The Structural Evolution of Galaxies at z < 7

This thesis utilises all five fields of the CANDELS survey to probe galaxy evolution out to redshift z ∼ 7 in the context of size, surface brightness, and structure. Both the optical and ultraviolet (UV) rest-frames are used depending on the context. We present a size analysis of a sample of ∼ 49,000 galaxies from the CANDELS GOODS-North and GOODS-South fields using redshift-independent relative surface brightness metrics to determine an unbiased measure of the differential size evolution of galaxies at 1 3) images that makes use of the Lyman break at 912 Å, in what we call `2D Lyman-break Imaging'. The images used are in the rest-frame optical at z 3. We separately consider a mass-selected sample (with masses in the range 10^9Mסּ < M* < 10^{10.5}Mסּ) and a number density-selected sample (using a constant number density of n = 1 x 10-4 Mpc-3). We use the redshift-independent Petrosian radius as a proxy for size. We use different definitions of the Petrosian radius to measure the inner and outer radii to determine where size change occurs within a galaxy. The evolution of the measured size can be described by a power-law of the form RPetr = α(1+z) kpc where β < 0. We find that the outer radius increases more rapidly, suggesting that as a galaxy grows, mass is added to its outer regions via an inside-out growth mechanism. This growth is stronger for the number density selected sample, with the outer radii changing by a factor of ∼ 3.4 compared to ∼ 2.6 for the inner radii. We test and confirm these results using a series of image simulations. Next, we investigate the rest-frame UV surface brightness (SB) evolution of galaxies up to z ∼ 6 using a variety of deep Hubble Space Telescope (HST) imaging. We find a very strong evolution in the intrinsic SB distribution with a decrease of 4-5 mag arcsec-2 between z = 6 to z = 1. This change is much larger than expected in terms of the evolution in UV luminosity, sizes or dust extinction and we demonstrate that this evolution is `unnatural' and due to selection biases. We also find no strong correlation between mass and UV SB and thus, deep HST imaging is unable to discover all of the most massive galaxies in the distant universe. Through simulations we show that only ∼ 16% of galaxies that we can detect at z = 2 would be detected at high-z. We furthermore explore possible origins of high SB galaxies at high-z by investigating the relationship between intrinsic SB and star formation rates. We conclude that ultra-high SB galaxies are produced by very gas rich dense galaxies which are in a unique phase of evolution, possibly produced by mergers. Analogues of such galaxies do not exist in the relatively nearby universe. Finally, we explore the quantitative non-parametric structural evolution of ∼ 16,000 galaxies up to z ∼ 3 in all five CANDELS fields. A fundamental feature of galaxies is their structure and how this evolves, yet we are just now deriving this evolution in quantitative ways. Our goal is to investigate how the structure changes with time with a focus on how the concentration and asymmetry of light evolve in the rest-frame optical. To interpret our galaxy structure measurements, we also run and analyse 300 simulation realisations from IllustrisTNG to determine the timescale of mergers for the CAS system. We measure that from z = 0-3, the median asymmetry merger timescale is 0.56^{+0.23}_{-0.18} Gyr, and find it does not vary with redshift. Using these data, we find that galaxies become progressively asymmetric at a given mass at higher redshifts and we derive merger rates which scale as ∼ (1+z)^{1.87\pm0.04}$ Gyr$^{-1}, which agrees well with recent machine learning and galaxy pair approaches, removing previous inconsistencies. We also show that far-infrared selected galaxies that are invisible to HST have a negligible effect on our measurements. Furthermore, we find that galaxies are more concentrated at higher redshifts. This is interpreted as a sign of how their formation occurs from a smaller initial galaxy that later grows into a larger one through mergers, consistent with the size growth of galaxies from 'inside-out', suggesting that the centres are the oldest parts of most galaxies