Intracluster medium

The intracluster medium, or ICM, is a vast cloud of superheated plasma. This plasma fills the immense spaces within galaxy clusters. Galaxy clusters are massive structures held together by gravity. While galaxies and stars are the most visible parts, they are not the main source of normal matter. Instead, the ICM accounts for most of the baryonic material in these clusters. Baryonic matter is the ordinary matter that makes up everything we can touch. The ICM is incredibly hot, reaching temperatures between 10 and 100 megakelvins. Because of this extreme heat, the gas emits powerful X-ray radiation.

The composition of the ICM is mostly ionized hydrogen and helium. These are the simplest elements in the universe. However, the plasma is also enriched with heavier elements, such as iron. In astronomy, the amount of these heavy elements compared to hydrogen is called metallicity. The metallicity in the ICM usually ranges from one-third to one-half of the Sun's metallicity. In some specific areas, like the Centaurus Cluster, the metallicity can even rise above that of our Sun. These heavy elements are ejected from exploding stars known as supernovae. The strong gravitational field of the cluster keeps this metal-rich gas trapped.

Understanding the density and temperature of the ICM is vital for astronomers. The gas is actually very thin. Typical values show only 10⁻³ particles per cubic centimeter. The mean free path, or the distance a particle travels before hitting another, is about one light-year. Despite being thin, the density rises sharply toward the cluster's center. Interestingly, the temperature also changes near the core. The temperature in the central regions typically drops to one-half or one-third of the value found in the outer regions. Once the density reaches a certain critical value, the ions interact enough to cause cooling via X-ray radiation.

Scientists use several methods to observe this invisible plasma. Because the ICM is so hot, it emits X-rays through a process called bremsstrahlung. It also produces X-ray emission lines from its heavy elements. Astronomers use X-ray telescopes to study this light. This data helps them determine the temperature, density, and metallicity of the plasma. They can also use hydrostatic equilibrium modeling to find the mass distribution. Another method involves the Sunyaev–Zel'dovich effect. This happens when relativistic electrons in the ICM interact with low-energy photons from the cosmic microwave background radiation. This interaction causes distortions in the spectrum of that background radiation.

The mass of the ICM provides a way to find dark matter. In the Virgo Cluster, the ICM contains roughly 3 × 10¹⁴ solar masses. The total mass of the Virgo Cluster is estimated at 1.2 × 10¹⁵ solar masses. The ICM makes up about 15% of a cluster's total mass. In contrast, stars and galaxies contribute only about 5%. Most of the mass in a cluster is actually dark matter, which we cannot see directly. However, mass distributions determined from ICM observations reveal much more mass than the visible stars. This is a strong indication that dark matter is present. A 2018 study even described the ICM as an accurate luminous tracer of dark matter.

One fascinating process in the ICM is the cooling flow. Because the density is highest at the center, the radiative cooling time is much shorter there. As the central gas cools, it can no longer support the weight of the hot gas above it. This creates a pressure gradient that drives a cooling flow. In this process, hot gas from the outer regions flows slowly toward the center. Historically, scientists thought this would create large regions of cold gas and new star formation. However, images from the Chandra X-ray Observatory do not show as much new star formation as expected.

Researchers are still looking for the reason why the gas does not cool as much as predicted. There are two popular explanations for this. One idea is feedback from active galactic nuclei. These are powerful centers that inject relativistic jets of plasma into the ICM. These jets can be seen with high-resolution telescopes like Chandra. Another idea is the "sloshing" of plasma during mergers between subclusters. A 2024 simulation by Eric Rohr and others used TNG-Cluster software to study 352 clusters. The simulation suggested that supermassive black holes might be the most important factor. Their kinetic energy might prevent the central ICM from cooling and forming stars.