The ALMA view of quasars host galaxies at the dawn of cosmic time

The unparalleled capabilities of ALMA have opened a new window to study galaxy formation even at the earliest cosmic times. High-resolution maps of bright ISM lines, such as [CII] or CO, allow us to map the gas kinematics in galaxies even at the largest redshifts. By surveying multiple ISM tracers, we can infer first quantitative constraints on the physical properties (density, temperature, ionization state, excitation mechanism) of the star-forming medium. In this talk, I will capitalize on both these approaches to study the early evolution of quasars and their host galaxies at z>4. We infer dynamical masses for a sample of z>4 quasar host galaxies observed at high angular resolution in [CII], and we compare them with the central black hole masses to test the onset of the black hole -- host galaxy relations observed in the local universe. We then study the impact of the luminous accreting black hole on the ISM properties of the host, by comparing the emission of various gas tracers (OH, H2O, [NII], high-J CO) in pairs of galaxies with and without a luminous quasar at z>6. Finally, I will discuss how these results fit within the co-evolution scenario and highlight their impact for our understanding of the early evolution of massive galaxies