Abstract :
Quasars at high-redshift (z > 3) are among the most luminous sources in the early Universe, powered by rapid gas accretion onto the central supermassive black hole and intense episode of star formation in their host galaxies. These extraordinary systems provide unique laboratories for studying the interstellar medium (ISM) and the baryon cycle of primeval massive galaxies under extreme conditions. Moreover, quasars serve as cosmic beacons, enabling unparalleled investigations of their circumgalactic medium in emission and the intervening intergalactic gas through absorption spectroscopy. Despite decades of effort, key questions about the formation and evolution of the first massive galaxies remain open: How do galaxies acquire their gas? What are the morphology and kinematics of the accreting gas, and how do these processes regulate the build-up of galaxies? While instruments like VLT/MUSE and JWST are revolutionizing our understanding of ionized gas and stellar populations, we are pushing ALMA capabilities to the limit to map the cold ISM at sub-kiloparsec scales and to detect faint, extended emissions around quasar hosts at the dawn of cosmic time. In this colloquium, I will review recent advances in uncovering the nature of quasar host galaxies and their environments during the Universe's first gigayears, highlighting the synergy of multiwavelength observations and the crucial role of cold gas in the assembly of today's most massive galaxies.
