Periodic Reporting for period 4 - FEEDGALAXIES (A new vantage point on how gas flows regulate the build-up of galaxies in the early universe)
Reporting period: 2021-05-01 to 2022-10-31
A fundamental prediction of the current cosmological model is that galaxies form in overdensities that are connected by a network of filaments, which composes the cosmic web. This picture emerges clearly from computer simulations, and it is supported by observational studies that probe the gas distribution in between galaxies, within the so-called intergalactic medium (IGM). Entangled within these filaments, galaxies are part of a cosmic ecosystem in which the interaction with the IGM shapes and drives their evolution. For this reason, the study of the denser 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 the distribution, kinematics and chemical properties of this gas at the interface between galaxies and the cosmic web has therefore been the technique of absorption line spectroscopy, thereby 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, a task that requires significant effort even at the largest telescopes. This action builds on ground-breaking technological developments in instrumentation that allow for a revolution in our view of the link between gas and galaxies. First, and foremost, the deployment of the MUSE instrument at the Very Large Telescope allows for the first time to conduct deep and complete surveys at optical wavelengths of every astrophysical object within a large field of view, particularly in regions where bright background sources can be used to map the CGM and IGM in absorption. The WFC3 instrument on board of the Hubble Space Telescope allows a similar study, but at infrared and ultraviolet wavelengths, thus extending the time interval over which this analysis can be conducted. By leading some of the most ambitious observational campaigns on these instruments, we aim to establish a novel and complete understanding of the co-evolution of galaxies and the surrounding gas over a significant part of the cosmic history. Excitingly, by pushing the limits of MUSE, we aim to glimpse at the CGM and IGM directly in emission, to obtain a more complete and systematic view of how gas is distributed and exchanged around and between galaxies. This project is therefore poised to add critical information to our appreciation of how galaxies assemble and evolve into the objects we see today in the Universe, contributing to paint a more complete picture of the events that lead to the assembly of the general galaxy populations, including our own Galaxy.
A significant effort to date has been in developing novel analysis techniques to exploit the ground-breaking nature of the MUSE data. Specifically, we have developed one of the most advanced end-to-end pipelines to maximise the information content of the MUSE data, including numerical techniques to improve the sensitivity of these observations. With this tool at hand, we have delivered a set of novel results in connecting the evolution of galaxies with the properties of their surrounding gas. In a series of publications, we have demonstrated the game-changing nature of MUSE for this type of studies. While previous work has been hindered by the difficulty of identifying galaxies near to the gas clouds mapped in absorption, our analysis technique applied to MUSE data has enabled us to obtain a very high detection rate for galaxies near these gas clouds, especially at early times in the cosmic history. With this powerful dataset, we have uncovered for the first time a population of star-forming galaxies near relatively unpolluted gas clouds. This is somewhat surprising, as most of the gas residing next to star-forming galaxies is expected to be polluted by heavy elements, a by-product of star formation. Our observations therefore offer new empirical evidence that the CGM and IGM is not homogenous around galaxies, and that relatively pristine gas clouds are still present near galaxies 10 billion years after the Big Bang. This is both challenging and adding constraints to most modern cosmological simulations, which demand that a large fraction of material enriched with heavy elements by stars is ejected to large distances from galaxies during their evolution.
Thanks to the first significant statistical samples, we have been able to develop a novel methodology of analysis that adds new information on the mass of the galaxies that host the gas clouds we observe in absorption, a largely unknown but key quantity to compare observations with the results of numerical simulations. We have further been able to expand this analysis to the most distant epoch in the evolution of the Universe where it is possible to connect gas and galaxies directly. Our pilot observations at this very early times have hinted at a different distribution of galaxies around gas clouds compared to later times, with galaxies preferentially associated with clouds enriched of heavy elements that are illuminated by ionizing radiation. This finding suggests that sites rich of ionized heavy elements are more likely to be pintointing sites where galaxies forms at early times than regions of dense but less enriched and ionized gas.
