| Izvleček: | <p>(Ultra) Luminous infrared galaxies (U/LIRGs) are amongst the most powerful star forming galaxies in the universe. They play a significant role in galaxy evolution and dominate the star formation rate density at the peak of cosmic star formation, at z ∼ 2. Therefore, understanding the properties of luminous infrared galaxies, and how these properties evolve through cosmic time, is necessary for testing our best theories of galaxy evolution, and for pushing the current understanding of cosmic star formation.</p>
<p>In this thesis, I show U/LIRGs at z ∼ 2 have underlying mechanisms driving their huge star formation rates (SFR) that are consistent with the high redshift normal star forming population, as opposed to the merger driven star formation seen in local U/LIRGs. I show that the general properties of the U/LIRG population have changed over the past 10 Gyr and that local U/LIRGs are not necessarily analogues for high-z luminous IR galaxies. Major mergers do not appear to be a necessary condition to drive the high observed SFR, as the properties of my sample of U/LIRGs are consistent with galaxies undergoing steady state star formation.</p>
<p>I explore a unique sample of U/LIRGs at z ∼ 0.4 that form a subpopulation that straddles the region between star forming galaxies and starbursts at this epoch. This sample shows a continuous change in star formation efficiency (SFE) between these two populations, as opposed to the suggested bi-modality in SFE between normal star forming galaxies and starbursts. It seems that interactions are responsible for the boost in the SFE, and it is the SFE, as opposed to the gas fraction, that may be driving a galaxy’s transition towards the starburst regime. Intriguingly, there is a possible hidden merger in one of the targets that was previously thought to be isolated based on rest frame optical observations, and also a potential CO dark ULIRG which would be a very unusual interstellar medium (ISM) for a luminous IR galaxy. These examples show how multi-wavelength observations are vital to fully understand a galaxy.</p>
<p>I use a new post-processing pipeline to make mock observations of a typical star forming galaxy at z ∼ 2. This pipeline adds mock spectra to hydrodynamical cosmological simulations, and I use the output from this pipeline to make mock High Angular Resolution Monolithic Optical and Near-infrared Integral field spectrograph (HARMONI) observations of the Hα emission line. I confirm the intrinsic physical properties of the simulated galaxy are correctly encoded into its spectrum. It is then shown how the presence of moderate dust doesn’t affect the analysis of the mock observations, which is vital as U/LIRGs are ubiquitous in the high-z universe.</p>
<p>Having shown the post-processing pipeline correctly captures the underlying properties of a galaxy, I investigate how the observational signatures change as a galaxy progresses through a minor merger, and whether these observables can accurately predict a galaxy’s stage of interaction. I show that a galaxy’s position on the main sequence is not always a reliable indicator of a merging versus non-merging galaxy. Neither is the ratio of rotational velocity to velocity dispersion, although the velocity dispersion does increase throughout the minor merger. I show that the SFE, as opposed to gas fraction, is driving the boost in SFR as the minor merger progresses. The morphology indicators are affected by dust, but are still useful to categorize the galaxy correctly under moderate dust attenuation. I conclude that high spatial resolution HARMONI observations may uncover minor mergers at cosmic noon that can be missed by seeing limited observations.</p>
<p>This thesis helps disentangle what drives star formation in high redshift luminous infrared galaxies, explores how the SFR is linked to the global properties of galaxies, shows how these properties evolve over time and probes how accurately observations capture the intrinsic properties of a galaxy.</p>
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