The role of high-redshift star-forming regions in galaxy evolution

The peak of cosmic star formation occurs at redshift 1-3. Galaxies at this redshift are heavily star-forming, with irregular disk-like morphologies. These galaxies are dominated by starbursting ultraviolet emitting structures known as clumps. The formation mechanism and contribution of clumps to galactic evolution is still uncertain, but it is believed clumps can form either from mergers or gravitational instabilities in the disk and have survival timescales long enough that they could migrate to the galactic centre and contribute to bulge formation. Despite being commonly detected in star formation tracers, clumps remain elusive in molecular gas tracers. Between redshifts 4 and 6 there is a 10x increase in the cosmic star formation rate density. During this period, galaxies grow rapidly from low-mass, primordial galaxies towards the more mature types of massive galaxies that we see at cosmic noon. Growth via gas accretion and mergers undoubtedly shape this evolution; however, there is considerable uncertainty at present over the contribution of each of these processes to the overall mass-assembly of galaxies. There exist very few high-resolution observations of main sequence galaxies in molecular gas tracers. In this thesis, we combine molecular gas data with ancillary data (tracing star formation/stellar mass) to understand the morphologies and kinematics of these galaxies at these two epochs. The first part of the thesis focuses on cosmic noon. We have extensively studied the main sequence galaxy BX610 at z = 2.21, using ALMA observations of the CO(4-3), CO(7-6) and dust continuum at high resolution. Combining these ALMA observations with multi-wavelength ancillary data, we derive the galaxy and clump physical properties, such as molecular gas mass, stellar mass, dust obscuration and star formation rate. We find that the star-forming clumps in BX610 have no detectable molecular gas and very low dust extinctions. Additionally, we find evidence of non-star forming clumps in molecular gas tracers - some of the first observations in unlensed galaxies of gas clumps at this redshift. Using these observations, we suggest different mechanisms of formation of these clumps. In the second part of this thesis, we utilise new high-resolution ALMA [CII] observations to analyse three main sequence (MS) galaxy systems at a redshift of 4.5 and at resolutions of down to $0.15''$. This approach enables us to investigate the morphology and kinematics on a kpc scale and understand the processes at play as well as the classifications of galaxies at high resolution. Thanks to this unique window, we are able to gain insights into the molecular gas of MS galaxies undergoing mass assembly in the early Universe. In general, we find that the high-resolution ALMA data reveal more complex morpho-kinematic properties and more mergers than previously observed. One galaxy that was previously classified as dispersion-dominated turned out to show two bright [CII] emission regions, which could either be classified as merging galaxies or massive star-forming regions within the galaxy itself. The high-resolution data for the other dispersion-dominated object also revealed clumps of [CII] that had not been identified previously. For a previously identified merger in our sample we identified interaction-induced clumps, demonstrating the profound effect that mergers have on the molecular gas in galaxies, which is consistent with what has been suggested by recent simulations. Within the sample, we also detect star-formation-powered outflows (or outflows from active galactic nuclei) that appear to be fuelling diffuse gas regions and enriching the circumgalactic medium. We also detect an example of a compact rotating disk of only 1.1 kpc diameter, indicating that galaxies at this redshift can also have mature molecular gas disks.