Simulations of structure formation and feedback at high redshift

Understanding how structure formation progressed in the time between the emission of the cosmic microwave background and the formation of the first galaxies is essential to our models of the reionisation of the universe. Processes which can impact the formation and abundance of small-scale structures are of particular interest, since it is these structures which are thought to, initially, be the primary drivers of reionisation. In this thesis, we present a range of works using numerical simulations to model structure formation, and associated feedback processes, in the high-redshift universe. To this end, we present our methodology for studying the impact of supersonic relative baryon-dark matter velocities on small-scale structure formation and demonstrate its effect on the halo baryon fraction and early star formation. We find a suppression in the baryon fraction of haloes and a delay in the onset of star formation, in qualitative agreement with previous works. This is the first simulation, to our knowledge, to self-consistently sample the relative velocity from a large box, making it useful for future works exploring the effect of the spatial fluctuations of the relative velocity. Extending previous works, we begin to model the impact of reionisation on the Local Group of galaxies, using extremely high-resolution radiation-hydrodynamics simulations. We ran an extremely high-resolution constrained dark matter-only simulation (containin 163843 effective particles in the zoom region—the highest- resolution simulation in the Hestia suite to date) of the Local Group down to z = 0, demonstrating excellent agreement with previous Hestia runs. Further, we presented preliminary work on calibrating the star formation and supernova feedback to produce a realistic ionisation history. Through the analysis of high-resolution N-body simulations, we assess the impact of initial small-scale suppression (due to interactions between radiation and dark matter in the very early universe) in the matter power spectrum on high-redshift halo formation and evolution. We find that the initial small-scale suppression is washed out to some extent, as power cascades from larger (less suppressed) scales down to small scales. We also find that the abundance of low-mass (M ? 1010 h-1 M?) haloes is reduced, and that the haloes in the interacting dark matter case accrete more of their mass later than in the standard cold dark matter case. Finally, we present analysis of structure formation in the latest in a series of state-of-the-art fully-coupled radiation-hydrodynamics simulations of reionisation, containing 81923 dark matter particles and cells. We present a comparison to a companion dark matter-only simulation, finding good agreement at low redshifts and high masses. We also identify cases of overlinking in the halo analysis, whereby two unbound structures have been spuriously linked.