The first stars, galaxies and black holes in the universe produced large quantities of ionising UV radiation; forming H II regions in the neutral gas before the Epoch of Reionisation (EoR). These ionisation fronts will have come into contact with overdensities in the surrounding Intergalactic Medium (IGM), including haloes which were in the process of collapse. Previous studies have shown that the feedback processes on these secondary haloes can either disrupt the gas, or induce further cooling from the formation of molecular hydrogen. The ionising source will eventually die and create a defunct H II region, which expands into the surrounding neutral IGM. Minihaloes at the edge of these defunct H II regions are particularly susceptible to positive feedback due to not having been photoevaporated like their counterparts further inside the ionised volume. In this thesis, numerical simulations of minihaloes at the edges of H II regions formed by the first luminous objects before the EoR are presented. A methodology of including secondary ionisations from high energy photons is also implemented into the radiation hydrodynamical code ZEUS-RT. The interaction of differing spectral index sources with minihaloes including secondary ionisation is discussed. The secondary ionisations show the greatest effect for hard spectral sources with a large fraction of high energy photons; where a decrease in photoheating and an increase in ionisation rate is found at the outer reaches of the ionisation front (I-front). The increased ionisation rate lowers the optical depth of the gas for subsequent photons, and thus reduces the trapping of I-fronts in high densities found in the minihalo cores. The ratio of the free electron fraction to the temperature in the core of the minihaloes is found to constrain the resulting evolution. A high ratio is correlated with the creation of molecular hydrogen, which can then induce further cooling and the collapse of the system.A large parameter suite of 3780 two-dimensional minihalo models utilising radiative hydrodynamical simulations with 12 species and a coupled reaction network, including H2 cooling, HD cooling, Lyman-Werner radiation and secondary ionisation is performed. The parameter space includes: the spectral index representing different sources such as quasars or galaxies, the mass of the minihaloes from 105 - 106 Mʘ, the redshift of ionisation from z ~ 10 - 30, and other factors which denote the position of the minihalo relative to the boundary of the H II region. Minihaloes with average core densities of n0 = 2 - 10 cm-3 show almost unanimous positive feedback, while the majority of minihaloes under this average density are disrupted. Minihaloes with core densities above this value generally would have collapsed in isolation anyway, but are found to not be delayed by the I-front. The H2 fraction in the minihalo gas is also increased in models with no blowout by factors between 2 - 100 times that of an isolated minihalo. This is especially significant for lower redshift, z ≤ 15, minihaloes. Finally, a parameter suite of larger 106 - 107 Mʘ minihaloes results in the creation of self-gravitating clumps of gas moving out of the dark matter potential. The gas core is compressed by the I-front, enriched with molecular hydrogen, and ejected by the pressure front after the source dies. These "baryon bullets" could be progenitors of primordial globular clusters found in the haloes of galaxies today. Properties such as old stellar populations and the lack of dark matter haloes can be explained by this radiative ejection method. The dynamical nature of these interacting systems and diversity of subsequent evolution suggest that minihaloes less than 108 Mʘ are important in the early formation history of the universe, and in determining the constraining parameters of the EoR. The feedback mechanisms investigated, and secondary ionisation physics, should be included in astrophysical simulations and analytical calculations determining the evolution of the universe at this critical epoch.

