School of Physics - Theses
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ItemStructure of dark matter in galaxies( 2004-12)The origin, nature and distribution of dark matter in the universe form some of the biggest questions in modern astrophysics. Dark matter is distributed on a wide range of scales in the universe. This thesis concentrates on galactic scales, attempting to lower the veil and probe the structure of dark matter in galaxies. (For complete abstract open document)
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ItemThe broad emission line region of quasars and gravitational lensing by early-type galaxies( 2012)This thesis has focused on predicting emission line flux ratios from the broad emission line region of quasars under different physical conditions, and measuring the dark matter fraction and total mass density slope within early-type galaxies using gravitational lensing. Quasars are the energetic cores of distant galaxies, and they reside in some of the oldest, most massive objects formed in the universe. Due to their incredible luminosity (as much as $10^5$ times greater than a typical galaxy), quasars can be observed at extremely large distances. Quasars have a unique spectrum, with bright, broad emission lines that are produced by photoionised gas that is close to the central super-massive black hole. Despite the prominence of these broad emission features, the gas physical conditions and the geometry of the emission region are poorly understood. Due to its small scale and large distance, the emission line region cannot be resolved directly — even with the most powerful telescopes — and simulations are required to understand the mechanism that produces the unique quasar spectrum. Using simulations of micro-physical processes, including photoionisation, the broad emission line flux ratios can be calculated for a range of gas densities and distances from the central black hole. Using the photoionisation code, Cloudy, hydrogen and helium line emission was over the range of possible broad emission line region conditions. The hydrogen and helium lines are of particular interest because the line emission has strong dependence on the gas number density and incident ionising flux, whilst having only a negligible dependence on several other free parameters of the model. These simulations were then used to find a set of interesting ratios that can be used to determine the limits on the upper limit on the gas number density, and outer radius of the emission region. This thesis demonstrates a new technique for determining the physical conditions of the broad line emitting gas in quasars, using optical and near-infrared hydrogen and helium emission lines. Near-infrared line ratios are advantageous, as they have a negligible dependence on the amount of internal dust. A locally optimally emitting cloud model of the broad emission line region was applied to four nearby (z $\sim$ 0.2) quasars from the Glikman et al. (2006) sample. By comparing simulated emission line ratios to measured ratios from optical and near-infrared spectroscopy, the physical conditions required to produce the observed emission lines were inferred. The model provides a good fit to three of the objects, and a fair fit to the fourth object. We find that low incident ionising fluxes ($phi <10^{18}$cm^-2 s^-1), and high gas densities (n>10^{12} cm^-3) are required to reproduce the observed line ratios. This analysis demonstrates that the use of composite spectra in photoionisation modelling is inappropriate; models must be fitted to the individual spectra of quasars. This thesis also derives properties of early-type galaxies using a joint gravitational lensing and stellar-dynamics analysis. The sample consists of 11 early-type galaxies from the Strong Lenses in the Legacy Survey (SL2S). The median deflector redshift is 0.5, making it the largest sample of intermediate redshift lenses that have been studied using a joint lensing and dynamics analysis. By combining measured redshifts and stellar velocity dispersions from Keck spectroscopy with lens models from Gavazzi et al. (2012, submitted), the total mass density slope inside the Einstein radius for each of the 11 lenses was derived. The average total density slope was found to be 2.16$\pm$0.9, with an intrinsic scatter of 0.25. The dark matter fraction for each lens within half the effective radius was also determined. The average projected dark matter mass fraction was found to be 0.42$\pm$0.08 with a scatter of 0.25 for a Salpeter initial mass function. By combining the SL2S results with those from previous studies, a mild trend in the cosmic evolution of the total mass density slope was found. This suggests that the total density profile of massive galaxies has become slightly steeper over cosmic time. If this result is confirmed by larger samples, it would indicate that either dissipative processes or off-axis major mergers play an important role in the growth of massive galaxies since a redshift of 1.
