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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
21

Cosmology with the Lyman alpha forest

Liske, Jochen, Physics, Faculty of Science, UNSW January 2000 (has links)
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.
22

Detecting cosmological reionization on large scales through the 21 cm HI line

Chippendale, Aaron Paul January 2009 (has links)
Doctor of Philosophy (PhD) / This thesis presents the development of new techniques for measuring the mean redshifted 21 cm line of neutral hydrogen during reionization. This is called the 21 cm cosmological reionization monopole. Successful observations could identify the nature of the first stars and test theories of galaxy and large-scale structure formation. The goal was to specify, construct and calibrate a portable radio telescope to measure the 21 cm monopole in the frequency range 114 MHz to 228 MHz, which corresponds to the redshift range 11.5 > z > 5.2. The chosen approach combined a frequency independent antenna with a digital correlation spectrometer to form a correlation radiometer. The system was calibrated against injected noise and against a modelled galactic foreground. Components were specified for calibration of the sky spectrum to 1 mK/MHz relative accuracy. Comparing simulated and measured spectra showed that bandpass calibration is limited to 11 K, that is 1% of the foreground emission, due to larger than expected frequency dependence of the antenna pattern. Overall calibration, including additive contributions from the system and the radio foreground, is limited to 60 K. This is 160 times larger than the maximum possible monopole amplitude at redshift eight. Future work will refine and extend the system known as the Cosmological Reionization Experiment Mark I (CoRE Mk I).
23

