The effect of dust in observing galaxies in the early Universe

The first stellar populations kick-started the process of reionisation and began to enrich the pristine interstellar medium (ISM), with the first supernovae starting the dust formation and destruction processes. The pristine ISM evolved significantly over the course of the next billion years, affecting the subsequent evolution of the galaxies. Understanding this phase in the early Universe will help us learn how the galaxies evolved into the demographics we see today. Even in this regime, dust is an essential ingredient: even though the average dust content of galaxies is very low compared to the local Universe, it still has a significant impact on deriving meaningful answers from observations. In this thesis I use a variety of numerical methods which include the semi-analytical methods (SAMs) and hydrodynamical simulations to study the evolution of dust in galaxies as well as its effect on the galaxy spectral energy distributions (SEDs). In the first section I use the L-Galaxies SAM to incorporate a self-consistent model of dust formation and evolution. A novel feature in this work compared to similar efforts that have been published for semi-analytic and hydro-dynamic models are (i) the more accurate consideration of the impact of molecular cloud chemistry on grain growth in dense molecular clouds (by separate tracking of dust in molecular and diffuse gas) and (ii) incorporating information on dust depletion fractions. I present the results of our implementation and compare it to the observational space. In the second section I introduce Flares (First Light And Reionisation Epoch Simulations), a suite of zoom simulations targeting a range of overdensities in the Epoch of Reionisation (EoR). The various overdensities were picked from a (3.2 cGpc)3 volume, giving us access to some of the large scale modes in the Universe, which are and will be probed by current and next generation surveys/telescopes. These region were re-simulated using the EAGLE simulation physics, a well tested model in the low-redshift Universe. Flares matches the stellar mass function and the star formation rate function of the current observations well. In the third section I show how we implement a simple line-of-sight (LOS) dust extinction model to retrieve the UV to near infrared SED including nebular emission from the Flare simulations in the EoR. I present the UV luminosity function, the UV continuum slope (ß) relations, the UV attenuation as well as the line luminosity and equivalent widths of some prominent nebular emission lines. The relative contribution of obscured and unobscured star formation is also explored, finding comparable contributions by z ~ 6. In the fourth section, I post-process the massive galaxies (= 109M) in Flares using the skirt radiative transfer code to study their dust properties such as the infrared luminosity function, the infrared excess - ß (IRX-ß) relation, various measures of luminosity-weighted dust temperatures. The Flares IRX-ß relation predominantly follows the local starburst relation. The luminosity-weighted dust temperatures increase towards higher redshifts, with the slope of the peak dust temperature - redshift relation showing a higher slope than the lower redshift relations obtained from previous observational and theoretical works.