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ItemDark matter indirect detection and Bremsstrahlung Processes( 2011)It is now well established that some form of Dark Matter (DM) makes a sizeable contribution to the total matter-energy abundance of the Universe, yet DM still evades detection and its particle properties remain unknown. Indirect detection provides an important probe of some of these fundamental properties. DM self-annihilation throughout the Universe is expected to lead to an observable signal of standard model particles at Earth, and any observed flux of standard model particles from a particular region acts as an upper limit on the annihilation signal from that region. In Chapter 1, we give an introduction to our current knowledge of DM. We begin with the historic and recent evidence for the existence of DM based on its gravitational effects, before describing our current knowledge of DM formation history and abundance. We then describe and compare a number of competing DM density profiles for our galaxy, highlighting the large uncertainties towards the Galactic center. There are currently a large number of DM candidates, sometimes called the `Candidate Zoo'. We briefly introduce several of the most popular candidates, describing their history and motivation. We then move on to describe current searches for DM, focusing on indirect detection, which aims to detect DM via an observable flux of its SM annihilation products. We detail the major constraints on the DM self-annihilation cross section, and examine some potential signals from DM annihilation. We also describe constraints on DM from direct detection and collider searches. Finally, we introduce bremsstrahlung processes in the context of DM annihilation, where a particle such as a gamma-ray is radiated from one of the DM annihilation products at the Feynman diagram level. In Chapter 2, we use gamma-ray data from observations of the Milky Way, Andromeda (M31), and the cosmic background to calculate robust upper limits on the dark matter self-annihilation cross section to monoenergetic gamma rays, $\langle v\sigma \rangle_{\gamma \gamma}$, over a wide range of dark matter masses. We do this in a model-independent and conservative way, such that our results are valid across a broad spectrum of DM models and astrophysical assumptions. In fact, over most of this range, our results are unchanged if one considers just the branching ratio to gamma rays with energies within a factor of a few of the endpoint at the dark matter mass. If the final-state branching ratio to gamma rays, $Br(\gamma \gamma)$, were known, then $\langle v\sigma \rangle_{\gamma \gamma} / Br(\gamma \gamma)$ would define an upper limit on the {\it total} cross section. %we conservatively assume $Br(\gamma \gamma) \gtrsim 10^{-4}$. In Chapter 3, we take advantage of the fact that annihilation to charged leptons will inevitably be accompanied by gamma rays due to radiative corrections to place similar limits on the the annihilation cross section to an electron-positron pair, $\langle v\sigma \rangle_{e^+e^-}$. Photon bremsstrahlung from the final state particles occurs at the Feynman diagram level, yet the gamma-ray spectrum per annihilation is approximately model independent, such that our analysis applies to a broad class of DM models. We compare the expected annihilation signal with the observed gamma-ray flux from the Galactic Center, and place conservative upper limits on the annihilation rate to an electron-positron pair. We also constrain annihilation to muon and tau lepton pairs. We again make conservative choices in the uncertain dark matter density profiles, and note that our constraints would only be strengthened if the density were more tightly constrained. The spectrum per annihilation produces hard gamma rays near the kinematic cutoff, and we find that the constraints on $\langle v\sigma \rangle_{e^+e^-}$ are weaker than those on $\langle v\sigma \rangle_{\gamma \gamma}$ only by a factor of $\sim 10^{-2}$, as expected since the $2\rightarrow 3$ process is suppressed relative to the $2\rightarrow 2$ process. Annihilation to leptons will also be accompanied by massive gauge bosons due to electroweak radiative corrections. In Chapter 4 we examine a case where DM annihilates exclusively to neutrinos at the $2\rightarrow 2$ level, and gamma rays, leptons and hadrons will inevitably be produced due to electroweak bremsstrahlung. We explicitly calculate the ratio of the rate for the three electroweak bremsstrahlung modes $\chi\chi\rightarrow \nu\bar\nu Z,\, e^+ \nu W^-\, e^- \bar \nu W^+$ to the rate for the $2\rightarrow 2$ process $\chi\chi\rightarrow \nu\bar\nu$. Electroweak bremsstrahlung plays a larger role in the special case where the annihilation rate to leptonic modes suffers helicity suppression. While it has long been known that photon bremsstrahlung can lift the helicity suppression, we show in Chapter 5 that electroweak bremsstrahlung is also capable of lifting this suppression, such that the branching ratio to the 3-body electroweak bremsstrahlung final states can greatly exceed the branching ratio to an electron-positron or neutrino pair. We explicitly calculate the electroweak bremsstrahlung cross section in a typical leptophilic model. In Chapter 6 we examine observational signatures of dark matter annihilation in the Milky Way arising from these electroweak bremsstrahlung contributions to the annihilation cross section. Here we calculate the spectra of stable annihilation products produced via $\gamma/W$/$Z$-bremsstrahlung. After modifying the fluxes to account for the propagation through the Galaxy, we set upper bounds on the annihilation cross section via a comparison with observational data. We show that stringent cosmic ray antiproton limits preclude a sizable dark matter contribution to observed cosmic ray positron fluxes in the class of models for which the bremsstrahlung processes dominate.
