Witnessing the intracluster medium assembly at the cosmic noon in JKCS 041

In this work, we study the intracluster medium (ICM) of a galaxy cluster at the cosmic noon: JKCS 041 at z = 1.803. A 28 h long Sunyaev-Zel'dovich (SZ) observation using MUSTANG-2 allows us to detect JKCS 041, even if intrinsically extremely faint compared to other SZ-detected clusters. We found that the SZ peak is offset from the X-ray centre by about 220 kpc in the direction of the brightest cluster galaxy, which we interpret as due to the cluster being observed just after the first passage of a major merger. JKCS 041 has a low central pressure and a low Compton Y compared to local clusters selected by their ICM, likely because the cluster is still in the process of assembly but also in part because of a hard-to-quantify bias in current local ICM-selected samples. JKCS 041 has a 0.5 dex fainter Y signal than another less massive z similar to 1.8 cluster, exemplifying how much different weak-lensing mass and SZ mass can be at high redshift. The observations we present provide us with the measurement of the most distant resolved pressure profile of a galaxy cluster. Comparison with a library of plausibly descendants shows that JKCS 041 pressure profile will likely increase by about 0.7 dex in the next 10 Gyr at all radii.