The transverse proximity effect in quasar spectra

Worseck, Gábor January 2007 (has links)
The intergalactic medium is kept highly photoionised by the intergalactic UV background radiation field generated by the overall population of quasars and galaxies. In the vicinity of sources of UV photons, such as luminous high-redshift quasars, the UV radiation field is enhanced due to the local source contribution. The higher degree of ionisation is visible as a reduced line density or generally as a decreased level of absorption in the Lyman alpha forest of neutral hydrogen. This so-called proximity effect has been detected with high statistical significance towards luminous quasars. If quasars radiate rather isotropically, background quasar sightlines located near foreground quasars should show a region of decreased Lyman alpha absorption close to the foreground quasar. Despite considerable effort, such a transverse proximity effect has only been detected in a few cases. So far, studies of the transverse proximity effect were mostly limited by the small number of suitable projected pairs or groups of high-redshift quasars. With the aim to substantially increase the number of quasar groups in the vicinity of bright quasars we conduct a targeted survey for faint quasars around 18 well-studied quasars at employing slitless spectroscopy. Among the reduced and calibrated slitless spectra of 29000 objects on a total area of 4.39 square degrees we discover in total 169 previously unknown quasar candidates based on their prominent emission lines. 81 potential z>1.7 quasars are selected for confirmation by slit spectroscopy at the Very Large Telescope (VLT). We are able to confirm 80 of these. 64 of the newly discovered quasars reside at z>1.7. The high success rate of the follow-up observations implies that the majority of the remaining candidates are quasars as well. In 16 of these groups we search for a transverse proximity effect as a systematic underdensity in the HI Lyman alpha absorption. We employ a novel technique to characterise the random absorption fluctuations in the forest in order to estimate the significance of the transverse proximity effect. Neither low-resolution spectra nor high-resolution spectra of background quasars of our groups present evidence for a transverse proximity effect. However, via Monte Carlo simulations the effect should be detectable only at the 1-2sigma level near three of the foreground quasars. Thus, we cannot distinguish between the presence or absence of a weak signature of the transverse proximity effect. The systematic effects of quasar variability, quasar anisotopy and intrinsic overdensities near quasars likely explain the apparent lack of the transverse proximity effect. Even in absence of the systematic effects, we show that a statistically significant detection of the transverse proximity effect requires at least 5 medium-resolution quasar spectra of background quasars near foreground quasars whose UV flux exceeds the UV background by a factor 3. Therefore, statistical studies of the transverse proximity effect require large numbers of suitable pairs. Two sightlines towards the central quasars of our survey fields show intergalactic HeII Lyman alpha absorption. A comparison of the HeII absorption to the corresponding HI absorption yields an estimate of the spectral shape of the intergalactic UV radiation field, typically parameterised by the HeII/HI column density ratio eta. We analyse the fluctuating UV spectral shape on both lines of sight and correlate it with seven foreground quasars. On the line of sight towards Q0302-003 we find a harder radiation field near 4 foreground quasars. In the direct vicinity of the quasars eta is consistent with values of 25-100, whereas at large distances from the quasars eta>200 is required. The second line of sight towards HE2347-4342 probes lower redshifts where eta is directly measurable in the resolved HeII forest. Again we find that the radiation field near the 3 foreground quasars is significantly harder than in general. While eta still shows large fluctuations near the quasars, probably due to radiative transfer, the radiation field is on average harder near the quasars than far away from them. We interpret these discoveries as the first detections of the transverse proximity effect as a local hardness fluctuation in the UV spectral shape. No significant HI proximity effect is predicted for the 7 foreground quasars. In fact, the HI absorption near the quasars is close to or slightly above the average, suggesting that the weak signature of the transverse proximity effect is masked by intrinsic overdensities. However, we show that the UV spectral shape traces the transverse proximity effect even in overdense regions or at large distances. Therefore, the spectral hardness is a sensitive physical measure of the transverse proximity effect that