Dissertations / Theses on the topic 'Intergalactic medium; Cosmology'

Clues about the timing of reionization and the nature of the ionizing sources responsible are imprinted in the ionization and thermal state of the IGM. In this thesis, I use high-resolution quasar spectra in conjunction with state-of-the-art hydrodynamical simulations to probe the IGM at high redshift, focusing on the ionization and thermal state of the gas. After reionization, the ionization state of the IGM is set by the intensity of the ultraviolet background(UVB), quantified by the hydrogen photoionization rate, Γ_bkg. At high redshifts this has been estimated by measuring the mean flux in the Lyα forest, and scaling Γ_bkg in simulations such that the simulated mean flux matches the observed value. In Chapter 3 I investigate whether the precision of these estimates can be improved by using the entire flux probability distribution function (PDF) instead of only the mean flux. Although I find it cannot improve the precision directly, the flux PDF can potentially be used to constrain other sources of error in observational estimates of Γ_bkg, and so may increase the precision indirectly. The ionizing output of a quasar will locally dominate over the UVB, and this leads to enhanced transmission bluewards of the quasar Lyα line, known as the proximity effect. In Chapter 4 I present the first measurements of Γ_bkg at z > 5 from the proximity effect. The UVB intensity declines smoothly with redshift over 4.6 < z < 6.4, implying a smooth evolution in the mean free path of ionizing photons. This suggests that reionization ends at z > 6.4. There is a drop in Γ_bkg by roughly a factor of five, which corresponds to a drop in the ionizing emissivity by about a factor of two. Such a redshift evolution in the emissivity cannot continue to much higher redshift without reionization failing to complete, which suggests that reionization cannot have ended much higher than z = 6.4. Estimates of Γ_bkg from the proximity effect and the mean flux are generally discrepant at z ~ 2-4, with those from the proximity effect systematically higher. This is generally attributed to effects of the quasar environment. I investigate the significance of several environmental biases on proximity effect measurements at z ~ 5-6 in Chapter 5. The biases are found to be small, and so the proximity effect is expected to give relatively unbiased estimates of Γ_bkg at z > 5, in contrast to lower redshifts. Photoionization heats the gas in the IGM, and so the thermal history of the IGM provides important constraints on reionization. The thermal state of the IGM is reflected in the level of small-scale structure in the Lyα forest. In Chapter 6 I quantify the small-scale structure using two independent statistics, the curvature and the peakiness, and convert these into a temperature by comparing with simulations. These are the first measurements of the temperature in the general IGM at z > 5. Both statistics show an increase in the temperature by a factor of roughly two from z = 4.4 to 5.6. This rise is sensitive, however, to any smoothing of the gas density distribution due to the thermal history spanning reionization. I find that this should only be a small effect, as otherwise the corrected temperatures at z ~ 4-5 are implausibly low. The temperature evolution therefore suggests a late reionization. The temperatures at z ≥ 4.8 are well fit by an adiabatic cooling curve, for which reasonable peak temperatures at the end of reionization are reached at 6 ≲ z ≲ 7. The temperatures at z ~ 4-5 are consistent with reionization being carried out by Pop II stars. In conclusion, the ionization and thermal state of the IGM at z ~ 5-6 suggest a late hydrogen reionization, driven by star-forming galaxies and ending around 6.5 ≲ z ≲ 7. This is consistent with other recent lines of observational evidence, and supports theoretical models that infer a late reionization from the observed star formation rate history.

We examine resonant scattering of Lyα (Lyman-alpha) photons in the neutral hydrogen Intergalactic Medium (IGM) at high redshift. Lyα scattering plays a key role in the 21cm emission/absorption against the Cosmic Microwave Background via the 'Wouthuysen-Field Effect' Knowledge of the strength of Lyα scattering induced by the first sources will constitute a significant step in predicting and understanding the eventual observations of the impact of these objects on the signal from the IGM during the Epoch of Reionisation (EoR), using planned facilities such as the Square Kilometre Array. A quantitative analysis of the scattering rate requires formulation and solution of the radiative transfer equation for the system. We consider radiative transfer of Lyα scattering far from a source in the homogeneous expanding IGM for photons that free stream until scattering in the blue wing of the local line profile: we describe an approximation that ignores spatial diffusion of photons and assumes a locally homogeneous scattering medium, allowing the calculation of simple analytic solutions to examine the dependence of the local scattering rate on various physical effects such as local expansion/contraction, and extend our approach to determine time-dependent solutions. The more complex problem of Lyα photons scattering in both frequency and space within a spherically symmetric medium is subsequently solved for several test problems using both Monte Carlo methods and a method based on the ray and moment radiative transfer equations following an approach due to Mihalas et al. [1975, 1976]. We examine local Lyα scattering around a continuum source in the homogeneous expanding IGM using both methods and compare our results with an analytic solution in the zero-temperature diffusion regime derived from an analogous solution for a monochromatic source found by Loeb and Rybicki [1999]. Our results are used to make estimates of the resulting size of the spherical region around the source that is rendered detectable via 21cm observations against the CMB background. We also examine cases with different density and velocity profiles and determine the effect on the scattering rate.

