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Title: The relationship between galaxies and their dark matter haloes over cosmic time
Author: Hatfield, Peter
ISNI:       0000 0004 6499 5174
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2017
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In this thesis I study and measure the spatial distribution of galaxies selected in optical and near-infrared surveys over cosmic time. By measuring the clustering of these sources, valuable insight can be gained into the role of environment in shaping galaxy evolution over the history of the Universe. I present a series of results from a clustering analysis of the first data release of the Visible and Infrared Survey Telescope for Astronomy (VISTA) Deep Extragalactic Observations (VIDEO) survey. VIDEO is the only survey currently capable of probing the bulk of stellar mass in galaxies at redshifts corresponding to the peak of star formation on degree scales. Galaxy clustering is measured with the two-point correlation function, which is calculated using a non para- metric kernel based density estimator. I use my measurements to investigate the connection between the galaxies and the host dark matter halo using a Halo Occupation Distribution (HOD) methodology, deriving bias, satellite fractions, and typical host halo masses for stellar masses between 109.35M and 1010.85M, at redshifts 0.5 < z < 1.7. I show that the typical halo mass increases with stellar mass (with moderate scatter) and bias also increases with stellar mass and redshift, consistent with previous studies. I find the satellite fraction increases towards low redshifts, from ∼ 5% at z ∼ 1.5, to ∼ 20% at z ∼ 0.6, also increasing for lower mass galaxies. I combine my results to derive the stellar mass to halo mass ratio for both satellites and centrals over a range of halo masses and find the peak corresponding to the halo mass with maximum star formation efficiency to be ∼ 2 × 1012M, finding no evidence for evolution. It has long been known that environment has a large effect on star formation in galaxies. There are several known plausible mechanisms to remove the cool gas needed for star formation, such as strangulation, harassment and ram-pressure stripping. It is unclear which process is dominant, and over what range of stellar mass. In this thesis, I find evidence for suppression of the cross-correlation function between massive galaxies and less massive star-forming galaxies, giving a measure of how less likely a galaxy is to be star-forming in the vicinity of a more massive galaxy. I develop a formalism for modelling environmental quenching mechanisms within the HOD formalism. I find that at z ∼ 2 environment is not a significant factor in determining quenching of star-forming galaxies, and that galaxies are quenched with similar probabilities in group environments as they are globally. However, by z ∼ 0.5 galaxies are much less likely to be star forming when in a group environment than when not. This increased probability of being quenched does not appear to have significant radial dependence within the halo, supportive of the quenching being caused by the halting of fresh inflows of pristine gas, as opposed to by tidal stripping. Furthermore, by separating the massive sample into passive and star-forming, I find that this effect is further enhanced when the central galaxy is passive, a manifestation of galactic conformity. Hydrodynamical cosmological simulations, with advances in computing power over the last decade, have recently made great advances in reproducing the galaxy population and understanding the underlying physical processes behind galaxy evolution. There is extensive research in the literature comparing predicted stellar mass functions from hydrodynamical simulations to observed stellar mass functions in data. In this thesis I extend these results, comparing clustering of galaxies in mock catalogues from the hydrodynamical cosmological simulation Horizon-AGN to clustering measurements from the VIDEO observations. Clustering and HOD modelling in the Horizon-AGN mock catalogue qualitatively recreates clustering measurements from the VIDEO data, but reflects the known excess stellar mass to halo mass ratio for low mass haloes in Horizon-AGN. This reinforces the need for stronger regulation of star formation in low mass haloes in the simulation. I extend my results into the high redshift regime by studying the large-scale structure of the bright high-redshift Lyman-break galaxy (LBG) population - gaining insight into the role of environment in galaxy formation physics in the early Universe. I measure the clustering of a sample of bright (−22.7 < MUV < −21.125) LBGs at z ∼ 6 and use a HOD model to measure their typical halo masses. I find that the clustering amplitude and corresponding HOD fits sug- gests that these sources are highly biased (b ∼ 10) objects in the densest regions of the high-redshift Universe. Coupled with the observed rapid evolution of the number density of these objects, my results suggest that the shape of high lu- minosity end of the luminosity function is related to feedback processes or the onset of dust obscuration - as opposed to a scenario where these sources are pre- dominantly rare instances of the much more numerous MUV ∼ −19 population of galaxies caught in a particularly vigorous period of star formation. Despite investigating several variations on the model, it was not possible to simultaneously fit both the number densities and clustering measurements. I interpret this as a signal that a refinement of the model halo bias relation at high redshifts or the incorporation of quasi-linear effects may be needed for future attempts at modelling the clustering and number counts. Finally, the difference in number density between the fields (UltraVISTA has a surface density ∼ 1.8 times greater than UDS) is shown to be consistent with the cosmic variance implied by the clustering measurements. Finally I discuss future data sets that will become available in the coming years, and future approaches to modelling large-scale structure. In summary I have shown that measuring the spatial distribution of galaxies on large-scales is a vital probe of galaxy evolution and an essential tool for understanding the connection between galaxies and their dark matter haloes over cosmic time.
Supervisor: Jarvis, Matt Sponsor: Science Technology Facilities Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Cosmology ; Astrophysics ; Near infrared astronomy ; Surveys ; Galaxies ; Large-scale structure