Erni, Peter: Early Nucleosynthesis Studies with Quasar Absorption Line Spectroscopy. - Bonn, 2007. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
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author = {{Peter Erni}},
title = {Early Nucleosynthesis Studies with Quasar Absorption Line Spectroscopy},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2007,
note = {Studying the light of celestial objects in different wavelengths is a fundamental approach and very common in astronomy, where large ground based telescopes and sophisticated high-resolution spectrographs became recently available to the scientific community. The smaller spectrographs aboard satellites are equally important because they are not restricted to the optical band and not affected by any atmospheric aberrations. Both, the large ground based instruments and satellites made it possible to extend the spectroscopic study of the Universe onto a cosmological scale.
The Universe is almost empty and has an average density of only about one atom per cubic meter. However, the density is clearly not uniform and ranges from regions almost devoid of any matter (voids) through regions with relatively high densities (galaxy clusters and galaxies) to objects with very high densities (stars). Only about 30% of the baryonic matter content in the present-day Universe is found in a condensed phase, i.e., in the form of stars and galaxies. In the past there were fewer stars and more matter was present in gaseous form. At a redshift of z ≈ 2−3, i.e., more than 1010 years ago, when the Universe had only about 15−25% of its present age, over 95% of the (ordinary) baryonic matter was solely present in the form of gas. This epoch is of particular interest because it marks the time when most of the galaxies started to form and to condense out of the intergalactic medium.
Quasars, a kind of cosmological beacons, are extremely bright and some of the most distant objects known to date. Their light was emitted at a time when the Universe had only about 10% of its present age. Hence, their light travels over vast distances before we observe it here on Earth. Light that encounters gas is absorbed in very well defined wavelength ranges. Following the laws of quantum mechanics, only photons with specific energies are absorbed and will cause specific absorption lines that arise mostly from the Lyman transitions of neutral hydrogen, the most abundant element in the Universe. The expansion of the Universe stretches the photons’ wavelengths (Hubble redshift) on their way to Earth. Since neutral hydrogen clouds at different distances will always encounter photons at different wavelengths, each individual cloud leaves its fingerprint in form of an absorption line at a different position, λ, in the observed spectrum. The Lyman absorption lines, and in particular lines that arise from the Lyman a transition, are so frequent and densely packed that it is common to speak of them as the Lyman a forest. With the Lyman a forest absorption lines it is possible to investigate the properties of the intergalactic medium, e.g., to determine the density, temperature or occurrence of the intervening gas clouds containing neutral hydrogen. Furthermore, the search for lines from other elements (matching in redshift), like carbon, nitrogen, oxygen, silicon, etc., allows us to study the abundance of heavier elements, too.
A damped Lyman a system is a gas cloud with a high column density of neutral hydrogen and shows broad absorption lines with typical Lorentzian damping wings. Number statistics of damped Lyman α systems imply that these objects dominate the neutral gas content of the Universe at z > 1, making them prime candidates to be the progenitors of present-day galaxies. The present work is about damped Lyman α absorption line systems and their involvement in early nucleosynthesis enrichment. Chapters 3−5 are a general introduction to the properties of the intergalactic medium, intergalactic absorption lines, and the technique of quasar absorption line spectroscopy. The subsequent chapters focus on the analysis and interpretation of particular lines of sight that had been studied in detail, except for Chapter 7 that extends the finding from the previous chapter and gives more general considerations on very early nucleosynthesis.
Chapter 3 – A short overview of the properties of the intergalactic medium is given. Different environments, ranging from low to high densities, are introduced. The common classification for quasar absorption line systems is given and I show that the gas density is closely related to the gas temperature. Further, the process of ionization is exemplified and I give clear arguments why the bulk of the gas in the intergalactic medium is ionized by photons from the UV background radiation, and why collisional ionization only becomes significant in hotter and denser regions such as the gas in galaxy clusters or the gas in galaxies.
Chapter 4 – In this chapter I discuss the nature of absorption lines that arise when the quasar’s light travels through intergalactic or interstellar gas. I derive the line profile (Voigt profile), a convolution of the Lorentz function that describes the finite width and the Maxwellian velocity distribution function that accounts for the intrinsic particle motion in the gas itself (Doppler velocity or temperature, respectively). Measures, such as the radiation intensity, the optical depth, the column density, and the Doppler parameter are introduced and set in context to each other. Finally, I discuss different effects of line broadening and saturated absorption lines.