is able to break the density degeneracy affecting the traditional searches. / Das intergalaktische Medium wird durch das intergalaktische UV-Hintergrundsstrahlungsfeld in einem hochgradig photoionisierten Zustand gehalten. Der UV-Hintergrund stammt von der gesamten Population von Quasaren und Galaxien. In der Nähe von leuchtkräftigen Quasaren, ist das UV-Strahlungsfeld lokal erhöht durch den Anteil der Quelle. Der höhere Ionisationsgrad ist beobachtbar als eine reduzierte Liniendichte oder allgemein als ein vermindertes Maß an Absorption im Lyman-alpha Wald des neutralen Wasserstoffs. Dieser sogenannte Proximity-Effekt ist bei leuchtkräftigen Quasaren mit hoher statistischer Signifikanz nachgewiesen worden. Falls Quasare fast isotrop strahlen, dann sollten Sichtlinien zu Hintergrundquasaren in der Nähe von Vordergrundquasaren eine Region mit verminderter Absorption zeigen. Trotz beträchtlichen Aufwands wurde solch ein transversaler Proximity-Effekt nur in wenigen Fällen entdeckt. Bisher waren Studien des transversalen Proximity-Effekts meist begrenzt durch die kleine Anzahl von geeigneten projizierten Paaren oder Gruppen von hochrotverschobenen Quasaren. Mit dem Ziel die Zahl der Quasargruppen in der Nähe von hellen Quasaren beträchtlich zu erhöhen, führen wir eine gezielte Suche nach schwachen Quasaren um 18 oft studierte Quasare durch. Unter den reduzierten und kalibrierten spaltlosen Spektren von 29000 Objekten auf einer Gesamtfläche von 4.39 Quadratgrad entdecken wir insgesamt 169 vorher unbekannte Quasarkandidaten anhand ihrer Emissionslinien. 81 potentielle z>1.7 Quasare werden ausgesucht zur Bestätigung mittels Spaltspektroskopie am Very Large Telescope (VLT). Wir können 80 von diesen als Quasare bestätigen. 64 der neu entdeckten Quasare liegen bei z>1.7. Die hohe Erfolgsrate der Nachfolgebeobachtungen deutet an, dass die Mehrzahl der verbleibenden Kandidaten ebenfalls Quasare sind. In 16 dieser Gruppen suchen wir nach dem transversalen Proximity-Effekt als eine systematische Unterdichte in der HI Lyman-alpha-Absorption. Wir nutzen eine neuartige Methode die zufälligen Absorptionsfluktuationen zu charakterisieren, um die Signifikanz des transversalen Proximity-Effekts abschätzen zu können. Weder schwach aufgelöste noch hoch aufgelöste Spektren von Hintergrundquasaren unserer Gruppen zeigen Anzeichen für einen transversalen Proximity-Effekt. Aufgrund von Monte Carlo Simulationen sollte der Effekt jedoch nur schwach in der Nähe von 3 Vordergrundquasaren detektierbar sein. Deshalb können wir nicht zwischen An- oder Abwesenheit des Effekts unterscheiden. Selbst in Abwesenheit von systematischen Effekten zeigen wir, dass eine statistisch signifikante Detektion des transversalen Proximity-Effekts mindestens 5 Hintergrundquasarspektren bei mittlerer Auflösung nahe Vordergrundquasaren erfordert, deren UV-Fluss den UV-Hintergrund um einen Faktor 3 übersteigt. Deshalb erfordern statistische Studien des transversalen Proximity-Effekts große Zahlen von geeigneten Quasaren. Zwei Sichtlinien zeigen HeII-Absorption. Ein Vergleich der HeII-Absorption mit der entsprechenden HI-Absorption liefert eine Abschätzung der Spektralform des UV-Strahlungsfelds, das typischerweise durch das HeII/HI Säulendichteverhältnis eta parameterisiert wird. Wir analysieren die fluktuierende spektrale Form des UV-Strahlungsfelds auf beiden Sichtlinien und korrelieren sie mit 7 Vordergrundquasaren. Auf der Sichtlinie zu Q0302-003 finden wir ein härteres Strahlungsfeld nahe 4 Vordergrundquasaren. In der direkten Umgebung der Quasare ist eta konsistent mit Werten von 25-100, wogegen bei großen Entfernungen zu den Quasaren eta>200 erforderlich ist. Die zweite Sichtlinie zu HE2347-4342 sondiert kleinere Rotverschiebungen. Wieder finden wir, dass das Strahlungsfeld nahe der 3 Vordergrundquasaren signifikant härter ist als im allgemeinen. Während eta trotzdem große Fluktuationen nahe den Quasaren aufweist, die wahrscheinlich von Strahlungstransport herrühren, ist das Strahlungsfeld in der Nähe der Quasare im Mittel härter als in großer Entfernung. Wir interpretieren diese Entdeckungen als die ersten Detektionen des transversalen Proximity-Effekts als eine lokale Fluktuation im spektralen Härtegrad. Kein signifikanter HI Proximity-Effekt ist für die 7 Vordergrundquasare vorhergesagt. Tatsächlich ist die HI-Absorption nahe den Quasaren nahe am oder etwas über dem Mittelwert, was darauf hindeutet, dass die schwache Signatur des transversalen Proximity-Effekts maskiert wird durch intrinsische Überdichten. Jedoch zeigen wir, dass der Härtegrad den transversalen Proximity-Effekt selbst in überdichten Regionen oder auf großen Distanzen sichtbar werden läßt. Deshalb ist der spektrale Härtegrad ein empfindliches physikalisches Maß für den transversalen Proximity-Effekt, der in der Lage ist, die Dichteentartung zu brechen, die die traditionelle Suche behindert.
24

