| There is a growing notion that the formation of massive galaxies must be a two-phase process, with a late phase that is dominated by galaxy mergers and an early phase that is driven by cold gas accretion.
I will present observational evidence for large (~100 kpc) reservoirs of very cold molecular gas in the halo-environments of proto-cluster radio-galaxies at z~2. We discovered that this cold molecular medium is related to a variety of important evolutionary processes, from cold gas accretion to jet-induced feedback and galaxy merging. These processes reveal the important role of the cold molecular inter-galactic medium (IGM) in the early formation of massive galaxies.
I will also show that studying the molecular IGM is only possible with radio interferometers that are sensitive to detecting low-surface-brightness emission of widespread cold gas. In this respect, I will explain how traditional "cm" instruments like the ATCA and VLA (which also operate in the 30-50 GHz regime) form a crucial complement to ALMA, and how technical considerations regarding surface-brightness sensitivity can optimise future radio interferometers for studying the cold molecular medium.