Chapter 5 – This chapter is dedicated to the data reduction techniques I have used in this work. The technical specifications and observing capabilities of the UVES/VLT spectrograph are given and the different setup configurations are explained. After column densities and Doppler parameters are measured in the observed spectra, using the previously described measuring methods, the data have to be corrected for altering effects due to photoionization and dust within the absorption line system itself. In general, only a few ionization states can be observed and, hence, the derived column densities contribute only partially to the total column density of an element (ionization correction). Dust also plays an important role because gas-phase atoms will stick onto dust grains and eventually cause a change in the observed metal abundance pattern (dust depletion). Finally, I give some details on logarithmic error calculus because in log-space the normal distribution and therefore the standard error calculus methods are no longer applicable.
Chapter 6 – In this chapter I report on the damped Lyman α system at zabs = 2.6183 toward the quasar Q0913+072. I have analyzed UVES/VLT highresolution data from the ESO archive in detail and discovered remarkable facts: not only is this damped Lyman a the most metal deficient absorption line system of its kind at z < 3, it was also possible to measure its CNO elements with high accuracy. This and the comparison of its characteristic metal abundance pattern with yields from model calculations allows us to conclude on the type of stars that must have previously enriched this absorption line system.
Chapter 7 – I discuss the mechanism for star formation in primordial or poorly enriched gas clouds, where the cooling function is of particular interest. The different fragmentation mechanism in pristine gas clouds implies that the first stars in the Universe might have been very massive with masses of up to 600M. Stars that are so massive do no longer explode as (typical) core-collapse supernovae but undergo a different explosion mechanism due to an electron-positron pair instability. This in mind, I make use of the same approach as in the previous chapter and compare the abundance pattern of very metal deficient objects with results from model calculations to derive constraints on the first stellar generations in the Universe. The method is applied to 59 damped Lyman α systems, to 21 extremely metal-poor stars, and to 2 hyper metal-poor stars.
Chapter 8 – The line of sight toward the quasar Q0420−388 represents a particular case of its own. At a redshift of zabs = 3.088 we can identify two interacting absorption line systems, only separated by 160kms−1 (Δz = 0.0005), that are manifestly different in their structure and their metal content. While one of these two absorption line systems shows a metal content of 5% solar and a clear structure with six individual subcomponents, we can identify in the other absorption line (almost) no substructure but a metal content of 35% solar. Additionally, this line of sight offers one of the rare opportunities to measure deuterium at high redshift. Even though the D/H ratio is nowadays measured by the cosmic microwave background radiation, it is important to study the deuterium abundance also directly. The deuterium abundance we derive is in very good agreement with the predictions from the standard big bang nucleosynthesis model and other measurements. Further, we detect highly ionized species in between the two absorption line systems that are most likely heated by collisional ionization – clear indications that we are witnessing the merging of two early galaxies at high redshift.
Chapter 9 – So-called dark clumps are putative non-luminous mass concentrations with masses on the orders of galaxies or galaxy clusters, found in large weak gravitation lensing surveys. Yet, there is no known mechanism that could efficiently prohibit star formation in such an object. Hence, the nature of dark clumps or their mere existence remains mysterious. The technique of quasar absorption line spectroscopy, completely independent of gravitational effects and the luminosity of the object under study, offers an elegant alternative approach to this problem. I compared 63 dark clump candidates with over 60 000 known quasars and found, by pure coincidence, a quasar with adequate redshift, brightness, and separation in projection, in order to detect any gas that would be associated with one of these dark clumps – if present. From the observational data we derive restrictive upper limits for the transition lines of single ionized magnesium (and other species). Thus, we conclude that the detected lensing signal in this particular case (line of sight toward the quasar 004345.8−294733) is most likely not due to the presence of a significant (baryonic) mass concentration but presumably the result of statistical inhomogeneities, i.e., a so-called statistical fluke.
Chapter 10 – The aim of our ESO proposal 077.B–0758(A), as presented in this chapter, is the investigation of the temperature and ionization of the intergalactic medium at a redshift of z ≈ 2. We have applied for very high-resolution (R ∼ 75000) observation with the UVES/VLT spectrograph that would allow us to resolve the subcomponent structures of C IV and O VI absorption lines toward the quasar PKS 1448–232. These subcomponent structures could not be resolved in the past because any previous high-resolution observation of such systems are carried out with resolutions on the order of typically R ∼ 45000. Although our observation was only partially carried out, we have obtained a spectrum with very high resolution but a somewhat modest signal-to-noise ratio. Yet, the quality of the data was already sufficient to resolve some subcomponents. The remaining observation blocks are reported to the forthcoming observation period P79. The complete data set will allow us accurate measures of the component structure and line widths of the intervening O VI and C IV absorbers.},

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