School of Physics - Theses
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ItemDark matter halos in the early Universe( 2016)We use high resolution N-Body simulations to study the properties of dark matter halos during the Epoch of Reionization. The halo concentration and spin parameters are measured in the mass range 10^8Msun/ h < M < 10^11M sun/h and redshifts 55 concentration-mass (c(M)) relation that is almost flat and well described by a simple power-law for both NFW and Einasto fits. The equilibrium state of the halo has a significant effect on the resulting concentrations. We also measure the spin distribution and spin mass relation, which has a weak dependence on equilibrium state. The spin virial mass relation has a mild negative correlation at high redshift. The correlation between the local density (the environment) of a halo and its formation history is examined. There is very little correlation between the formation time of a halo with local density, but some correlation between environment and the number of mergers the halo has experienced since formation.
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ItemThe accretion history of dark matter halos( 2016)The goal of this thesis is to (i) explore the physics that drives universal accretion history of dark matter halos; (ii) determine the relation between the halos accretion history and the halos internal structure; and (iii) disentangle the impact of halos accretion history on galaxy evolution. To address these topics, we first use the extended Press-Schechter (EPS) formalism to derive the halo mass accretion history (MAH) from the growth rate of initial density perturbations. We show that the halo MAH can be well described by an exponential function of redshift in the high-redshift regime. However, in the low-redshift regime the mass history growth slows down because the growth of density perturbations is halted in the dark energy dominated era due to the accelerated expansion of the Universe. As a result, in the low-redshift regime the halo MAH can be described by a power-law function of redshift. We complement this study with the analysis of MAHs of dark matter halos using a suite of cosmological simulations. We explore the relation between the density profile of dark matter halos and their MAHs, and confirm that the formation time, defined as the time when the virial mass of the main progenitor equals the mass enclosed within the scale radius, correlates strongly with concentration. We combine both analysis, analytic and numerical, to show that the halo MAH is the link between halo concentration and the initial density perturbation field. The connection found between the halo MAH and its density profile reached in these studies was vital to derive a semi-analytic, physically motivated model for dark matter halo concentration as a function of halo mass and redshift. Because the semi-analytic model is based on EPS theory, it can be applied to wide ranges in mass, redshift and cosmology. The resulting concentration-mass (c-M) relations are found to agree with the simulations, and because the model applies only to relaxed halos, they do not exhibit the upturn at high masses or high redshifts found by some recent works. We predict a change of slope in the z~0 c-M relation at a mass scale of 10^11 solar masses. We find that this is due to the change in the functional form of the halo MAH, which goes from being dominated by an exponential (for high-mass halos) to a power-law (for low-mass halos). During the latter phase, the core radius remains approximately constant, and the concentration grows due to the drop of the background density. We then connect the evolution of dark matter halos to the evolution of galaxies. We investigate the hot hydrostatic halo formation and its dependence on feedback mechanisms. We find that in the presence of energy sources like stellar feedback, the hot halo mass increases and the mass scale of hot halo formation is reduced. Active galactic nuclei (AGN) do not affect the hot halo as strongly. We develop a semi-analytic approach that makes use of both, the hot halo mass and the fraction of shock-heated gas, to calculate a `critical mass scale' for hot halo formation. We find that this mass scale, where the heating rate produced by accretion shocks equals cooling, is the point in mass above which halos develop a stable hot atmosphere. In the redshift range z=0-4, the critical mass is 10^11.7 solar masses, but it then increases for increasing redshift, in very good agreement with our numerical results. Finally, we investigate the physics that drives the gas accretion rate onto galaxies at the center of dark matter halos. We separately analyze the gas accretion rate onto the interstellar medium (ISM) and onto the galaxy. We find that the accretion rate onto the ISM remains roughly constant in halos larger than 10^11.7 solar masses, whereas the accretion rate onto the galaxy increases with increasing halo mass and flattens in the halo mass range 10^11.7-10^12.7 solar masses, and at redshifts z<2. The flattening is produced by the presence of the hot halo atmosphere that acts as a preventive feedback mechanism. We derive a physically motivated model of gas accretion onto galaxies that accurately reproduces the gas accretion rates from simulations. The model depends on the rate of gas cooling from the hot halo, on the fraction of shock-heated gas, and on the rate of cold gas accretion. We show that the rate of gas cooling from the hot halo is driven by the cooling radius, that it does not continuously decrease with increasing halo mass as generally thought. Instead, it decreases in the halo mass range 10^11.5-10^13 solar masses, and then increases with increasing halo mass, meaning that high-mass halos develop a hot halo cooling flow. We find that the upturn in the cooling radius is due to the change in the gas density profile, which is characterized by an evolving radial slope with halo mass.
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ItemLyα emitters as a probe of galaxy formation and ionisation history( 2016)Current observations suggest that the reionisation of hydrogen in the intergalactic medium had begun by z ∼ 10 and was completed around z ∼ 6. Directly observing this epoch is not possible with existing instrumentation, making it difficult to infer how the increased ionising background during this period affected galaxy formation. This thesis aims to put constraints on the galaxy formation history of the Universe with existing instruments, by modelling and observing the number densities of observed Lyα emitters in the ionised environments around z ∼ 2 − 3 quasars to mimic conditions found during the epoch of reionisation. The main work presented is a model for the ionisation state of the intergalactic medium around star forming galaxies in the vicinity of a luminous quasar, tuned by empirical relationships from conditions at z ∼ 2 − 3. This model suggests that the intense ionising radiation from a quasar offsets the increased density of the intergalactic medium found around it, implying that the direct detection of star forming galaxies by their Lyα emission in the vicinity of z ∼ 2 − 3 quasars is less obstructed by the intergalactic medium than galaxies in the field. The accuracy of this model is compared to existing Lyα galaxy surveys and found to be in good agreement. Discrepancies exist between the expected number of Lyα emitting galaxies this model predicts and the surveyed region around the super-luminous quasar PKS 0424-131, in which no Lyα emission was detected. The modelling done suggests that in order to be consistent with this null detection at the 68% (90%) level, galaxies below 2.5×10^12 M⊙ (4.2×10^12 M⊙) must be omitted. These results suggest that considerable radiative suppression of galaxy formation by PKS 0424-131 is taking place. This hypothesis is tested using observations made on the Baade telescope at the Las Campanas Observatory with the Maryland Magellan Tunable Filter. The unique suitability of tunable filters for the detection of high-redshift galactic Lyα emission is described in detail, along with their idiosyncratic calibration and data reduction processes. The adverse seeing conditions make it impossible to put limits on the impact of ionising radiation of galaxy formation using these observations, and an analysis of the factors that prevented detection is provided. Finally, suggestions are made for ways to improve the chance of success for future observations of this effect using tunable filters, as well as ways to remove spurious ghost reflections in the data analysis that are unique to tunable filter observations.