School of Physics - Theses
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ItemThe fundamental plane and peculiar velocities from the 6dF galaxy survey( 2012)Early-type galaxies (ellipticals and lenticulars) are observed to populate the relation known as the Fundamental Plane that links their effective radius, R_e, stellar velocity dispersion, σ, and mean surface brightness, I_e. We have measured Fundamental Plane parameters in the near-infrared J, H and K passbands for ~10^4 of the brightest early-type galaxies in the 6dF Galaxy Survey (6dFGS). We improve upon previous regression techniques used to derive the Fundamental Plane by developing a robust maximum likelihood algorithm for fitting the galaxy distribution in Fundamental Plane space with a 3D Gaussian model. We exploit this large near-infrared-selected sample of galaxies to investigate trends in the Fundamental Plane with stellar population, morphology and environment. The 6dFGS galaxies exhibit clear stellar population trends in Fundamental Plane space, with age varying most strongly orthogonal to the plane. Remarkably, none of the stellar population parameters vary along the long axis of the plane, which corresponds to luminosity density. The Fundamental Plane slopes show little variation with either morphology or environment, but the Fundamental Plane size zeropoint is systematically larger for galaxies in lower density environments and for early-type spiral bulges. We speculate that age drives all the trends with residuals about the plane through its correlation with environment, morphology and metallicity. Using the Fundamental Plane, we measure distances and peculiar velocities for ~10^4 6dFGS galaxies to form the largest and most homogeneous peculiar velocity sample to date. Using a maximum-likelihood approach, we measure the overall bulk galaxy motions from the 6dFGS velocity field for the local volume of the universe, finding broad agreement with the predicted velocity field constructed from the 2MASS Redshift Survey. The local volume out to 16 120 km/s is found to have a bulk motion of 337 km/s in the direction (l,b) = (313°±9°,14°±10°), in good agreement with the results of other recent studies. A comparison of the observed and predicted fields is used to constrain parameters relating the distribution of galaxies and matter. We obtain a redshift-space distortion parameter β = 0.29±0.06 and a bias parameter for the 6dFGS velocity sample of b = 1.69±0.36. The 6dFGS velocity field provides an independent probe of cosmological parameters defining models of large-scale structure formation. Next steps include: (i) combining the 6dFGS sample in the south with the SDSS sample in the north to obtain an all-sky velocity field; (ii) deriving additional constraints on cosmological parameters from the velocity power spectrum analysis; and (iii) comparing the Fundamental Plane distances and velocities for early-type galaxies with the Tully-Fisher distances and velocities for spiral galaxies that will be obtained with the WALLABY survey on the Australian SKA Pathfinder.
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ItemStudying the epoch of hydrogen reionisation in redshifted 21-cm radiation( 2011)The measurement of the spatial distribution of neutral hydrogen in the high-redshift intergalactic medium, through its 21-cm hyperfine transition, will revolutionise our understanding of the period in the evolution of the early Universe known as the cosmic dark ages. This period began once hot ionised gas, formed in the Big Bang, combined to form neutral gas, and lasted until the first stars, galaxies and quasars reionised most of the neutral hydrogen sometime between 400 000 to 700 million years after the Big Bang. The epoch of reionisation, driven by these first sources of light, is arguably the least understood, but one of the most important, periods in the evolution of the Universe. This thesis is an attempt to explain and explore some of the techniques we may employ in order to advance our understanding of this period and prepare for the first observations to come from a new generation of low-frequency instruments. Using a new efficient semi-numerical ionisation model to create simulations of the ionisation state of the intergalactic medium during the epoch of reionisation, we analyse the impact a percolating intergalactic medium has on redshifted 21-cm observations of high-luminosity quasar-generated regions of ionised hydrogen. We also investigate how a population of quasars modifies the 21-cm power spectrum during this period. The study of the reionisation history of hydrogen and, indirectly, the first galaxies will be limited not by raw sensitivity to the signal, but rather, by bright foreground radiation from Galactic and extragalactic radio sources and the Galactic continuum. We analyse the effect of non-polarised foreground subtraction on redshifted 21-cm observations, and demonstrate a method for removing polarised foregrounds using Faraday tomography.
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ItemDetermining the characteristic mass of DLA host haloes from 21cm fluctuations( 2010)Absorption profiles are found in the observed spectra from quasars, and the most prominent of these are the Damped Lyman-alpha Absorbers (DLAs). They are caused by large collections of neutral hydrogen (HI) gas, which are thought to be housed in galaxies that lie along the line-of-sight to quasars. HI gas associated with DLAs contains most of the HI gas in the Universe during 2 < z < 5, and hence details about DLAs are important for understanding the history of star formation, as well as the formation and evolution of galaxies. Wyithe (2008) proposed a method of determining the characteristic mass of dark matter haloes that host DLAs. This involves generating an analytic power spectrum of the fluctuations in 21cm brightness temperature caused by the HI gas in the Universe. Calculating this analytic 21cm power spectrum requires a formalism for the HI mass weighted clustering bias of DLAs on both large and small scales. We include this DLA clustering bias by firstly generating an analytic galaxy power spectrum using the halo model of Peacock & Smith (2000), as well as including the occupation of haloes by galaxies -- using the Halo Occupation Distribution (HOD) weighting of Peacock (2003). This weighting is then adapted to account for the occupation of haloes by HI gas. We then fit the analytic 21cm power spectrum generated using this formalism to a simulated 21cm power spectrum, with the characteristic mass of DLA host haloes being used as a fitting parameter. The DLA host halo mass is in turn dependent upon two parameters in our model: the minimum mass of haloes M_{min} included in our formalism, and the HI weighting index alpha_{HI}. The neutral hydrogen fraction is another parameter, which we can choose to be the same as that from our simulation volume. If we also choose a value for alpha_{HI} that is motivated by analysis of the dark matter and HI gas content of the haloes in the simulation, then we are able to fit the 21cm power spectrum at both large and small scales, with an M_{min} that is the same or similar to the lowest mass in the simulation's halo catalogue. This in turn gives a similar value for the DLA host halo mass that is known to be the case in the simulation. This demonstrates the viability of the Wyithe (2008) method for determining the DLA host halo mass using observations of 21cm fluctuations. However, degeneracies in the free parameters of our analytic formalism would hinder an accurate determination of the DLA host halo mass from actual future observations. This is due to the fact that the real space, spherically averaged 21cm power spectrum is used throughout this thesis. However, extending our analytic formalism to the redshift space, angular-dependent 21cm power spectrum should be capable of breaking the degeneracy between DLA host halo mass and neutral hydrogen fraction.