The Physical Processes that Drive Galaxy Evolution - from Massive Galaxies to the Dwarf Regime
The study of galaxy formation and evolution is a cornerstone in astrophysics, as galaxies connect together all scales of the Universe. The physical processes that govern galaxies therefore needs to be fully understood if we are to understand how our Universe works. The various morphologies of galaxies we see today indicate that there are multiple processes at play, that act both on and within galaxies. In our currently accepted paradigm, galaxies are expected to assemble mass through merging and gas accretion, therefore these are critical processes that must be understood across the range of galaxy masses. We can now combine observational data from wide-are surveys with state of the art cosmological simulations to study how these processes drive galaxy evolution across cosmic time. In this thesis we have used cosmological simulations (Horizon-AGN and NewHorizon) to study how physical processes shape galaxies. Firstly, we explore an unusual population of extremely massive (M∗ >1011M0) spheroids, in the Horizon-AGN simulation, which exhibit anomalously low ex-situ mass fractions. This indicates that they form without recourse to significant merging, contrary to the common belief. These systems form in a single minor-merger event (with typical merger mass ratios of 0.11 - 0.33), where the satellite orbit is virtually co-planar with the disc of the massive galaxy. The merger triggers a catastrophic change in morphology, over only a few hundred Myrs, coupled with strong in-situ star formation. While this channel produces a minority (∼5 per cent) of such galaxies, our study demonstrates that the formation of at least some of the most massive spheroids need not involve major mergers - or any significant merging at all - contrary to what is classically believed. We also look at how extremely massive disc galaxies form in both simulations and in the nearby Universe. We show that extremely massive (M∗ > 1011.4 M0) discs are created via two channels. In the primary channel (accounting for 70% of these systems and 8% of massive galaxies) the most recent, significant merger (mass ratio > 1:10) between a massive spheroid and a gas-rich satellite ‘spins up’ the spheroid by creating a new rotational stellar component, leaving a massive disc as the remnant. In the secondary channel (accounting for 30% of these systems and 3% of massive galaxies), a system maintains a disc throughout its lifetime, due to an anomalously quiet merger history. These massive discs have similar black-hole masses and accretion rates to massive spheroids, providing a natural explanation for why some powerful AGN are surprisingly found in disc galaxies. In an observational follow-up we use UV-optical and HI data of massive galaxies, from the SDSS, GALEX, DECaLS and ALFALFA surveys to test the predictions from Horizon-AGN. Observed massive discs form ∼13 per cent of the population, in good agreement with the simulation (∼11 per cent). ∼64 per cent of massive discs exhibit tidal features indicative of recent minor mergers in the deep DECaLS images. The incidence of these features is at least four times higher than in low-mass discs, indicating that minor mergers play a significant role in the formation of these systems. The empirical star-formation rates agree well with Horizon-AGN’s predictions and, for a small galaxy sample with HI detections, the HI masses/fractions are consistent with the range predicted. The good agreement between theory and observations indicates that extremely massive discs are indeed remnants of recent minor mergers between spheroids and gas-rich satellites. We also look into the origins of dwarf low surface brightness galaxies (LSBGs), which are expected to dominate the galaxy number density. Using NewHorizon, a high-resolution cosmo- logical simulation, we study the origin of LSBGs and explain why, at similar stellar mass, they exhibit a large observed spread in surface brightness. NewHorizon galaxies populate a well- defined locus in the surface brightness - stellar mass plane, with a spread of ∼3 mag arcsec−2, in agreement with deep SDSS Stripe data. We find that galaxies with fainter surface brightnesses today are born in regions of higher dark-matter density. This results in faster gas accretion and more intense star formation at early epochs. The stronger resultant supernova feedback flattens gas profiles at a faster rate which, in turn, creates shallower stellar profiles (i.e. more diffuse systems) more rapidly. A small minority of dwarfs depart from the main locus towards high surface brightnesses, making them detectable in past wide surveys (e.g. standard-depth SDSS images). These systems have anomalously high star-formation rates, triggered by recent, fly-by or merger-driven starbursts. We note that objects considered extreme/anomalous at the depth of current datasets, e.g. ‘ultra-diffuse galaxies’, actually dominate the predicted dwarf population and will be routinely visible in future surveys like LSST. Finally, we look at a population of galaxies that go against our current theoretical paradigm. Recent observations have found local dwarfs with extremely low dark matter content, potentially bringing the validity of the standard model into question. We use NewHorizon to demonstrate that sustained stripping of dark matter, in tidal interactions between a massive galaxy and a dwarf satellite, naturally produces dwarfs that are dark-matter-deficient. The degree of stripping is driven by the closeness of the orbit of the dwarf around its massive companion, and produces dwarfs with halo-to-stellar mass ratios consistent with the findings of recent observational studies. ∼30 per cent of dwarfs show some deviation from normal dark matter fractions due to dark matter stripping, with 10 per cent showing high levels of dark matter deficiency (Mhalo/M*<10). The creation of these galaxies is, therefore, a natural by-product of galaxy evolution and their existence is not in tension with the standard paradigm. This thesis examines galaxy populations that appear to go against the currently accepted paradigm, in both the massive and dwarf galaxy regime. We show that galaxy evolution is a very complicated problem and that it is possible to form different galaxy populations through many processes. By using a cosmological simulation based on ACDM we also show that the existence of these populations do not pose a challenge to this model and are, in fact, readily explained by it.
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