Constraining the UV background with the proximity effect

Dall'Aglio, Aldo January 2009 (has links)
After the epoch of reionisation the intergalactic medium (IGM) is kept at a high photoionisation level by the cosmic UV background radiation field. Primarily composed of the integrated contribution of quasars and young star forming galaxies, its intensity is subject to spatial and temporal fluctuations. In particular in the vicinity of luminous quasars, the UV radiation intensity grows by several orders of magnitude. Due to an enhanced UV radiation up to a few Mpc from the quasar, the ionised hydrogen fraction significantly increases and becomes visible as a reduced level of absorption in the HI Lyman alpha (Ly-alpha) forest. This phenomenon is known as the proximity effect and it is the main focus of this thesis. Modelling the influence on the IGM of the quasar radiation, one is able to determine the UV background intensity at a specific frequency (J_nu_0), or equivalently, its photoionisation rate (Gamma_b). This is of crucial importance for both theoretical and observational cosmology. Thus far, the proximity effect has been investigated primarily by combining the signal of large samples of quasars, as it has been regarded as a statistical phenomenon. Only a handful of studies tried to measure its signature on individual lines of sight, albeit focusing on one sight line only. Our aim is to perform a systematic investigation of large samples of quasars searching for the signature of the proximity effect, with a particular emphasis on its detection on individual lines of sight. We begin this survey with a sample of 40 high resolution (R~45000), high signal to noise ratio (S/N~70) quasar spectra at redshift 2.1<z<4.7, publicly available in the European Southern Observatory (ESO) archive. The extraordinary quality of this data set enables us to detect the proximity effect signature not only in the combined quasar sample, but also along each individual sight line. This allows us to determine not only the UV background intensity at the mean redshift of this sample, but also to estimate its intensity in small (Delta z~0.2) redshift intervals in the range 2<z<4. Our estimates (J_nu_0~ 3x10^{-22} erg s^{-1} cm^{-2} Hz^{-1} sr^{-1}) are for the first time in very good agreement with different constraints of its evolution obtained from theoretical predictions and numerical simulations. We continue this systematic analysis of the proximity effect with the largest search to date invoking the Sloan Digital Sky Survey (SDSS) data set. The sample consists of 1733 quasars at redshifts z>2.3. In spite of the low resolution and limited S/N we detect the proximity effect on about 98% of the quasars at a high significance level. Thereby we are able to determine the evolution of the UV background photoionisation rate within the redshift range 2<z<5 finding Gamma_b~ 1.6x10^{-12} s^{-1}. With these new measurements we explore literature estimates of the quasar luminosity function and predict the stellar luminosity density up to redshift of about z~5. Our results are globally in good agreement with recent determinations inferred from deep surveys of high redshift galaxies. We then compare our measurements on the UV background photoionisation rate inferred from the two samples at high and low resolution. While these data sets show extreme differences, our determinations are in considerable agreement at z<3.3, even though they show less agreement at higher redshifts. We suspect that this may be caused by either the small number of high resolution quasar spectra at the highest redshifts considered or by some systematic effect due to the limited data quality of SDSS. Complementary to the observational investigation of the proximity effect on high redshift quasars, we exploit some theoretical aspects linked to and based on the results on this phenomenon. We employ complex numerical simulations of structure formation to achieve a better representation of the Ly-alpha forest. Modelling the signature of the proximity effect on randomly selected sight lines, we prove the advantages of dealing with individual lines of sight instead of combining their signal to investigate this phenomenon. Furthermore, we develop and test novel techniques aimed at a more precise determination of the proximity effect signal. With this investigation we demonstrate that the technique developed and employed in this thesis is the most accurate adopted thus far. Tighter determinations of the UV background are certainly based on suitable methods to detect its signature, but also on a deeper understanding of the environments in which quasars form and evolve. We initiate an investigation of complex numerical simulations including the radiative transport of energy to model in a more detailed way the proximity effect. Such a simulation may lead to the characterisation of the quasar environment based on the comparison between the observed