Scientists have directly observed hot intracluster gas in the protocluster SPT2349-56 at redshift 4.3, a finding that challenges existing theoretical models of galaxy cluster formation. Using the Atacama Large Millimeter/submillimeter Array (ALMA), the research team detected the thermal Sunyaev-Zeldovich (SZ) effect, a phenomenon where cosmic microwave background photons are scattered by hot electrons in the intracluster medium (ICM). This observation, reported in the journal Nature, indicates the presence of a substantial amount of hot gas, approximately 10^61 erg of thermal energy, in the core of SPT2349-56.

The discovery provides crucial insights into the early stages of galaxy cluster assembly. Galaxy clusters, the largest gravitationally bound structures in the universe, contain vast amounts of hot gas, known as the ICM, which accounts for a significant portion of the cluster's baryonic mass. Cosmological simulations have suggested that the ICM's mass and temperature should decrease at earlier times, as the gas is still in the process of being assembled and heated. However, the observation of hot ICM at such an early epoch (z=4.3) suggests that significant heating mechanisms were already at play much earlier than previously anticipated.

"This measurement implies a thermal energy about 10 times more than gravity alone should produce," the researchers stated in their Nature publication, highlighting the unexpected energy content of the ICM in SPT2349-56. The protocluster, located approximately 12 billion light-years away, also hosts a large reservoir of molecular gas and three radio-loud active galactic nuclei (AGN) within a relatively small region of about 100 kiloparsecs. These AGN may be contributing to the heating of the ICM through powerful outflows and radiation.

The Sunyaev-Zeldovich effect, the key to this discovery, is a powerful tool for detecting hot gas in galaxy clusters. It arises from the inverse Compton scattering of cosmic microwave background (CMB) photons by the hot electrons in the ICM. This scattering causes a slight distortion in the CMB spectrum, which can be detected by sensitive radio telescopes like ALMA. The strength of the SZ effect is directly related to the thermal pressure of the ICM, providing a measure of its temperature and density.

The implications of this finding extend to our understanding of the processes that govern the formation and evolution of galaxy clusters. The early presence of hot ICM suggests that feedback mechanisms, such as those from AGN, may play a more significant role in heating the gas than previously thought. These feedback processes can regulate star formation within the cluster and influence the overall distribution of matter in the universe.

Future research will focus on studying other high-redshift protoclusters to determine whether the early heating of ICM is a common phenomenon or unique to SPT2349-56. Further observations with ALMA and other telescopes will help to characterize the properties of the ICM in these systems and to identify the sources of heating. These studies will provide valuable constraints on cosmological simulations and help to refine our understanding of the complex interplay between gravity, gas dynamics, and feedback processes in the early universe.