In the context of cosmological structure formation sheets, filaments and eventually halos form due to gravitational instabilities. It is noteworthy, that at all times, the majority of the baryons in the universe does not reside in the dense halos but in the filaments and the sheets of the intergalactic medium. While at higher redshifts of z > 2, these baryons can be detected via the absorption of light (originating from more distant sources) by neutral hydrogen at temperatures of T ~ 10^4 K (the Lyman-alpha forest), at lower redshifts only about 20 % can be found in this state. The remain (about 50 to 70 % of the total baryons mass) is unaccounted for by observational means. Numerical simulations predict that these missing baryons could reside in the filaments and sheets of the cosmic web at high temperatures of T = 10^4.5 - 10^7 K, but only at low to intermediate densities, and constitutes the warm-hot intergalactic medium (WHIM). The high temperatures of the WHIM are caused by the formation of shocks and the subsequent shock-heating of the gas. This results in a high degree of ionization and renders the reliable detection of the WHIM a challenging task. Recent high-resolution hydrodynamical simulations indicate that, at redshifts of z ~ 2, filaments are able to provide very massive galaxies with a significant amount of cool gas at temperatures of T ~ 10^4 K. This could have an important impact on the star-formation in those galaxies. It is therefore of principle importance to investigate the particular hydro- and thermodynamical conditions of these large filament structures. Density and temperature profiles, and velocity fields, are expected to leave their special imprint on spectroscopic observations. A potential multiphase structure may act as tracer in observational studies of the WHIM. In the context of cold streams, it is important to explore the processes, which regulate the amount of gas transported by the streams. This includes the time evolution of filaments, as well as possible quenching mechanisms. In this context, the halo mass range in which cold stream accretion occurs is of particular interest. In order to address these questions, we perform particular hydrodynamical simulations of very high resolution, and investigate the formation and evolution of prototype structures representing the typical filaments and sheets of the WHIM. We start with a comprehensive study of the one-dimensional collapse of a sinusoidal density perturbation (pancake formation) and examine the influence of radiative cooling, heating due to an UV background, thermal conduction, and the effect of small-scale perturbations given by the cosmological power spectrum. We use a set of simulations, parametrized by the wave length of the initial perturbation L. For L ~ 2 Mpc/h the collapse leads to shock-confined structures. As a result of radiative cooling and of heating due to an UV background, a relatively cold and dense core forms. With increasing L the core becomes denser and more concentrated. Thermal conduction enhances this trend and may lead to an evaporation of the core at very large L ~ 30 Mpc/h. When extending our simulations into three dimensions, instead of a pancake structure, we obtain a configuration consisting of well-defined sheets, filaments, and a gaseous halo. For L > 4 Mpc/h filaments form, which are fully confined by an accretion shock. As with the one-dimensional pancakes, they exhibit an isothermal core. Thus, our results confirm a multiphase structure, which may generate particular spectral tracers. We find that, after its formation, the core becomes shielded against further infall of gas onto the filament, and its mass content decreases with time. In the vicinity of the halo, the filament's core can be attributed to the cold streams found in other studies. We show, that the basic structure of these cold streams exists from the very beginning of the collapse process. Further on, the cross section of the streams is constricted by the outwards moving accretion shock of the halo. Thermal conduction leads to a complete evaporation of the cold stream for L > 6 Mpc/h. This corresponds to halos with a total mass higher than M_halo = 10^13 M_sun, and predicts that in more massive halos star-formation can not be sustained by cold streams. Far away from the gaseous halo, the temperature gradients in the filament are not sufficiently strong for thermal conduction to be effective.