and simulated statistical properties of the proximity effect signature. / Nach dem kosmologischen Zeitalter der Reionisation wird der hohe Photoionisationsgrad des intergalaktische Mediums (IGM) durch die kosmische UV-Hintergrundstrahlung aufrecht erhalten. Zur Intensitaet der Hintergrundstrahlung tragen hauptsaechlich Quasare und jungen Galaxien bei. Daher entstehen sowohl raeumliche als auch zeitliche Fluktuationen, wobei die Intensistaet insbesondere in der Naehe von leuchtkraeftigen Quasaren um mehrere Groessenordnungen ansteigt. Aufgrund der erhoehten UV-Strahlung in einer Entfernung von bis zu einigen Mpc von einem Quasar wird ein groesserer Anteil des intergalaktischen Wasserstoffs ionisiert, was als reduzierte Absorption im Lyman alpha (Ly-alpha) Wald sichtbar wird. Dieses Phaenomen wird proximity effect genannt und ist das Hauptthema dieser Arbeit. Durch Modellierung des Einflusses des Quasars auf das IGM kann die Intensitaet des UV-Hintergrunds bei einer bestimmten Frequenz (J_nu_0) bzw. die entsprechende Photoionisationrate (Gamma_b) bestimmt werden. Dies ist sowohl fuer die theoretische als auch fuer die beobachtende Kosmologie eine wichtige Groesse. Bisher wurde der Proximity-Effekt als ein statistisches Phaenomen untersucht, wobei die Signale vieler einzelner Quasare kombiniert wurden. Nur in wenigen Analysen wurde versucht, den Effekt in einzelnen Sehlinien zu detektieren. Das Ziel dieser Arbeit ist eine systematische Untersuchung des Proximity-Effekts in einer grossen Anzahl von Quasaren, wobei der besonderen Schwerpunkt auf seiner Detektion in einzelnen Sehlinien liegt. Zunaechst werden 40 Quasare im Rotverschiebungsbereich 2.1<z<4.7 untersucht, deren Spektren mit hoher Aufloesung (R=45000) und hohem Signal-zu-Rausch-Verhaeltnis (S/N~70) im Archiv des European Southern Observatory (ESO) vorliegen. Die ausserordentlich gute Qualitaet dieser Daten ermoeglicht die Detektion des Proximity-Effekts nicht nur als kombiniertes Signal aller Quasare sondern auch in jeder einzelnen Sehlinie. Daher konnten wir nicht nur die Intensitaet des UV-Hintergrunds bei der mittleren Rotverschiebung ermitteln sondern auch in kleineren Rotverschiebungsintervallen (Delta z~0.2) im Bereich 2<z<4. Unsere Ergebnisse (J_nu_0}~3x10^{-22} erg s^{-1} cm^{-2} Hz^{-1} sr^{-1}) stimmen zum ersten Mal gut anderen Bestimmungen ueberein, die auf theoretischen Voraussagen und auf numerischen Simulationen beruhen. Unsere systematische Analyse des Proximity-Effekts wird mit dem bisher groessten Datensatz bestehend aus 1733 Quasaren mit Rotverschiebungen z>2.3 aus dem Sloan Digital Sky Survey (SDSS) fortgefuehrt. Trotz der niedrigen Aufloesung und dem begrenzten S/N detektieren wir den Proximity-Effekt mit einer hohen Signifikanz in etwa 98% der Sehlinen. Dabei kann die Entwicklung der Photoionisationsrate Gamma_b~1.6x10^{-12} s^{-1} im Rotverschiebungsbereich 2<z<5 bestimmt werden. Mit diesen neuen Messungen diskutieren wir verschiedene Quasar-Leuchtkraftfunktionen aus der Literatur und berechnen die stellare Emissivitaet bis z~5. Unsere Ergebnisse stimmen im Allgemeinen gut mit denen von neueren Himmelsdurchmusterungen nach hochrotverschobenen Galaxien ueberein. Dann vergleichen wir die auf den hoch bzw. niedrig aufgeloesten Spektren basierenden Photoionisationsraten miteinander. Obwohl die Datensaetze sehr unterschiedlich sind, fuehren sie bei z<3.3 zu den selben Ergebnissen, waehrend die Resultate bei hoeheren Rotverschiebungen weniger gut uebereinstimmen. Wir vermuten, dass dies entweder durch die kleine Anzahl von hochaufgeloesten Quasarspektren bei den hoechsten Rotverschiebungen, oder durch systematische Effekte der geringen SDSS Datenqualitaet hervorgerufen wird. Ergaenzend zu der Auswertung der Beobachtungsdaten fuehren wir basierend auf unseren Ergebnissen einige theoretische Untersuchugen durch. Wir benutzen komplexe Strukturbildungssimulationen, um eine bessere Beschreibung des Ly-alpha-Walds zu gewinnen. Mit Hilfe der Modellierung des Proximity-Effekts in zufaellig ausgesuchten Sehlinien zeigen wir den Vorteil auf, den die Analyse einzelner Sehlinien im Vergleich zur kombinierten Auswertung mehrerer Spektren hat. Ausserdem entwickeln und testen wir neue Ansaetze zur genaueren Bestimmung des Proxmity-Effekts. Dabei zeigen wir, dass die im Rahmen dieser Arbeit entwickelte und angewendete Methode bisher zu den genauesten Ergebnissen fuehrt. Fuer eine noch bessere Bestimmung des UV-Hintergurnds brauchen wir neben der optimalen Detektionsmethode auch ein tieferes Verstaendnis der Umgebung, in der Quasare entstehen und sich entwickeln. Wir beginnen eine Analyse komplexer numerischer Simulationen, die auch Strahlungstransportrechungen beinhalten, um weitere Details des Proximity-Effekts zu verstehen. Der Vergleich der statistischen Eigenschaften des Proximity-Effekts in solchen Simulationen mit Beobachtungen koennte in Zukunft zu einer genaueren Beschreibung der Umgebung von Quasaren fuehren.
25

