shaped our current universe.

The largest known structures formed through this cosmic evolution are galaxy clusters. These immense conglomerations of galaxies are held together by the powerful gravitational pull of dark matter, an invisible and mysterious substance that makes up most of the Universe’s mass. However, the dark matter and galaxies alone are not the dominant components of these clusters—significant mass exists in the form of gas, composed of hydrogen and helium gas left over from the Big Bang. As this primordial gas falls into a galaxy cluster, the immense gravitational energy converts it into superheated gas at temperatures of tens of millions of degrees. At such extreme temperatures, the gas emits X-rays, making X-ray observations essential for studying the evolution and dynamics of galaxy clusters. The mass of this hot gas is significantly greater than that of the galaxies themselves, meaning that understanding galaxy clusters requires understanding this high-energy component.

One of the great astrophysical puzzles has been why the hot gas in the center of a galaxy cluster does not cool over time. Theoretically, the gas should gradually lose energy through X-ray emission, cooling down in a process known as radiative cooling. However, previous observations have shown that, contrary to expectations, the gas remains persistently hot. This discrepancy suggests an unknown heating mechanism is at work, preventing the hot gas from cooling as expected. Unraveling this mystery is crucial for understanding the formation and evolution of the Universe’s largest structures.

From December 2023 to January 2024, the research team used XRISM to observe the nearby galaxy cluster, the Centaurus Cluster, located approximately 100 million light-years from Earth. The goal was to investigate the motion of the hot gas in the core of the galaxy cluster.

Figure 2:X-ray spectrum of the central region of the Centaurus Cluster taken by Resolve onboard XRISM. In the background is an image of the same area acquired by the Chandra X-ray Observatory. (JAXA)

Figure 2 displays the X-ray spectrum of the cluster’s center, captured by Resolve, the cutting-edge soft X-ray spectrometer onboard XRISM. By analyzing this spectrum, the research team aimed to gain deeper insights into how gas moves within the cluster. Precise spectroscopic measurements of emission lines—the sharp, spike-like features in Figure 2—are essential to study this motion. Resolve achieves an energy resolution about 30 times higher than conventional instruments, making it particularly adept at measuring gas velocity with unprecedented accuracy.

Observations revealed that the hot gas at the center of the Centaurus Cluster flows toward Earth at speeds of 130 to 310 km per second (Figure 3). This oscillating (“sloshing”) motion is believed to stir the surrounding hot gas, preventing it from cooling and maintaining the high temperatures observed in the cluster’s core.

Figure 3: Explanation of the high-temperature gas flow derived from XRISM observations. The galaxy NGC 4696 is located at the center of the Centaurus cluster of galaxies. (JAXA)

The research team compared XRISM’s observations with numerical simulations and concluded that past collisions and mergers of clusters are responsible for the observed hot gas sloshing. Figure 4 illustrates this mechanism: the Centaurus Cluster has undergone multiple interactions with smaller clusters, and the hot gas in its core is still sloshing due to these past collisions. XRISM’s observations reveal that these motions stir the gas, preventing it from cooling and maintaining the cluster’s high central temperature.

Figure 4: Illustration showing the mechanism of the sloshing motion of high-temperature gas in the center of a galaxy cluster. (JAXA)

This research has unveiled the gas motions within a galaxy cluster with unprecedented precision. XRISM’s observations provide direct evidence of how clusters evolve through collisions and mergers, offering a crucial missing piece in our understanding of cosmic history. By capturing the velocity of hot gas in such detail, this study marks a major leap forward in our knowledge of large-scale cosmic evolution. The discovery of these dynamic gas velocities not only deepens our insight into galaxy clusters but also holds the potential to greatly advance our understanding of the formation and evolution of other celestial bodies in the Universe.

Paper Information

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