Sunyaev-Zeldovich detection of hot intracluster gas at redshift 4.3

Most baryons in present-day galaxy clusters exist as hot gas (≳10⁷ K), forming the intracluster medium (ICM)¹. Cosmological simulations predict that the mass and temperature of the ICM decline towards earlier times, as intracluster gas in younger clusters is still assembling and being heated^2-4. To date, hot ICM has been securely detected only in a few systems at or above z ≈ 2, leaving the timing and mechanism of ICM assembly uncertain^5-7. Here we report the direct observation of hot intracluster gas via its thermal Sunyaev-Zeldovich signature in the protocluster SPT2349-56 with the Atacama Large Millimeter/submillimeter Array. SPT2349-56 hosts a large molecular gas reservoir and three radio-loud active galactic nuclei (AGN) within an approximately 100-kpc region at z = 4.3 (refs. ^8-11). The measurement implies a thermal energy of  about 10⁶¹ erg in the core, about 10 times more than gravity alone should produce. Contrary to current theoretical expectations^3,4,12, the hot ICM in SPT2349-56 demonstrates that substantial heating can occur very early in cluster assembly, depositing enough energy to overheat the nascent ICM well before mature clusters become common at z ≈ 2.