A detailed view of filaments and sheets of the warm-hot intergalactic medium

Klar, Jochen January 2012 (has links)
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.
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The History of Enrichment of the Intergalactic Medium Using Cosmological Simulations

Oppenheimer, Benjamin Darwin January 2008 (has links)
I investigate the chemical evolution of the Universe in a series of cosmological hydrodynamic simulations with the purpose of finding a self-consistent evolutionary scenario of galaxy formation able to reproduce key observables focusing on the enrichment of the intergalactic medium (IGM). The most successful models I run and analyze use the scalings of momentum-driven feedback whereby UV photons generated during the Main Sequence stage accelerate dust-driven winds while providing a significantly larger energy budget than from supernovae alone. The success of this outflow model relies on its ability to drive highly mass-loaded winds from small galactic haloes. These feedback relations, supported by observations of local starburst, are inserted into simulations at all epochs, reproducing observables including the C IV column density and linewidth distributions at z=6->1.5 and the O VI forest at z=0-0.5. Outflows at z>=5 propagate early nucleosynthetic products traced by C IV and lower ionization species into an otherwise metal-free IGM. Continual outflows at the peak ages of star formation (z=5->1.5) produce a non-evolving cosmic mass density of C IV despite continual enrichment increasing IGM metallicity by a factor of ten. The z=0-0.5 O VI forest is composed of weaker absorbers tracing photo-ionized diffuse IGM metals, sometimes injected by primordial galaxies, and stronger absorbers tracing recently injected metals, often unable to escape their parent haloes and sometimes collisionally ionized. Tracking the individual histories of metals in outflows shows the average outflow travels ~100 physical kpc and returns to galaxies on an average timescale of 1-2 Gyr; this result implies metals in superwinds do not remain in the IGM for a Hubble time and are more likely to rejoin galaxies. Metal absorbers aligned with Lyman-alpha are examined in detail, finding that the two often trace different phases of gas with the former tracing an inhomogeneous distribution of metals exhibiting turbulence imparted during the outflow phase dissipating on a Hubble timescale. I find this is the first model to self-consistently reproduce the wide range of IGM observables spanning the history of heavy metal production while being consistent with key galaxy observables. The link between star formation and galactic superwinds requires that a successful model of galaxy formation reproduces both the evolution of galaxies and the IGM.
27

Detecting cosmological reionization on large scales through the 21 cm HI line

Chippendale, Aaron Paul January 2009 (has links)
Doctor of Philosophy (PhD) / This thesis presents the development of new techniques for measuring the mean redshifted 21 cm line of neutral hydrogen during reionization. This is called the 21 cm cosmological reionization monopole. Successful observations could identify the nature of the first stars and test theories of galaxy and large-scale structure formation. The goal was to specify, construct and calibrate a portable radio telescope to measure the 21 cm monopole in the frequency range 114 MHz to 228 MHz, which corresponds to the redshift range 11.5 > z > 5.2. The chosen approach combined a frequency independent antenna with a digital correlation spectrometer to form a correlation radiometer. The system was calibrated against injected noise and against a modelled galactic foreground. Components were specified for calibration of the sky spectrum to 1 mK/MHz relative accuracy. Comparing simulated and measured spectra showed that bandpass calibration is limited to 11 K, that is 1% of the foreground emission, due to larger than expected frequency dependence of the antenna pattern. Overall calibration, including additive contributions from the system and the radio foreground, is limited to 60 K. This is 160 times larger than the maximum possible monopole amplitude at redshift eight. Future work will refine and extend the system known as the Cosmological Reionization Experiment Mark I (CoRE Mk I).
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Ανίχνευση και μελέτη υπολειμμάτων υπερκαινοφανών και εξωγαλαξιακής σκόνης

