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Description
The Intracluster medium (ICM) is the dominant visible matter in galaxy clusters, existing primarily in the form of hot, metal-rich ionized plasma, and providing the most robust measurements of the cluster mass scale. This hot gas can be characterized by its X-ray emission and the ability to scatter the photons from the Cosmic Microwave Background (CMB), known as the thermal Sunyaev Zeldovich (tSZ) Effect. X-ray observations of nearby galaxy clusters suggest that this hot ICM gas can be established well before the time of cluster formation (``ICM-preheating'').
However, due to the faint observational signature at early times, it is still unclear when and how this hot gas was heated prior to the gravitational collapse of the cluster progenitor, known as a protocluster. Here we report the robust detection of the tSZ decrement in an intense protocluster core SPT2349-56 at $z=4.3$, which is the highest galaxy overdensity yet discovered in the early universe. The tSZ signal in SPT2349-56 is too strong to be solely produced by the gravitational energy of the protocluster, and requires substantial additional energy injection. Our finding suggests that ICM-preheating plays a crucial role in the early establishment of the hot intracluster gas in galaxy clusters. The energetic processes in such early protocluster cores (extreme star formation and excess radio-loud AGN), coupled with the rapid and massive cold flows of gas into the cluster center, can apparently lead to a much {\it stronger} tSZ signal than expected at $z>4$.
Our study suggests that observing the hot ICM using the tSZ at $z>4$ may be a new emerging field, offering previously unforeseen insights into the formation of massive galaxy clusters.