A fundamental prediction of the current cosmological model is that galaxies form in overdensities connected by a network of filaments composing the cosmic web. This picture emerges clearly from computer simulations, and it is supported by observational studies that probe the gas distribution between galaxies within the so-called intergalactic medium (IGM). Entangled within these filaments, galaxies are part of a cosmic ecosystem where the interaction with the IGM shapes and drives their evolution. For this reason, the study of the denser gas regions surrounding galaxies within the circumgalactic medium (CGM) has emerged as a powerful tool for studies of galaxy evolution in connection with the inflows and outflows of gas, which are two of the key processes that regulate the galaxies' ability to form stars. Due to the diffuse nature of the CGM, which is much less dense than the gas inside galaxies, it has been extraordinarily challenging to see this medium directly even with the most powerful telescopes. The best way to study this gas's distribution, kinematics, and chemical properties at the interface between galaxies and the cosmic web has, therefore, been the technique of absorption line spectroscopy, by which the gas we want to study is probed in silhouette against bright and unrelated background sources. This powerful technique is, however, probing gas along a very narrow pencil beam, thus limiting the amount of information we can recover about its spatial distribution. Moreover, to relate the gas probed in absorption with the properties of the galaxies, very deep and complete surveys of the galaxies surrounding the detected gas clouds are needed, requiring significant effort even at the largest telescopes. This action has built on ground-breaking technological developments in instrumentation that have revolutionized our view of the link between gas and galaxies.
As part of this action, we have reached three transformative goals by leading some of the most ambitious observational campaigns on novel instruments. First, we have acquired direct images of the shape and chemical content of the gas filaments and gas envelopes near galaxies. Second, we have obtained new evidence on how this gas phase – an essential ingredient for the assembly and evolution of galaxies – evolves with time and changes with the number of close neighbors near galaxies. Finally, we have expanded our view of the gas-galaxy connection into a novel region of parameter space, reaching smaller galaxies that have remained elusive in the past but that account for most of the galaxies that populate the Universe. This project has thus added critical information to our appreciation of how galaxies assemble and evolve into the objects we see today in the Universe, contributing to a more complete picture of the events that lead to the assembly of the general galaxy populations, including our own Galaxy.
