The evolution of galaxy structure in 3D

: (Alternative Format Thesis)

Abstract

The evolutionary histories of galaxies leave an imprint on their structure. A full understanding of the physical mechanisms behind galaxy evolution relies on observing galaxy structure back to earlier cosmic times. In this thesis, we pair novel methodologies with unprecedented large observational samples of galaxies spanning a wide range in mass and redshift.

Following a description of the broader field and context within which this thesis work frames, we start by studying the intrinsic 3D shapes of quiescent galaxies (QGs) over the last half of cosmic history. We recover such intrinsic 3D shape distributions statistically based on their projected shapes seen from random viewing angles. We construct a sample of unprecedented size (478,677 QGs), exploiting multi-wavelength u-to-Ks photometry from the deep wide-area surveys KiDS + VIKING paired with high-quality i-band imaging from HSC-SSP. The dependences of the shapes on mass, redshift, photometric bulge prominence and environment are considered. For comparison, the intrinsic shapes of quenched galaxies in the IllustrisTNG simulation are analysed and contrasted with their formation history. We find that over the full 0 < z < 0.9 range, and in both simulations and observations, spheroidal 3D shapes become more abundant at M∗ > 1011M⊙, with the effect being most pronounced at lower redshifts. In the IllustrisTNG simulation, the most massive galaxies feature the highest ex-situ stellar mass fractions, pointing to violent relaxation via mergers as the mechanism responsible for their 3D shape transformation. Larger differences between observed and simulated shapes are found at low to intermediate masses. At any mass, the most spheroidal quiescent galaxies in TNG feature the highest bulge mass fractions, and, conversely, observed quiescent galaxies with the highest bulge-to-total ratios are found to be intrinsically the roundest. Finally, we detect an environmental influence on galaxy shape, at least at the highest masses, such that at fixed mass and redshift, quiescent galaxies tend to be rounder in denser environments.

The second half of the thesis focuses on the star-forming galaxies (SFGs), with a similar 3D-intrinsic shape modelling framework but further considering the effect of dust attenuation. Previous studies are often limited by focusing either exclusively on the structure of galaxies with disregard of dust effects, or by analyzing tracers of their ISM (dust attenuation or dust mass) without tying in constraints on galaxy size, shape or inclination. We analyse the joint distribution of dust attenuation and projected axial ratios, together with galaxy sizes and surface brightness profiles, to infer lessons on the dust content and star/dust geometry within SFGs. To do so, we make use of large observational datasets from KiDS+VIKING+HSC-SSP and extend the analysis out to redshift z = 2.5 using the HST surveys CANDELS and 3D-DASH (439,965 SFGs). We construct suites of SKIRT radiative transfer models for idealized galaxies observed under random viewing angles with the aim of reproducing the aforementioned distributions, including the level and inclination dependence of dust attenuation. We find that attenuation-based dust mass estimates are at odds with constraints from farinfrared observations, especially at higher redshifts, when assuming smooth star and dust geometries of equal extent. We demonstrate that UV-to-NIR and far-infrared constraints can be reconciled by invoking clumpier dust geometries for galaxies at higher redshifts and/or very compact dust cores. We discuss implications for the significant wavelength- and redshift-dependent differences between half-light and half-mass radii that result from spatially varying dust columns within -especially massive- SFGs. This work has important implications for and synergies with ongoing observational studies using the James Webb Space Telescope (JWST) as well as (sub)mm interferometers such as ALMA and NOEMA.

A discussion of questions prompted by the investigation in this thesis and prospects offered by state-of-the-art and upcoming facilities is presented in the final, outlook chapter of the thesis.