Star formation in cosmological simulations of Milky Way-like galaxies

Detailing how the cosmological environment surrounding a galaxy (large-scale structure) influences the local star-forming regions (small-scale structure) in the galaxy, and viceversa, remains an open question in the field of galaxy formation and evolution. Observations of massive star-forming galactic discs at extremely high redshifts have made this problem all the more pressing, putting current star formation models under careful scrutiny. To pin down these observations in theory, multi-scale simulations with well- calibrated sub-grid models are the way forward.My work as a PhD student, compiled in this thesis, has been carried out in the context of the VINTERGATAN model, generating high resolution cosmological zoom-in simulations of Milky Way-like galaxies. An integral part to unravel the mysteries of galaxy formation and evolution is to understand how they turn their gas reservoirs into stars. In VINTERGATAN this is modeled via a well- motivated star formation law that depends on the density, turbulence, and temperature of the gas at the resolution limit of the simulation, i.e. 20 pc.Using VINTERGATAN I demonstrate that the star formation history of Milky Way-type galaxies can be explained by the response of the interstellar medium (ISM) to the cosmological environment around the galaxy at its different stages of evolution. In particular, I show how the assembly of a galactic disc in combination with a merger-driven environment paints a picture where the galaxy shifts between more quiescent and starbursty modes of star formation caused by changes in its global gas depletion timescale. At the scale of giant molecular clouds (GMCs), starburst epochs coincide with tidal compression of larger volumes of gas, shifting the density and turbulence distributions towards higher values. However, the density and turbulence structure of the ISM evolve so that the star formation law implemented predicts a local universal efficiency of 1%. My results are in line with current surveys targeting star- forming spiral galaxies, e.g. PHIBSS, as well as star-forming GMCs in them, e.g. PHANGS.Changing the large-scale structure around a galaxy can lead to differences in its star formation history. The genetic modifications technique enables systematic changes like increasing or decreasing the mass of the last major merger (LMM) in a cosmological simulation. Using a genetically modified VINTERGATAN whose LMM is four times more massive, I propose a formation channel for kinematically misaligned low surface brightness structures around early-type galaxies.