Αλικάκος, Ιωάννης 26 April 2012 (has links)
Η διδακτορική διατριβή βασίζεται σε οπτικές παρατηρήσεις που πραγματοποιήθηκαν από τα τηλεσκόπια του αστεροσκοπείου του Σκίνακα (το οποίο βρίσκεται στην Κρήτη) και από το τηλεσκόπιο Ισαάκ Νιούτον (που βρίσκεται στη Λα Πάλμα στα Κανάρια νησιά). Από τις παρατηρήσεις αυτές, (οπτικές εικόνες και φάσματα) ανακαλύφθηκαν σε μία περιοχή εβδομήντα τετραγωνικών λεπτών του τόξου της μοίρας, έξι υπολείμματα υπερκαινοφανών αστέρων τα οποία δεν αναγράφονται σε καμία βιβλιογραφία. Τα υπολείμματα αυτά, λόγω της ίδιας περίπου απόστασης που απέχουν, ενδεχομένως να προέρχονται από αλληλεπιδράσεις υπερκαινοφανών αστέρων, οπού η έκρηξη του ενός αστέρα, επιταχύνει την έκρηξη του πλησιέστερου σε αυτόν αστέρα που βρίσκεται στο τελευταίο στάδιο, όταν το ωστικό κύμα διέλθει από αυτόν, δημιουργώντας έτσι μια φυσαλίδα υπολειμμάτων υπερκαινοφανών. Το σημαντικό με αυτή την μελέτη είναι ότι για πρώτη φορά παρατηρήθηκαν στον Γαλαξία μας, στο οπτικό μέρος του φάσματος, μια περιοχή με τόσα υπολείμματα υπερκαινοφανών, παρέχοντάς μας την δυνατότητα να μελετήσουμε τον ρυθμό των εκρήξεων των υπερκαινοφανών με αυτόν της δημιουργίας των υπολειμμάτων τους, και να εξάγουμε συμπεράσματα για τον ρυθμό αστρογένεσης στον Γαλαξία μας. Παράλληλα μελετώντας την ομάδα Μ81, ανιχνεύτηκε για πρώτη φορά στο οπτικό μέρος του φάσματος, σκόνη στην περιοχή μεταξύ των γαλαξιών. Η μελέτη βασίζεται στην σύγκριση του δείκτη χρώματος των γαλαξιών υποβάθρου, των υποψηφίων περιοχών, με τον δείκτη χρώματος γαλαξιών υποβάθρου σε περιοχές που είναι απομακρυσμένες από την ομάδα Μ81, και αποτελούν πεδία ελέγχου. Η συστηματική ερύθρωση που παρουσίαζαν οι γαλαξίες υποβάθρου στις περιοχές πλησίον της ομάδας Μ81 μπορεί να ερμηνευτεί μόνο με την παρουσία μεγάλων ποσοτήτων σκόνης που εμπεριέχονται στο μεσογαλαξιακό χώρο. Η ποσότητα της σκόνης στις περιοχές αυτές υπολογίστηκε ότι είναι περίπου 50 εκατομμύρια ηλιακές μάζες, όσο δηλαδή και η σκόνη που διαθέτει ένας τυπικός σπειροειδής γαλαξίας. Η προέλευση της σκόνης πιθανολογείται ότι οφείλεται σε ένα μέλος της ομάδας (τον γαλαξία Μ82), ο οποίος είναι ένας γαλαξίας με περιοχές έντονης αστρογένεσης και εκτοξεύει μεγάλες ποσότητες σκόνης στο μεσογαλαξιακό χώρο ή στις παλιρροϊκές δυνάμεις που αναπτύχθηκαν κατά την αλληλεπίδραση των γαλαξιών (που έγινε πριν από 200 εκατομμύρια χρόνια) και εκτόξευσαν στον μεσογαλαξιακό χώρο τεράστιες ποσότητες αερίου και μαζί με αυτό και σκόνη. Οι προβλέψεις για την ύπαρξη σκόνης στις περιοχές αυτές, επιβεβαιώθηκε πρόσφατα από τις υπέρυθρες εικόνες που λήφθηκαν από το διαστημικό τηλεσκόπιο Herschel. / This thesis is based on deep optical CCD images which large have been obtained in the light of Hα+[N II], [O III] and [S II]. The resulting mosaic covers an area of 1.4º ´ 1.0º, where filamentary and diffuse emission was discovered, suggesting the existence of more than one supernova remnants (SNRs) in the area. Deep long slit spectra were also taken at eight different regions. Both the flux calibrated images and the spectra show that the emission of the filamentary structures originates from shock-heated gas, while photo-ionization mechanism is responsible for the diffuse emission. In most case, the optical emission is found to be well correlated with the radio at 1420 MHz and 4850 MHz, suggesting their association. The presence of the [O III] 5007 emission line in one of the candidate SNRs suggests shock velocities into the interstellar "clouds" of >100 Km/s, while the absence in the other indicates slower shock velocities. For all candidate remnants the [S II] λλ 6716/6731 ratio indicates electron densities below 270 cm-3, while the Hα emission has been measured to be between 0.6 to 41´10-17 erg s-1 cm-2 arcsec-2. The detected optical emission could be part of a number of supernovae explosions and the possibility that it is within an OB association can not be ruled out. It will then be the first optical discovery of SRNs within a bubble or superbubble in our Galaxy. Further, the study of those areas, also provides information for the star formation history of the Galaxy. Galactic dust constitutes approximately half of the elements more massive than helium produced in stellar nucleosynthesis. Notwithstanding the formation of dust grains in the dense, cool atmospheres of late-type stars, there still remain huge uncertainties concerning the origin and fate of galactic stardust. In this Letter, we identify the intergalactic medium (i.e., the region between gravitationally bound galaxies) as a major sink for galactic dust. We discover a systematic shift in the color of background galaxies viewed through the intergalactic medium of the nearby M81 group. This reddening coincides with atomic, neutral gas previously detected between the group members. The dust–to–H I mass ratio is high (1/20) compared to that of the solar neighborhood (1/120), suggesting that the dust originates from the center of one or more of the galaxies in the group. Indeed, M82, which is known to be ejecting dust and gas in a starburst-driven superwind, is cited as the probable main source.
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Discovery of a Protocluster Associated with a Ly α Blob Pair at z = 2.3