Im Rahmen der kosmologischen Strukturbildung entstehen durch Gravitationsinstabilitäten Flächen, Filamente und schließlich Halos. Interessanterweise befinden sich zu jedem Zeitpunkt der kosmologischen Entwicklung der Großteil der Baryonen nicht in den Halos, sondern in den Filamenten und Ebenen des intergalaktischen Mediums. Während diese Baryonen bei höheren Rotverschiebungen (z ~ 2) noch in Form durch die Absorbtion von Licht (von weit entfernteren Quellen) durch neutralen Wasserstoff bei einer Temperatur von T ~ 10^4 K beobachtbar sind (Lyman-Alpha Wald), gilt dies bei niedrigeren Rotverschiebungen für nur noch ca. 20 % der Baryonen. Der überwiegende Teil (ca. 50-70 % der gesamten baryonischen Masse) sind bisher noch nicht direkt beobachtbar. Numerische Simulationen sagen jedoch voraus, das sich diese Baryonen in den Filamenten und Flächen des kosmischen Netzes befinden. Die entsprechende Gasverteilung zeichnet sich durch hohe Temperaturen T = 10^5 - 10^7 K und geringe bis mittlere Dichten aus und wird als warm-heißes intergalaktisches Medium (WHIM) bezeichnet. Die hohen Temperaturen entstehen in Folge der Bildung von Stoßwellen und der darauf folgenden Erhitzung des Gases (shock-heating). Das WHIM ist daher hochgradig ionisiert und sein verlässlicher Nachweis stellt eine große Herausforderung für die beobachtende Kosmologie dar. Neuere hydrodynamische Simulationen zeigen, dass sich bei höheren Rotverschiebungen von z ~ 2 Gasströmungen entlang der Filamente bilden, die massive Galaxien mit erheblichen Mengen an relativ kaltem Gas (T ~ 10^4 K) versorgen können. Dies hätte einen erheblichen Einfluss auf die Sternentstehung in diesen Galaxien. Es ist daher von grundsätzlichem Interesse, die spezifischen hydro- und thermodynamischen Bedingungen in den Strukturen des WHIM zu untersuchen. Sowohl Dichte- und Temperaturprofile als auch Geschwindigkeitsfelder prägen spektroskopische Beobachtungen. Eine mögliche Mehrphasenstruktur des WHIM könnte daher als Indikator in beobachtenden Studien dienen. Im Zusammenhang mit den kalten Strömen ist es besonders interessant, Prozesse zu untersuchen die den Zufluss von kaltem Gas zu den Galaxien regulieren. Dies umfasst die Zeitentwicklung des Anteils an kaltem Gas in den Filamenten, sowie mögliche Mechanismen, die zum Versiegen des Zuflusses von kaltem Gas auf die Galaxienscheibe führen. Um diese Zusammenhänge zu erforschen, führen wir spezielle hydrodynamische Simulationen mit sehr hoher Auflösung durch, die zu ausgewählten, wohldefinierten Strukturen führen, die das WHIM charakterisieren. Wir beginnen mit einer ausführlichen Untersuchung des eindimensionalen Kollaps einer sinusförmigen Störung (pancake formation). Hierbei untersuchen wir den Einfluss von Strahlungkühlung, Heizung durch den intergalaktischen UV Hintergrund, Wärmeleitung, sowie von kleinskaligen Störungen, welche dem kosmologischen Störungsspektrum folgen. Wir benutzen hierbei eine Reihe von Simulationen, welche die Längenskala der anfänglichen Störung L als Parameter verwenden. Für L ~ 2 Mpc/h führt der Kollaps zur Ausbildung einer Stoßwelle. Zusätzlich entsteht als Folge der Strahlungskühlung und der Heizung durch den UV Hintergrund ein relativ dichter und kalter isothermer Kern. Mit ansteigendem L wird dieser Kern dichter und kompakter. Durch Wärmeleitung reduziert sich die räumliche Ausdehnung des Kerns. Für L ~ 30 Mpc/h führt dies zu einem Verschwinden des Kerns. Mit der Erweiterung unserer Methodik auf dreidimensionale Simulationen, entsteht nun eine Konfiguration, welche aus wohldefinierten Flächen, Filamenten und einem gasförmigen Halo besteht. Für L > 4 Mpc/h, erhalten wir Filamente, die vollständig durch Akkretionsschocks begrenzt sind. Wie in unseren eindimensionalen Simulationen weisen auch sie einen isothermen Kern auf. Dies legt nahe, dass das WHIM eine Mehrphasenstruktur besitzt und mögliche Spektralsignaturen erzeugen kann. Nach seiner Entstehung ist der Kern gegen weiteren Zufluss von Gas abgeschirmt und seine Masse reduziert sich mit der Zeit. In der direkten Umgebung des Halos entspricht der Kern des Filamentes den oben angesprochenen kalten Strömen. Unsere Untersuchung zeigt, dass diese während der gesamten Entwicklung des Halos existent sind. In der weiteren Entwicklung werden sie durch den expandierenden Akkretionsschock des Halos verengt. Ab einer Skala von L > 6 Mpc/h kann Wärmeleitung zu einem Verschwinden des Zustroms von kaltem Gas führen. Diese Skala entspricht Halos mit einer Gesamtmasse von M_halo = 10^13 M_sun. Galaxien, die sich in noch massiveren Halos bilden, können daher nicht durch kalte Ströme mit Gas für die Sternentstehung versorgt werden. Im Filament, weit außerhalb des gasförmigen Halos, sind die Temperaturgradienten zu klein, um effiziente Wärmeleitung zu ermöglichen.