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ItemQuasar microimaging( 2010)Observations of gravitationally microlensed quasars offer a unique opportunity to probe quasar structure on extremely small scales. In this thesis, we conduct extensive microlensing simulations and compare with observational data to constrain quasar accretion discs, and conduct preliminary probes of broad emission line region structure. This analysis is done using a new single-epoch imaging technique that requires very little telescope time, and yet produces results that are comparable to those obtained from long-term monitoring campaigns. We begin by exploring the impact of variable smooth matter percentage and source size on microlensing simulations. Adding a smooth matter component affects minimum and saddle point images differently, broadening the magnification distribution for the saddle point image significantly. However, increasing the radius of the background source washes out this difference. The observation of suppressed saddle point images can therefore only be explained by microlensing with a smooth matter component if the background source is sufficiently small. We use these simulations, in combination with I-band imaging of the lensed quasar MG 0414+0534 to constrain the radius of the quasar source. This demonstrates the viability of a single-epoch imaging method for constraining quasar structure. This technique is then expanded to single-epoch multi-band observations, in order to constrain the radial profile of quasar accretion discs as a function of observed wavelength. We present new Magellan observations of two gravitationally lensed quasars: MG 0414+0534 and SDSS J0924+0219. We also analyse two epochs of Q2237+0305 data obtained from the literature. Our results are compared with four fidicial accretion disc models. At the 95 per cent level, only SDSS J0924+0219 is inconsistent with any of the accretion disc models. When we combine the results from all three quasars -- a first step towards assembling a statistical sample -- we find that the two steepest accretion disc models are ruled out with 68 per cent confidence. In addition, we are also able to use our microlensing simulations to constrain the smooth matter percentages in the lenses at the image positions. In both MG 0414+0534 and SDSS J0924+0219 we find smooth matter percentages that are inconsistent with zero. A smooth matter percentage of approximately 50 per cent is preferred in MG 0414+0534, and approximately 80 per cent in SDSS J0924+0219. Q2237+0305 is usually assumed to have a smooth matter percentage of zero at the image positions, as they lie in the bulge of the lensing galaxy. Though our measurement is consistent with a zero smooth matter percentage, there is a peak in the probability distribution at a value 20 per cent. This is perhaps a hint of additional intervening structures along the line of sight to the background quasar. We test the sensitivity of our accretion disc constraints to a range of modelling parameters. These include errors in lens modelling, Bayesian prior probability selection, errors in observational data, and the microlens mass function. Constraints on the power-law index relating source radius to observed wavelength are found to be relatively unaffected by changes in the modelling parameters. Constraints on source radii are found to be more strongly affected. Finally, the broad emission line region of Q2237+0305 is examined. Gemini IFU observations are presented clearly showing differential microlensing across the velocity profile of the CIII] emission line. To analyse this signature, we present three simple broad emission line region models: a biconical outflow, a Keplerian disc, and spherical infall. A method is developed to compare the shapes of simulated flux ratio spectra with the observed spectrum. We are unable to discriminate between the biconical outflow and Keplerian disc models when we average over all viewing angles and orientations. The spherical infall model, however, does not fit the observed data. We also find that for the non-spherically symmetric geometries, low inclination angles are essentially incompatible with the observations. This analysis offers hope that with sufficiently high signal-to-noise observations, differential microlensing signatures may allow us to constrain the geometry and kinematics of this poorly understood portion of quasar structure.