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), researchers detected the thermal Sunyaev-Zeldovich (SZ) effect, a signature of hot gas, within this early cluster of galaxies.

The observation, detailed in a recent Nature publication, reveals a thermal energy of approximately 10^61 erg in the core of SPT2349-56. This energy level is about ten times higher than what gravitational forces alone would be expected to produce. According to the study, this suggests that substantial heating mechanisms were at play very early in the cluster's assembly, potentially overheating the intracluster medium (ICM) before it fully formed.

The intracluster medium, composed of hot (10^7 K) gas, contains most of the baryons, or ordinary matter, within galaxy clusters. Cosmological simulations have previously suggested that the mass and temperature of the ICM should decrease at earlier times in the universe, as the gas is still in the process of assembling and heating. Prior to this discovery, secure detections of hot ICM were limited to systems at or above redshift 2, leaving uncertainty around the timing and mechanisms of ICM assembly.

SPT2349-56, located at a redshift of 4.3, hosts a significant reservoir of molecular gas and three radio-loud active galactic nuclei (AGN) within a region spanning roughly 100 kiloparsecs. These characteristics make it a unique environment for studying early cluster formation. The detection of the SZ effect in this protocluster provides direct evidence that significant heating can occur much earlier than previously anticipated.

The Sunyaev-Zeldovich effect is a phenomenon where cosmic microwave background (CMB) photons are scattered by hot electrons in the ICM, resulting in a distortion of the CMB spectrum. By measuring this distortion, scientists can infer the temperature and density of the hot gas.

"This discovery provides crucial insights into the early stages of galaxy cluster formation," said one of the lead researchers on the project. "It suggests that current theoretical models may need to be revised to account for the rapid heating observed in this system."

The findings have implications for our understanding of how galaxies and large-scale structures formed in the early universe. The presence of such a hot ICM at this early epoch suggests that feedback processes, such as those from AGN, may have played a more significant role in regulating the growth of galaxies and clusters than previously thought.

Future research will focus on studying other high-redshift protoclusters to determine whether the rapid heating observed in SPT2349-56 is a common phenomenon or an outlier. Further observations with ALMA and other telescopes are planned to probe the properties of the ICM and the role of AGN in heating the gas. These studies will help refine our understanding of the complex processes that shaped the universe we observe today.