Another significant advancement of our action has been in demonstrating the importance of the surrounding environment in shaping the gas distribution around galaxies. Thanks to the very complete surveys enabled by MUSE, we have been able to uncover several instances of multiple galaxies that are associated to gas clouds. Our analysis convincingly shows that the cases of multiple galaxies have a different gas distribution compared to isolated galaxies. This shows that in rich environments galaxies can interact more prominently with each other, and this shapes the gas distribution around the galaxies. In line with findings obtained when studying the stellar content of galaxies, our analysis of the CGM confirms that the evolution of galaxies in rich environments follows therefore a different path compared to galaxies that live in isolation. Finally, a very exciting result of this action, which has led to a high-profile publication in the Science magazine, is the first glimpse at the emission from gas in between galaxies (see attached figure). Using MUSE, we have in fact obtained a first view of the filaments connecting the CGM of rapidly star-forming galaxies in a rich cluster of galaxies that was assembling when the Universe was 10 billion years old. This study confirms the great promise of MUSE and of this action to re-shape the view of how gas is distributed in between galaxies.
Thanks to the first significant statistical samples, we have been able to develop a novel methodology of analysis that adds new information on the mass of the galaxies that host the gas clouds we observe in absorption, a largely unknown but key quantity to compare observations with the results of numerical simulations. We have further been able to expand this analysis to the most distant epoch in the evolution of the Universe where it is possible to connect gas and galaxies directly. Our pilot observations at this very early times have hinted at a different distribution of galaxies around gas clouds compared to later times, with galaxies preferentially associated with clouds enriched of heavy elements that are illuminated by ionizing radiation. This finding suggests that sites rich of ionized heavy elements are more likely to be pintointing sites where galaxies forms at early times than regions of dense but less enriched and ionized gas.
Another significant advancement of our action has been in demonstrating the importance of the surrounding environment in shaping the gas distribution around galaxies. Thanks to the very complete surveys enabled by MUSE, we have been able to uncover several instances of multiple galaxies that are associated to gas clouds. Our analysis convincingly shows that the cases of multiple galaxies have a different gas distribution compared to isolated galaxies. This shows that in rich environments galaxies can interact more prominently with each other, and this shapes the gas distribution around the galaxies. In line with findings obtained when studying the stellar content of galaxies, our analysis of the CGM confirms that the evolution of galaxies in rich environments follows therefore a different path compared to galaxies that live in isolation. Finally, a very exciting result of this action, which has led to a high-profile publication in the Science magazine, is the first glimpse at the emission from gas in between galaxies (see attached figure). Using MUSE, we have in fact obtained a first view of the filaments connecting the CGM of rapidly star-forming galaxies in a rich cluster of galaxies that was assembling when the Universe was 10 billion years old. This study confirms the great promise of MUSE and of this action to re-shape the view of how gas is distributed in between galaxies.
Having completed a work-intensive technical phase, we are now in a strong position to fully exploit our unrivalled dataset to address the key questions underlying this action. We are now moving from the analysis of small samples to the study of the entire sample of observations collected with MUSE to date. This represents more than a ten-fold increase in sample size compared to the current state of the art. At the end of this project, we will obtain a very solid statistical view of how gas is distributed around galaxies and how the properties of the CGM change with galaxy properties, with their environment, and with time. By systematically combining experiments conducted at different cosmic epochs, we will be in a strong position to better understand the impact of the CGM in the assembly of galaxies, particularly bracketing the epoch at which galaxies are most active in the formation of their stars. We are also in the process of collecting some of the deepest observations ever obtained in a single region of the Universe. With these unique data, we will detect some of the faintest galaxies, to better appreciate how galaxies of low mass evolve in connection to their CGM, and to test whether there is any systematic difference compared to galaxies of higher mass. These very deep data will also enable a sensitive search for the faint glow expected from the CGM and IGM, and will get us closer to a full and detailed view of the spatial distribution of gas around galaxies. Finally, we are assembling a library of cosmological simulations that build on the state-of-the-art EAGLE cosmological simulation, but extend the simulated volume by 8 times so to capture with adequate statistics the different environments probed by our observations. These simulations also implement variations of the physical model that describes the evolution of galaxies, thus enabling controlled experiments in which we can link observable effects to a specific physical process. By systematically comparing these simulations with our observations, we will obtain a complete theoretical framework to understand the co-evolution of galaxies and the CGM over cosmic time.