At redshifts of z=2 to 3, the epoch of the peak star formation and black hole activity in the Universe, the first giant dark matter halos were also growing very rapidly, and baryons falling into their deep potential wells induced prodigiously vigorous activity leading to the major phases of galaxy and black hole assembly, often hidden by dust. These early phases for the evolution of galaxies are expected to be crucial to lead to the formation of their dominant early type galaxy population (via quenching mechanisms still poorly understood) and well relaxed hot gas atmospheres, as observed in local massive galaxy clusters (via some sort of yet unknown feedback between galaxies and the hot gas, leading to energy and entropy injection affecting its thermodynamic evolution). The overall physical processes relevant for galaxies and structures evolution in the first forming clusters are still largely poorly mapped, and yet not well understood. But increasing interest and efforts in the community coupled with emerging observational results and prospects for future mission (e.g., JWST, Euclid and Athena) make this research field one of the most hot and promising in the current domain of galaxies and structure formation. I propose a PhD thesis in Saclay in this research field, based on new observations with ALMA, NOEMA, Herschel, HST and Keck of two dense structures discovered by our research groups at z=2 and 2.5 (Gobat et al 2011; 2013; Wang et al 2016) and a new forming cluster found at z=2.91. The student will be responsible of the final reduction, analysis and interpretation of a substantial amount of data we obtained
with the new and revolutionary Keck Cosmic Web Imager, allowing for the first time 3D spectroscopy in the blue over large fields, down to wavelengths not accessible to the MUSE instrument at the VLT. These Keck data have revealed giant clouds of cold gas extending over 100kpc or more at the cluster cores, detected from their Lya emission. The high level goal of the thesis will be to observationally characterise and understand the nature, origin and fate of these giant reservoirs of cold gas. This will be done in particular in connection with galaxy activity present in the clusters, that will be probed by HST multicolour imaging (to reveal morphologies, stellar populations and merging rates possibly connected to galaxy stripping and production of inter cluster material) and with NOEMA, ALMA and Herschel (to study gas reservoirs, star formation hidden by dust and the state of the interstellar medium). The cold gas might eventually result to be a first convincing smoking gun of cold flow accretion to massive dark matter halos required by theory to justify the vigorous galaxy activity present at high redshift. Such smoking gun has long been sought observationally at high redshifts but never convincingly detected yet. Possible evidences leading to this could be connected with the morphology of the Lya gas, large kinematics and metal enrichment, that we will be able to investigate with existing data and with future observations. The student will be, in fact, involved during the PhD in a vigorous effort of proposing for Keck, VLT and ALMA/NOEMA time, to foster this science, with a resulting expertise in all aspects of observational astronomy, including experience with dealing with truly multi-wavelength datasets.