Our overall aim is to understand the role of environment in regulating galaxy growth from the first overdensities at z~6, cosmic dawn, to the galaxy clusters forming at the epoch of peak star formation, z~2, cosmic noon. To accomplish this goal, we have and will assemble a vast data set spanning a wide range of frequencies from the UV to the radio, - we (co)lead three large observing program at ESO, ATCA and IRAM NOEMA. Our selected redshift range implies the understanding of the galaxies that reionized the Universe, as well as the processes that resulted in the formation of galaxies both in the field and in forming clusters (protoclusters). In particular, we will study 1) the conversion of molecular gas in stars, and 2) how the characteristics of star formation depend on the environmental density. Specifically, we intend with this proposal to:

1) understand star formation during the formation and most rapid phase of mass elliptical galaxy growth. We will achieve such understanding by probing the cold molecular gas, the fuel of star formation, and dust, the by-product of the evolution of stars, in distant (proto-)clusters. These will be done by observing these structures in the far-IR and radio. Through radio interferometry we will search for large reservoirs of molecular gas in a well selected sample of galaxies in (proto-)clusters and the field.

2) probe the environmental-dependent evolution of galaxies. We will compare the properties of cluster and available field galaxies. The combination of a proven but novel technique, proper target selection and an excellent dataset promises a breakthrough in our understanding of the formation of the progenitors of elliptical galaxies which dominate local galaxy clusters.

3) investigate the claim that protoclusters play a crucial, perhaps dominate role in the reionization of the Universe. We will conduct a study of overdensities up to the epoch of reionization and we will use observations from the GTC and as well as those available in the literature for this purpose. We aim to understand the sizes of ionized bubbles generated by the combined impact of galaxies in overdensities to test their role in reionizing the universe and, additionally, characterize the earliest stages of the formation of local galaxy clusters.