Speaker
Description
Galaxy clusters are the largest virialized structures in the Universe, yet their assembly at z>2 remains poorly understood. Their progenitors, protoclusters, span tens of Mpc hosting gas-rich, dust-obscured galaxies in diffuse, low-surface-brightness environments. (Sub-)millimeter observations uniquely trace the molecular gas and dust fueling rapid galaxy growth, but current facilities lack the mapping speed, surface-brightness sensitivity, and field of view to probe these scales. A next-generation 50m-class single-dish telescope such as AtLAST would enable wide-area, high-sensitivity spectral-imaging surveys of high-redshift protoclusters, capturing both extreme (sub-)millimeter galaxies and the bulk of star-forming systems in feasible integration times. Crucially, AtLAST would recover extended emission inaccessible to interferometers, providing the first complete view of the large-scale baryon cycle in forming clusters. Uniform multiline CO, [CI], and [CII] observations, including the CO SLED, across statistically significant samples will quantify the cold gas budget and its spatial distribution, revealing where star formation occurs and tracing gas accretion and cooling. I will highlight the key challenges limiting protocluster studies and show how AtLAST, in synergy with wide-field optical-to-NIR surveys, overcomes them to deliver the multi-scale, multiphase view needed to understand how galaxies, gas, and dark matter assemble in the densest regions of the early Universe.