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A new vantage point on how gas flows regulate the build-up of galaxies in the early universe

Periodic Reporting for period 5 - FEEDGALAXIES (A new vantage point on how gas flows regulate the build-up of galaxies in the early universe)

Periodo di rendicontazione: 2022-11-01 al 2024-01-31

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.
This action has led to three transformative results.

First, we have uncovered the emission from the gas surrounding galaxies, revealing for the first time gas filaments connecting multiple galaxies feeding from the cosmic fuel through which they actively form stars. The cosmic web we observed resembles the filaments predicted by computer simulation based on the current cosmological model, thus confirming this prediction strongly. Furthermore, we have detected, for the first time, the light from the heavy elements produced within stars and subsequently ejected in their gaseous envelopes. This new detection opens an exciting prospect for studying the poorly constrained feedback processes: an ensemble of mechanisms that regulate the efficiency with which galaxies convert gas in stars. This is a vital step for the build-up and evolution of galaxies.

Second, we have unveiled how the galaxy environment shapes the halo gas distribution. Galaxies are well-known to exhibit different properties according to the number of close neighbors, whereas systems living in the busiest environment have older stars and are devoid of gas. Our observations have added fresh clues to understanding the origin of these different properties by detecting a clear excess of gas near galaxies in rich environments compared to isolated systems across a large interval of cosmic times. For more evolved galaxies, this excess can be attributed to mechanisms actively removing gas from the galaxies. In contrast, for the galaxies observed in the younger universe, we linked this excess to a more intense gas accretion phase, leading to a rapid evolution.

Finally, our study has provided an expanded view of the gas environment of low-mass galaxy populations that have remained elusive in the past but that account for the large majority of the galaxies that inhabit the Universe. Our study has led to a picture in which these low-mass galaxies are connected by gas filaments that contain the majority of the dense gas. These are the most likely channels through which galaxies acquire gas. Extended pockets of more diffuse and enriched gas are also present, which we link to past galaxy assembly episodes that have spread heavy elements in the surrounding environment. With modeling from simulations, this action has, therefore, broadened our understanding of the baryon cycle of galaxies across approximately 6 billion years of cosmic time.

From the technical point of view, a significant contribution of this action has been the development of novel analysis techniques to exploit the ground-breaking nature of integral field spectrographs at large telescopes (in particular, the MUSE instrument at VLT). Specifically, we have developed one of the most advanced end-to-end pipelines to maximize the information content of the MUSE data, including numerical techniques to improve the sensitivity. Furthermore, we have put forward the basis of new ways to consistently connect detailed modeling of the gas surrounding galaxies with observations, thus paving the ground for more systematic and self-consistent comparisons between data and models.
The imaging of the intergalactic and circumgalactic medium through Lyα and metal emission is a significant advancement in exploring the impact of the baryon cycle on galaxy evolution. Previously, our knowledge of the gas environment of distant galaxies was limited to a more indirect technique, spectroscopy in absorption. With our new ability to image the diffuse gas surrounding galaxies, we have gained insights into the shape of filaments and halos, previously inaccessible to our observations. Additionally, the ability to directly study the metal content of halos through emission provided powerful constraints for the uncertain models of galaxy feedback. A significant breakthrough has been made in studying the gas environment of star-forming galaxies at cosmic noon. Before the ERC program, we could only study massive galaxies. With the ability to reach the smaller, low-mass systems, we can now study one of the most abundant galaxy populations in the universe. These galaxies are responsible for the bulk enrichment we observe in the cosmos outside of galaxies. Our study has opened up new paths for future large surveys at new-generation telescopes like the European ELT. These telescopes will soon be able to explore even deeper into the dwarf galaxy regime, closing the gap between all the heavy elements observed in the cosmos and the sources that created them.
Map showing emission from the gas in the cosmic web connecting multiple star-forming galaxies