Monsters in the dark: modelling the formation of bright galaxies in the distant Universe

The study of the high-redshift Universe lies at the frontier of extragalactic as- trophysics. This critical period of the Universe’s history encompasses the formation of the first stars and Super-Massive Black Holes (SMBHs), the reionisation of the Universe, and the production of the first heavy elements. Observationally this era is now being opened up by the revolutionary JWST and will soon be complemented by the Euclid spacecraft. The last 10 years have also seen dramatic progress with the development of hydrodynamical simulations. Unlike their predecessors which fo- cused solely on gravitational effects these self-consistently model baryonic processes including cooling, star formation, SMBH formation and growth, and feedback. This thesis explores several topics in both observational and theoretical galaxy formation.By utilising forward modelling simulations and spectral synthesis codes, I inves- tigate various aspects of galaxy formation. I make use of analytic models that allow quick exploration of a wide range of intrinsic galaxy properties and hydrodynamical simulations. In the latter case, I utilise a novel suite of hydrodynamical simulations called First Light And Reionisation Simulations (Flares).The thesis introduces a software library aimed at studying galaxy scaling rela- tions and distribution functions, EritLux. The functionality of EritLux is described along with examples of the various empirically motivated models that are used in simulating galaxy photometry. By default EritLux uses analytic models for galax- ies, but is also built to accommodate more complex models. The code includes components for selecting variety of models, simulating photometry, measuring com- pleteness in surveys and inferring evolution of key high-redshift galaxy observables.I also investigate the co-evolution of Super-Massive Black Holes (SMBHs) with galaxies in Flares, forward modelling Active Galactic Nuclei (AGN) spectra to model their photometric properties. I present the numbers and mass functions of Flares SMBHs as well as the properties of their host galaxies. I also introduce photometric properties of the simulated AGN in terms of their bolometric, UV and ionising emission. I present the Flares Galaxy+AGN UV luminosity functions across the reionisation epoch and investigate the impact of AGN on star formation.Complementing JWST is the Euclid spacecraft, set to launch in 2023. I present predictions for Euclid-accessible reionisation epoch galaxies based on Flares. I present the predicted numbers of galaxies detectable by Euclid, the corresponding UV luminosity function constraints and the physical and photometric properties of these galaxies. These predictions will further inform us of the early galaxy formation and evolution when eventually compared to the observations in the Euclid surveys.</p

First light and reionization epoch simulations (FLARES) IX: the physical mechanisms driving compact galaxy formation and evolution

In the First Light And Reionization Epoch Simulations (FLARES) suite of hydrodynamical simulations, we find the high-redshift (z > 5) intrinsic size–luminosity relation is, surprisingly, negatively sloped. However, after including the effects of dust attenuation, we find a positively sloped UV observed size–luminosity relation in good agreement with other simulated and observational studies. In this work, we extend this analysis to probe the underlying physical mechanisms driving the formation and evolution of the compact galaxies driving the negative size–mass/size–luminosity relation. We find the majority of compact galaxies (R1/2, * </p

First light and reionisation epoch simulations (FLARES) - VIII. The emergence of passive galaxies at z ≥ 5

Passive galaxies are ubiquitous in the local universe, and various physical channels have been proposed that lead to this passivity. To date, robust passive galaxy candidates have been detected up to z ≤ 5, but it is still unknown if they exist at higher redshifts, what their relative abundances are, and what causes them to stop forming stars. We present predictions from the first light and reionisation epoch simulations (flares), a series of zoom simulations of a range of overdensities using the eagle code. Passive galaxies occur naturally in the eagle model at high redshift, and are in good agreement with number density estimates from Hubble Space Telescope (HST) and early JWST results at 3 ≤ z ≤ 5. Due to the unique flares approach, we extend these predictions to higher redshifts, finding passive galaxy populations up to z ∼8. Feedback from supermassive black holes is the main driver of passivity, leading to reduced gas fractions and star forming gas reservoirs. We find that passive galaxies at z ≥ 5 are not identified in the typical UVJ selection space due to their still relatively young stellar populations, and present new rest-frame selection regions. We also produce mock NIRCam and MIRI fluxes, and find that significant numbers of passive galaxies at z ≥ 5 should be detectable in upcoming wide surveys with JWST. Finally, we present JWST colour distributions, with new selection regions in the observer-frame for identifying these early passive populations.</p

First light and reionization epoch simulations (FLARES) X iii: the lyman-continuum emission of high-redshift galaxies

The history of reionization is highly dependent on the ionizing properties of high-redshift galaxies. It is therefore important to have a solid understanding of how the ionizing properties of galaxies are linked to physical and observable quantities. In this paper, we use the First Light and Reionization Epoch Simulations (Flares) to study the Lyman-continuum (LyC, i.e. hydrogen-ionizing) emission of massive () galaxies at redshifts z = 5 - 10. We find that the specific ionizing emissivity (i.e. intrinsic ionizing emissivity per unit stellar mass) decreases as stellar mass increases, due to the combined effects of increasing age and metallicity. Flares predicts a median ionizing photon production efficiency (i.e. intrinsic ionizing emissivity per unit intrinsic far-UV luminosity) of, with values spanning the range. This is within the range of many observational estimates, but below some of the extremes observed. We compare the production efficiency with observable properties, and find a weak negative correlation with the UV-continuum slope, and a positive correlation with the [O iii] equivalent width. We also consider the dust-attenuated production efficiency (i.e. intrinsic ionizing emissivity per unit dust-attenuated far-UV luminosity), and find a median of. Within our sample of galaxies, it is the stellar populations in low mass galaxies that contribute the most to the total ionizing emissivity. Active galactic nuclei (AGN) emission accounts for 10 - 20 per cent of the total emissivity at a given redshift, and extends the LyC luminosity function by ∼0.5 dex.</p