Des systèmes d'absorption Lorentziens, qui sondent les nuages de gaz HI de plus hautes densités de colonne, servent ici à sonder les environnements de galaxies hôtes de noyaux actifs à grand décalage spectral (z > 2). Ceci permet d'étudier l'effet des mécanismes de rétroaction des noyaux actifs sur les galaxies hôtes, tel que les vents à haute vitesse et l'ionisation intense. J'implémente deux techniques pour identifier les systèmes Lorentziens au décalage spectral du quasar dans les données du Sloan Digital Sky Survey III Baryon Oscillation Spectroscopic Survey.Un tel système Lorentzien fait office de coronographe naturel puisqu'il absorbe complètement le rayonnement provenant du noyau actif. Parfois une raie Lyα étroite émise par la galaxie est superposée à l'absorption. J'étudie un échantillon statistiquement complet et je caractérise l'émission. Les systèmes Lorentziens qui révèlent les raies étroites d'émission Lyα les plus lumineuses proviennent de nuages denses et compacts dans la galaxie hôte. Les autres sont dus à des galaxies voisines du quasars.Une deuxième technique consiste à observer des paires de quasars ayant une petite séparation angulaire pour sonder les environnements des galaxies hôtes à des distances transverses inférieures à 90 kpc. J'analyse les propriétés du gaz pour des paires où un système Lorentzien apparait dans le spectre du quasar d'arrière plan coïncident avec le pic d'émission Lyα du quasar de premier plan.Dans une des paires, je détecte une sur-densité de systèmes absorbants à z = 2.69 dans une région correspondant à 6.4 Mpc en distance propre. Les propriétés de cette région suggèrent un filament du milieu intergalactique
Damped Lyman-α absorbers (DLAs), the highest column density HI Lyman-α (Lyα) absorptions, are used in this thesis to study the environments of high-redshift (z > 2) quasar host galaxies. This is essential for determining how feedback mechanisms from active galactic nuclei (AGN), including high-velocity winds and intense ionizing radiation, impact the host galaxies. Thanks to the large number of quasar sight-lines from the Sloan Digital Sky Survey III Baryon Oscillation Spectroscopic Survey, I implement two techniques to identify DLAs that occur at the quasar redshift.Along the sight-line, these DLAs act as natural coronagraphs and completely absorb the broad Lyα emission from the central AGN. In some cases, a narrow Lyα emission line from the quasar host galaxy is superimposed on the DLA trough. I compare coronagraph DLAs that reveal narrow Lyα emission with those that do not in a statistically complete sample and characterize the emission. DLAs with the most luminous narrow Lyα emission peaks may arise from dense, compact clouds in the host galaxy, while the others may be due to neighboring galaxies. With a second technique, I use pairs of quasars with small angular separations to investigate host galaxy environments at distances of less than 90 kpc in the transverse direction. I analyze the gas properties for pairs where a DLA appears in the background quasar spectrum coincident with the foreground quasar Lyα emission peak.In one of the pairs, I also detect an overdensity of Lyman-limit system absorbers at z = 2.69 in a region spanning 2000 km/s (6.4 Mpc proper distance) along the two sight-lines. The overdense region properties suggest an intergalactic medium filament

In this thesis we investigate the large-scale distribution of Ly alpha forest absorption, the effect of ionizing radiation from QSOs on their surrounding intergalactic medium and the primordial abundance of deuterium. We develop a new technique for detecting structure on Mpc scales in the Ly alpha forest. This technique does not rely on identifying individual absorption lines but is rather based on the statistics of the transmitted flux. We demonstrate that the new method is significantly more sensitive to the presence of large-scale structure in the Ly alpha forest than a two-point correlation function analysis. We apply this method to 2 A resolution spectra of ten QSOs which cover the redshift range 2.2 < z < 3.4. The QSOs form a closely spaced group on the sky and are concentrated within a 1-deg^2 field. We find evidence for large-scale structure in the distribution of Ly alpha