Quenching of star formation in the local universe : a comparison of hydrodynamical simulations with spectroscopic observations

Understanding the physical processes responsible for ceasing, or 'quenching', star formation in galaxies is one of the most important unsolved questions in the field of galaxy evolution. Over the past two decades multiple mechanisms were suggested as potential drivers of the transition between the star-forming and quiescent galaxy categories, painting a very complex picture of quenching and its potential physical drivers. In this thesis we take on the challenge of identifying and characterising the physics of galaxy quenching by combining traditional statistical methods with machine learning techniques to embrace the complexity of all physical processes at play. We first show how both gas fraction and star formation efficiency (SFE) decrease in Sloand Digital Sky Survey (SDSS) galaxies at z = 0 as they depart from the star-forming Main Sequence (MS) towards quiescence. We further show that both quantities correlate similarly strongly with the departure from the MS, implying the need for any physical model of quenching to invoke a change in both gas fraction and SFE. We then move on to extract theoretical predictions of the observable consequences of Active Galactic Nucleus (AGN) feedback quenching from three state-of-the-art cosmological simulations: EAGLE, Illustris and IllustrisTNG. Our machine learning (ML) classification reveals supermassive black hole mass (MBH) as the most predictive parameter in determining whether a galaxy is star forming or quenched at redshift z = 0 in all three simulations. This prediction is met overwhelmingly well in the observations, where it is true for a range of indirect estimates of MBH via proxies as well as its dynamical measurements. In simulations we further demonstrate that it is the integrated power output of the AGN, rather than its instantaneous activity, which causes galaxies to quench. We thus mark our most important contribution to the field by providing direct evidence in favour of the AGN quenching paradigm, together with suggestions on how to best observe the effect supermassive black holes have on their galactic hosts. Finally, we analyse the trends in molecular gas content with MBH, finding that the SDSS observations more closely follow trends predicted by IllustrisTNG, rather than the Illustris suite. In our current ongoing work we also analyse spatially resolved properties of the cold ISM, rigorously testing the ability of current cosmological simulations to successfully recover realistic ISM on kpc scales. As a result of our study we further conclude that a viable AGN feedback prescription can be achieved by a combination of preventative feedback and turbulence injection which together quench star formation in central galaxies.