The elusive baryon cycle and prospects for future ALMA and AtLAST observations

The baryon cycle drives the formation and evolution of structures from stellar core on sub-pc scales to the largest overdensities in the Universe on Mpc scales. This implies that all these physical scales are connected and linked physically to each other, and such link extends over different time scales and multiple gas phases. However, all the manifestations and observables of such baryon cycle, from star forming filaments in the ISM, to the massive molecular outflows and accreting filaments enriching the circumgalactic medium and, possibly, the intergalactic medium, are: 1) diffuse and low surface brightness in nature; 2) extended on large angular scales (arcmin/degrees), and 3) multi-phase, with a crucial role of gas phases (molecular, atomic) that are not visibile in the optical/IR but require sensitive sub-mm instruments. These characteristics make it very challenging to observe the baryon cycle with current or planned telescopes. This issue motivates, among other science drivers, the construction of a new, sensitive, large-aperture (50 meter), wide field of view (1-2 deg), single dish telescope on the Chajnantor plateau: the Atacama Large Aperture Submillimeter Telescope (AtLAST). AtLAST will obviate the technical limitations of interferometers and will be a fundamental complement to ALMA, by providing the much needed coverage of short UV baselines needed for ALMA+single dish data combination, as well as the high mapping speed and throughput to survey large areas of the sky and so discover new targets for ALMA follow ups. After an introduction on the crucial discoveries relevant to the baryon cycle enabled by ALMA, I will outline the main results of the EU-funded AtLAST design study, which will be completed in August 2024. The study is investigating all aspects of building such new international research infrastructure such as the selection of the optimal site, the power supply through renewable energy, and the governance and operations models.