Abstract:  The South Pole Telescope (SPT) has discovered a large sample of millimetre point sources in the 2500 deg^2 SPT-SZ survey, and the recently completed ‘SPT-3G’ third generation survey has dramatically expanded this sample through almost 5x deeper surveys. Follow-up observations with ALMA show that about 90% of the brightest sources are strongly lensed dusty star-forming galaxies, while the remaining ~10% are compact groups of galaxies at z~3-7, representing protocluster cores caught during their most intense phase of star formation. These systems are ideal laboratories for studying how intracluster gas first emerges and interacts with extreme star formation and AGN activity.

In this talk, I will introduce the SPT-selected protocluster sample and highlight our recent ALMA detection of hot intracluster gas at z=4.3 via the thermal Sunyaev–Zeldovich effect – a completely unexpected result which reverses our view of cluster ICM in the first 3 billions years of the Universe. I will discuss the implications and briefly outline future prospects for high-redshift ICM studies, including opportunities with the upcoming facilities such as ALMA Wideband Sensitivity Upgrade (WSU), AXIS, and Athena. I will conclude with the status of the CCAT telescope (first light this year) and its Prime-Cam instrument, and the prospects for continued work in this field with this next generation submm survey facility.