Exploring the Universe with Quasar Absorption Spectra: correlations among tracers of the mass density field and the impact of ionizing background intensity fluctuations

The work we present in this manuscript revolves around the use of quasar spectra to probe the Universe. Absorption features in the electromagnetic spectrum of bright distant quasars by ionized hydrogen give us access to the distribution of gas in the Universe. Because the light from distant quasars is redshifted as it travels through the Universe, photons emitted with an energy below the Lyα transition will be absorbed along their journey, at the point where their wavelength has been redshifted to the transition wavelength of 1216 A. As a result, an observed spectrum and its features, referred to as Lyα Forest, depend on the density of neutral hydrogen as a function of position along the line of sight from the quasar. Similar studies can be made with photons blueward of the emission line of the triply ionized carbon. Studying the distribution of this ionized gas on cosmological scales allows us to trace the underlying matter density field while at the same time gaining insight on the thermal state of the intergalactic medium by studying the small-scale correlations of the Lyα absorption. Part I of this manuscript, Introduction to the Universe introduces all the physical and observational notions necessary for the understanding of this doctoral work. In chapter 1, we put in context the field of cosmology and its motivations as well as acquaint ourselves with the standard models that made consensus as best describing our Universe at this time. Chapter 2 presents the formalism and notions of cosmology used in the following chapters. Additional clarifications are available for the interested reader in appendix A. Chapter 3 introduces useful concepts for the use of quasars and their spectra as observables for cosmological surveys. Part II starts with chapter 4, On the effect of the ionizing background on the Lyα forest autocorrelation function. In this work, we present an analytical framework to understand the effects of a fluctuating intensity of the cosmic ionizing background on the correlations of the Lyα Forest transmission fraction measured in quasar spectra. In the absence of intensity fluctuations, , and in the limit of large scales at which the linear regime is applicable, the Lyα power spectrum should have the expected cold dark matter power spectrum with redshift distortions in the linear regime, with a bias factor bδ and a redshift distortion parameter β that depend on redshift but are independent of scale. We found that the intensity fluctuations introduce a scale dependence in both bδ and β, but keeping their product bδβ fixed. We also showed that these fluctuations do not bias current or future measurements of the baryon acoustic oscillation (BAO) scale from Lyα Forest. Only the amplitude of the Lyα autocorrelation is affected, the position of the BAO peak is not. Chapter 5 is titled Quasar - CIV Forest cross-correlation. In a similar fashion that the Lyα Forest is used to probe neutral hydrogen clouds, we use the absorption features of triply ionized carbon (CIV) to (a) probe metal enriched gas at redshift 1:4 < z < 4:2, (b) trace structures at z < 2 that were not accessible with the Lyα Forest. Thanks to the data provided by the full BOSS survey, there is a sufficient increase in quasar number density, compared to previous data sets available, to offset the lower sensitivity of the CIV Forest. As a result, we have made the first detection using the continuum transmission approach of a signal from the Quasar - CIV forest cross-correlation.This detection allows to measure the transmission CIV bias factor at redshift z = 2:3:(1 + βCIV )bCIV = -0.024 ± 0.003