Bădescu, Toma, Yang, Yujin, Bertoldi, Frank, Zabludoff, Ann, Karim, Alexander, Magnelli, Benjamin 23 August 2017 (has links)
Bright Ly alpha blobs (LABs)-extended nebulae with sizes of similar to 100 kpc and Ly alpha luminosities of similar to 10(44) erg s(-1)-often reside in overdensities of compact Ly alpha emitters (LAEs) that may be galaxy protoclusters. The number density, variance, and internal kinematics of LABs suggest that they themselves trace group-like halos. Here, we test this hierarchical picture, presenting deep, wide-field Ly alpha narrowband imaging of a 1 degrees x. 0 degrees.5 region around a LAB pair at z = 2.3 discovered previously by a blind survey. We find 183 Lya emitters, including the original LAB pair and three new LABs with Ly alpha luminosities of (0.9-1.3) x 10(43) erg s(-1) and isophotal areas of 16-24 arcsec2. Using the LAEs as tracers and a new kernel density estimation method, we discover a large-scale overdensity (Bootes J1430+3522) with a surface density contrast of delta(Sigma) = 2.7, a volume density contrast of delta similar to 10.4, and a projected diameter of approximate to 20 comoving Mpc. Comparing with cosmological simulations, we conclude that this LAE overdensity will evolve into a present-day Coma-like cluster with log(M/M-circle dot) similar to 15.1 +/- 0.2. In this and three other wide-field LAE surveys re-analyzed here, the extents and peak amplitudes of the largest LAE overdensities are similar, not increasing with survey size, and implying that they were indeed the largest structures then and today evolve into rich clusters. Intriguingly, LABs favor the outskirts of the densest LAE concentrations, i.e., intermediate LAE overdensities of delta(Sigma) = 1-2. We speculate that these LABs mark infalling protogroups being accreted by the more massive protocluster.
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Probing the Intergalactic Medium with high-redshift quasars

Calverley, Alexander Peter January 2011 (has links)
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.

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