Periodic Reporting for period 4 - MUSTANG (Multi-scale Star Formation Across Nascent Galaxies)
Okres sprawozdawczy: 2021-10-01 do 2023-01-31
One of the fundamental questions in modern astrophysics is how gas is converted into stars during galaxy formation, and how this may have changed with galactic environment and cosmic time. It is widely accepted that the unknown physics of star formation (SF) in molecular clouds and feedback from massive stars represent the main uncertainties in our understanding of galaxy formation. The key open questions are: which physical mechanisms govern the time-evolution of gas towards SF and its subsequent expulsion by feedback? How do these physics change with environment or cosmic time? How did the most extreme stellar systems in the Universe (i.e. globular clusters) form at high redshift? How do the cloud-scale physics of SF and feedback connect cold dark matter cosmology to the observable galaxy population? It is now possible to answer these questions thanks to two crucial developments. (1) The latest generation of billion-euro observatories allow the necessary observational data to be taken. (2) The techniques needed to solve the problem have been developed, tested, and validated, through major theoretical efforts that led by the PI of this project. This team is answering the above questions with a unique combination of world-leading observational data, fundamental theory, innovative analysis techniques, and state-of-the-art simulations. The team is using these to formulate a multi-scale description of SF and feedback, focussing on the cloud-scale physics and advancing the field beyond the phenomenology of galactic scaling relations. The team is introducing an empirically-motivated, physical model for cloud-scale SF and feedback in galaxy formation simulations, thereby overcoming their main limitation and making the crucial step of linking the observable galaxy population to cold dark matter cosmology. This ambitious programme bridges observations, theory, and simulations, and has the end goal of explaining galaxy formation from the bottom up, using cloud-scale descriptions for SF and feedback.
Our team has developed a new methodology for reconstructing the evolutionary timeline of cloud-scale star formation and feedback from pairs of galaxy maps. We obtained the necessary observational data and applied this method to high-resolution observations of molecular gas and star formation tracers in nearby galaxies to empirically characterise cloud evolution, star formation, and feedback. This has provided first insight in how these processes change with the host galaxy properties. To complement and help interpret this observational work, we have developed a new theory for the molecular cloud lifecycle. We have tested this theory against high-resolution numerical simulations of different disc galaxies with properties chosen to isolate each cloud evolutionary mechanism. Partially based on these results, we have implemented a new model for cloud-scale star formation in numerical simulations of galaxy formation and evolution. This model accounts for cloud dynamics explains the low degrees of star formation in galaxy centres and bulges, including the centre of the Milky Way. We have also developed a similar, observation-based model for stellar feedback. In addition to these efforts, we have developed a comprehensive analytical theory of stellar cluster formation, which has been implemented in numerical simulations of galaxy formation as descriptions for stellar cluster formation and dynamical evolution. We are currently applying these novel cluster models in cosmological zoom-in simulations of galaxy formation, with a complete description of the cold interstellar medium, as well as the tracking of more than 30 chemical elements, so that the simulations will eventually show how the debris of stellar clusters can be identified through their chemical abundances and be used to reconstruct the formation and evolution of the host galaxy. Using similar methods, we have used the globular cluster population of the Milky Way to reconstruct its assembly history, showing that the Milky Way must have undergone 15 mergers with other galaxies. This work also predicts that the most major of these mergers took place with a galaxy that we named "Kraken", the debris of which has since been discovered using the Gaia telescope.
By the end of the project, we expect to have unified the above results in cosmological simulations of galaxy formation and evolution, with complete models for the cold interstellar medium, star formation, stellar feedback, and star cluster formation and evolution. These simulations are not only aimed at predicting how galaxies form and evolve within their dark matter haloes, but also how the relics of that process can still be detected in old stellar populations orbiting galaxies at the present day.