The growth of mass in star-forming galaxies at high redshift

Abstract

This thesis presents an analysis of star-forming galaxies across the cosmic noon (z~1-4) spanning a wide range in physical properties, from strongly dust-obscured systems, to more typical 'main-sequence' galaxies. Consistent analysis and methodology, together with the large sample sizes, allow us to compare and contrast populations of different stellar, gas and dark matter masses to assess their evolutionary trends and probe the mass build up across the 'main-sequence'.

For consistent analysis throughout the thesis, we test and check the reliability of using an SED fitting code MAGPHYS (da Cunha et al. 2015) to derive photometric redshifts as well as key physical properties (such as stellar mass, far-infrared luminosity, star-formation rate) for high redshift star-forming galaxies in Chapter 2. Knowing the capabilities and shortcomings of the modelling code, in Chapters 3, 4 and 5, we model multi-wavelength photometric data to derive physical properties of galaxies across the 'main-sequence'.

In Chapter 3, we analyse the physical properties of a large, homogeneously selected sample of ALMA-located sub-millimetre galaxies (SMGs) from the AS2UDS survey (Stach et al. 2019), which identified 707 SMGs across the ~1deg^2 field. We determine a median redshift of z=2.61+-0.08, and the redshift distribution is well fit by a model combining evolution of the gas fraction in halos with the growth of halo mass past a critical threshold of M_h~6x10^12Mo, thus SMGs may represent the highly efficient collapse of gas-rich massive halos. We suggest that almost all galaxies with M_*>3x10^11Mo have passed through an SMG-like phase. SMGs are broadly consistent with simple homologous systems in the far-infrared, such as a centrally illuminated starburst. Overall, this study provides strong support for an evolutionary link between the active, gas-rich SMG population at z>1 and the formation of massive, bulge-dominated galaxies across the history of the Universe.

We analyse the physical properties of 121 sub-millimetre galaxies (SMGs) from the STUDIES 450-um survey in Chapter 4, and compare the results to 850-um-selected SMGs from Chapter 3 to understand the fundamental physical differences between the two populations at the observed depths. The 450-um sample has a median redshift of z=1.85+-0.12. The fainter 450-um-selected sources have higher space density than the brighter 850-um sample at z<2, and a lower space density at z>3, suggesting LIRGs are the main obscured population at z~1-2, while ULIRGs dominate at higher redshifts.
Using a uniform ~180um-selected sample at z=1-2 and z=3-4, we suggest that higher-redshift sources have higher dust densities due to smaller inferred dust continuum sizes at a given dust mass, leading to higher dust attenuation. We suggest that the dust content of galaxies is governed by a combination of both the variation of gas content and dust destruction timescale.

Finally, in Chapter 5 we present an on-going analysis of a sample of star-forming galaxies at z~1.5 from the KMOS Ultra-deep Rotational Velocity Survey (KURVS). We construct spatially-resolved 2D stellar mass maps from deep HST observations and use them to constrain the baryonic mass profile of the sample galaxies. Using these, together with the dynamical mass profiles from the deep H_alpha observations, we derive the dark matter profiles, which reveal high dark matter fractions (~0.8), even at effective radius.