A large thermal energy reservoir in the nascent intracluster medium at a redshift of 4.3

Most baryons in present-day galaxy clusters exist as hot gas (≳10^7K), forming the intracluster medium (ICM) [1]. Cosmological simulations predict that the mass and temperature of the ICM rapidly decrease with increasing cosmological redshift, as intracluster gas in younger clusters is still accumulating and being heated [2–4]. The thermal Sunyaev-Zeldovich (tSZ) effect arises when cosmic microwave background (CMB) photons are scattered to higher energies through interactions with energetic electrons in hot ICM, leaving a localized decrement in the CMB at a long wavelength [5, 6]. The depth of this decrement is a measure of the thermal energy and pressure of the gas [7]. To date, the effect has been detected in only three systems at or above z~2, when the Universe was 4 billion years old, making the time and mechanism of ICM assembly uncertain [8–10]. Here, we report observations of this effect in the protocluster SPT2349−56 with Atacama Large Millimeter/submillimeter Array (ALMA). SPT2349−56 contains a large molecular gas reservoir, with at least 30 dusty star-forming galaxies (DSFGs) and three radio-loud active galactic nuclei (AGN) in a 100-kpc region at z=4.3, corresponding to 1.4 billion years after the Big Bang [11–14]. The observed tSZ signal implies a thermal energy of ~10^61 erg, exceeding the possible energy of a virialized ICM by an order of magnitude. Contrary to current theoretical expectations [3, 4, 15], the strong tSZ decrement in SPT2349−56 demonstrates that substantial heating can occur and deposit a large amount of thermal energy within growing galaxy clusters, overheating the nascent ICM in unrelaxed structures, two billion years before the first mature clusters emerged at z~2.