Some of the most fundamental measurements we can make of the Universe are where and when stars formed in galaxies. In recent years, astronomers have converged on a picture in which the star formation rate density of the Universe peaks at approximately redshift (z) 2, when the Universe is around a quarter of its present age. There, star-forming galaxies harbour large reservoirs of molecular gas, assemble stellar mass rapidly, and typically display disordered morphologies.
In this thesis, I study the evolution of galaxies on large and small scales, with a particular focus on the epoch around the peak of cosmic star formation. My overarching aim is to understand the physical processes that drive and quench star formation in galaxies over cosmic time. In the first half of this thesis, I focus on global measurements of star formation, using the High-z Emission line survey (HiZELS), a deep, near-infrared narrow-band survey, which identifies star-forming galaxies at z=0.8-2.2. I characterise the dark matter halo environments of these galaxies via a clustering analysis, along with a Halo Occupation Distribution model fitting procedure, then study the relationships between host dark matter halo environment and galaxy properties.
I show that the clustering strength and the host dark matter halo masses of the HiZELS galaxies increase linearly with H-alpha luminosity (and, by implication, star formation rate) at all three redshifts. The typical galaxies in our samples are star-forming centrals, residing in dark matter haloes of mass ~10^12M_sol. I find a remarkably tight redshift-independent relation between the H-alpha luminosity scaled by the characteristic luminosity, L_H-alpha/L_H-alpha*(z), and the host dark matter halo mass of central galaxies. Simple analytic modelling suggests that this is consistent with a model in which the dark matter halo environment is a strong driver of galaxy star formation rate and therefore of the evolution of the star formation rate density in the Universe. I investigate this further by distinguishing the stellar mass and star formation rate dependencies of the clustering of HiZELS galaxies. I compare my observational results to the predictions of a pioneering cosmological hydrodynamical simulation, the Virgo Consortium's Evolution and Assembly of GaLaxies and their Environments project, known as EAGLE.
In the subsequent chapters of this thesis, I focus more heavily on simulations of galaxy formation, which are important tools for constraining and understanding the physics at play in galaxies. I use EAGLE to investigate the quenching of star formation in simulated galaxies via a novel application of Principal Component Analysis. I show that the key relations between halo mass, stellar mass and star formation rate are in good agreement with observed low-redshift galaxies.
Having studied the global properties of star-forming galaxies, I then turn to smaller scales, investigating what we can learn from spatially-resolved imaging. I present a detailed study of the spatially-resolved dust continuum emission of realistic simulated high-redshift galaxies. These galaxies, drawn from the FIRE-2 simulations, reach Milky Way masses by z~2. Post-processing them using radiative transfer techniques, I obtain predictions for the full rest-frame far-ultraviolet to far-infrared Spectral Energy Distributions of these simulated galaxies, as well as maps of their emission across the wavelength spectrum. As has been observed in distant galaxies, the rest-frame far-infrared emission of the simulated galaxies is compact, spanning half-light radii of ~0.5-4kpc. The derived morphologies of simulated galaxies are notably different in different wavebands; a galaxy can appear clumpy and extended in the far-ultraviolet yet compact at far-infrared wavelengths.
Finally, I perform a multi-wavelength study of a single observed galaxy, SHiZELS-14 (z=2.24), drawn from the HiZELS survey and subsequently imaged at 0.15'' resolution at multiple wavelengths. The data comprise kpc-resolution imaging in three different widely used tracers of star formation: the H-alpha emission line (from SINFONI/VLT), rest-frame far-ultraviolet continuum (from HST F606W imaging), and the rest-frame far-infrared (from ALMA), as well as the rest-frame optical (from HST F140W imaging). SHiZELS-14 displays a compact, dusty centre, as well as extended emission in both H-alpha and the rest-frame FIR. The ultraviolet emission is spatially offset from the extended dust emission, and appears to trace holes in the dust distribution. I find that the dust attenuation varies across the spatial extent of the galaxy, reaching up to ~5 magnitudes of extinction at H-alpha wavelengths in the most dusty regions. Global star formation rates inferred using standard calibrations to the different tracers vary from ~10-1000M_sol, and are particularly discrepant in the galaxy's dusty centre. This galaxy highlights the biased view of galaxy evolution provided by short-wavelength data in the absence of long-wavelength data, and is in line with my simulations
