The interstellar medium at high redshift

The gaseous content of galaxies is one of the key ingredients of galaxy evolution: It fuels star formation and nuclear activity, and at the same time it is the main receiver of any feedback mechanism from these two processes. However, we have only very limited knowledge of the gas content and of its physical properties in galaxies beyond the local universe. As a result, no description of galaxy formation can be fully validated without a better understanding of 1) how much gas was available for star formation in different cosmic times; 2) how the star formation efficiency, i.e., the capability of a galaxy to form stars from a given amount of gas, evolve through cosmic history, and which are physical conditions that governed this evolution; 3) what was the interplay between the interstellar medium, star formation, and nuclear activity. These questions are particularly critical at the very high redshifts (z>6, when the universe was <1 Gyr old), where our census of galaxies is highly incomplete, especially at the high-mass end, and galaxies followed very different evolution according to the mass of their initial gas reservoir. In this talk I will present the results of various observational campaigns aimed at addressing these questions, in particular by capitalizing on the unprecedented capabilities offered by the Atacama Large Millimeter Array and the Northern Extended Millimeter Array. These findings will guide the next generation of simulations and theoretical models, thus marking a transformational step in our understanding of galaxy evolution.