forest absorption at the > 99 per cent confidence level. Along the line of sight we find overdense Ly alpha absorption on scales of up to 1200 km s^-1. There is also strong evidence for correlated absorption across line of sight pairs separated by < 3 h^-1 Mpc. For larger separations the cross-correlation signal becomes progressively less significant. Using the same technique and dataset we confirm the existence of the proximity effect. We derive a value for the mean intensity of the extragalactic background radiation at the Lyman limit of J = (3.6^+3.5_-1.3) x 10^-22 ergs s^-1 cm^-2 Hz^-1 sr^-1. This value assumes that QSO redshifts measured from high ionization lines differ from the true systemic redshifts by Delta v = 800 km s^-1. Allowing for known QSO variability we find evidence at a level of 2.1 sigma that the significance of the proximity effect is correlated with QSO Lyman limit luminosity. From the complete sample we find no evidence for the existence of a foreground proximity effect, implying either that J > 20 x 10^-22 ergs s^-1 cm^-2 Hz^-1 sr^-1 or that QSOs emit at least a factor of 1.4 less ionizing radiation in the plane of the sky than along the line of sight to Earth. We do, however, find one counter-example where a foreground QSO apparently depletes the absorbing gas in four surrounding lines of sight. We discuss the feasibility of pre-selecting absorption systems from low resolution data for a measurement of the primordial deuterium abundance. We present a new, low resolution spectroscopic survey of 101 high redshift QSOs aimed at identifying candidate D/H systems. We further present an echelle spectrum of a Lyman limit system at z = 2.917. We find that this system is most likely heavily contaminated and does not yield an interesting limit on D/H.

Theory of cosmic structure formation outlines how stars, galaxies, clusters of galaxies, and large-scale structures formed out of primordial density fluctuations. It presents us a picture of cosmic mass assembly, and places strong constraints on cosmological model. Both observations and theories suggest that structures formation follows a "bottom up" process, in which small, low-mass component form first, and gradually develop into larger, more massive systems. This dissertation focuses on three crucial stages of cosmic structure formation: first generation stars, quasar host galaxies and the large-scale galaxy overdensities. In Chapter 1, I present an overview of structure formation, acquainting readers with a general picture from first object in the Universe to large-scale structures at later epochs. In Chapter 2 and Chapter 3, I derive strong constraints to the star formation rates (SFRs) of very massive Population III (Pop III) stars in two high redshift galaxies at z = 7. By probing the He II emission lines for both galaxies, I conclude that the contributions of very massive Pop III stars to total the SFRs are less than 3%. In Chapter 4, I move to more massive systems, quasar host galaxies at z ~ 3. Using damped Lyman alpha absorption systems as natural coronagraphs, I report that rest-frame far-UV emission of quasar host galaxy correlates strongly with quasar luminosity. This result suggests a co-evolution of supermassive black holes and their host galaxies. In Chapter 5, I develop a novel method for searching the most massive protoclusters at z = 2-3, by utilizing intergalactic Lyman alpha absorption. My investigations suggest that large intergalactic Lyman alpha absorption systems effectively trace the most overdense regions at large scale of ~ 15 h⁻¹ Mpc. In Chapter 6, I present our imaging observations of an extreme galaxy overdensity (protocluster) BOSS1441+4000, which is discovered using the techniques developed in Chapter 5. Furthermore, I report an intergalactic-scale Lyman alpha nebula detected at the density peak of BOSS1441+4000. This discovery, together with previously discovered Slug nebula, provide us a first look of intergalactic medium in emission in the early Universe. In the Chapter 7, I give a summary of this dissertation and discuss several future prospects.

In this Thesis work, we analyse 21cm line observations taken with the Precision Array to Probe the Epoch of Reionization (PAPER) in the 120-180MHz range (6