Scientists have directly observed hot intracluster gas in a protocluster called SPT2349-56 at a redshift of 4.3, a period when the universe was less than 1.5 billion years old, using the Atacama Large Millimeter/submillimeter Array (ALMA). This discovery, reported in Nature, challenges existing theoretical models about the formation of galaxy clusters and the heating of the intracluster medium (ICM).

The observation was made possible through the detection of the thermal Sunyaev-Zeldovich (SZ) effect, a phenomenon where photons from the cosmic microwave background (CMB) gain energy as they pass through hot electrons in the ICM. This effect provides a direct way to probe the hot gas that permeates galaxy clusters. SPT2349-56, located in the early universe, hosts a significant reservoir of molecular gas and three active galactic nuclei (AGN) within a relatively small region of approximately 100 kiloparsecs.

The measured thermal energy in the core of SPT2349-56 is approximately 10^61 ergs, which is about ten times greater than what gravity alone would be expected to produce. This suggests that significant heating mechanisms were at play very early in the cluster's assembly. According to the research paper, this finding contradicts current theoretical expectations that predict a decline in the mass and temperature of the ICM towards earlier times, as the gas is still in the process of assembling and being heated.

Galaxy clusters are the largest known gravitationally bound structures in the universe. They contain hundreds or even thousands of galaxies embedded in a diffuse plasma of hot gas, the ICM. This ICM contains most of the baryonic matter (ordinary matter made of protons and neutrons) in the cluster and is heated to temperatures of millions of degrees Kelvin. Understanding how the ICM forms and evolves is crucial for understanding the overall evolution of the universe.

Cosmological simulations have long been used to model the formation of galaxy clusters. These simulations predict that the ICM should be less massive and cooler at earlier times, as the gas is still accreting onto the cluster and being heated by gravitational processes and feedback from AGN. However, the new observation of SPT2349-56 suggests that substantial heating can occur much earlier than previously thought, potentially due to the intense activity of the AGN within the protocluster.

The implications of this discovery are significant for our understanding of galaxy cluster formation. It suggests that the processes that heat the ICM, such as AGN feedback, may be more efficient or more prevalent in the early universe than previously assumed. Further observations of similar protoclusters at high redshifts are needed to confirm these findings and to better understand the mechanisms that drive the early heating of the ICM. These observations could involve using other telescopes and instruments to study the properties of the gas and galaxies within SPT2349-56 in more detail.