In recent years the formation of primordial galaxies, cosmic metal enrichment, and hydrogen reionisation have been studied using both refined observations and powerful numerical simulations. High-redshift quasars have become a ubiquitous tool in the study of this era with more than 115 quasars now spectroscopically confirmed at z > 6.0. In this thesis, I use spectra of high-redshift quasars to provide improved observational constraints through a mixture of existing and new techniques. I first investigate the claim of neutral gas around the most distant known quasar, ULASJ1120+0641(J1120), with a cosmological redshift of z=7.1. Its spectrum shows a relatively weak Lyman-α emission line, which has been interpreted as evidence of absorption by neutral gas. Attributing this to a Gunn-Peterson damping wing has led to claims that the intergalactic medium is at least 10% neutral at that redshift. However, these claims rely on a reconstruction of the unabsorbed quasar emission. Initial attempts using composite spectra of lower-redshift quasars mismatched the CIV emission line of J1120, a feature known to correlate with Lyman-α and which is strongly blueshifted in J1120. I attempt to establish whether this mismatch could explain the apparently weak Lyman-α emission line. I find that among a C IV-matched sample the Lyman-α line of J1120 is not anomalous. This raises doubts as to the interpretation of absorbed Lyman-α emission lines in the context of reionisation. I then use a high quality X-Shooter spectrum of the same z=7 quasar to measure the abundances of diffuse metals within one billion years of the Big Bang. I measure the occurrence rates of CIV, CII, SiII, FeII and MgII, producing the first measurement at z > 6 for many of these ions. I find that the incidence of CIV systems is consistent with a continuing decline in the total mass density of highly ionized metals, a trend seen at lower redshifts. The ratio CII/CIV, however, seems to remain constant or increase with redshift, in line with predictions from models which include a decline of the ionising ultraviolet background. The evolution in MgII appears somewhat more complex; while the number density of strong systems continues to decline at high redshift,the number density of weak systems remains high and may even increase. This could signal an increase with redshift in the cross-section of low-ionisation metals. Large numbers of weak MgII systems are also seen at z∼2, suggesting they were already in place when reionisation was ending. I use this X-Shooter spectrum to study metal absorbers associated with the z=7 quasar itself. I find that one such absorber shows signs of only partially covering the line-of-sight, and investigate the possible implications for the quasar’s environment. Finally, I investigate the evolution of the intergalactic medium’s Lyman-α opacity using spectra of quasars at 5.7 < z < 7.1. I assemble a sample of 92 quasar spectra, more than 3 times larger than previous samples. The sample consists of quasars drawn from DES-VHS, SDSS and SHELLQs, new reductions of archival data, and new data. I develop methods to quantify the opacity distribution, providing measurements of the distribution function up to z=6.1. I find that the Lyman-α opacity evolves strongly with redshift. The scatter may be even larger than previously appreciated, posing a serious challenge for models of reionisation.

Cette thèse s'inscrit dans le domaine d'étude de l'Univers lointain, en particulier sur l'étude d'une période méconnue de notre Univers à savoir « la ré-ionisation ». Le choix de la sonde utilisée pour l'étude d'un Univers aussi lointain est quelque peu restreint. En effet il faut que la sonde soit visible à cette grande distance et donc que ce soit un objet extrêmement lumineux. Ainsi on se sert de galaxies très énergétiques avec un noyau galactique actif (appelées quasars) en tant que sonde. En étudiant le rayonnement de ces quasars on pourra alors caractériser l'environnement qui les entoure. Ces quasars étant des objets rares, il nous faut rechercher sur une grande surface du ciel pour espérer en obtenir quelques uns. Il existe beaucoup de projets d'observation dédiés à la cosmologie, un des futurs projets majeurs nommé Euclid sera un grand pas pour l'étude de la ré-ionisation. Ce futur satellite de l'ESA (lancement prévu en 2020) sera particulièrement performant dans l'infrarouge, permettant ainsi de détecter des quasars à des distances actuellement insondées. Afin de pouvoir bien préparer cette mission il nous faut établir et anticiper les performances attendues d'Euclid. Pour ce faire, il faut mener une étude similaire sur des données existantes puis extrapoler la performance d'Euclid. Pour ce faire, durant ma thèse, j'ai utilisé des données issues du télescope terrestre du CFHT (Canada France Hawaii Telescope) situé à hawaii au mont Mauna Kea. / The main topic of this thesis is the study of the distant Universe, particularly the study of a quite unknown period of our Universe namely "reionization." The probe's choice used in the study of a universe so far is somewhat restricted. Indeed it is necessary that the probe be visible at this distance and therefore the probe must be a very bright objet. Thus we used high-energy galaxies with an active galactic nuclei (called quasars) as a probe. By studying the emission of these quasars we can then characterize the environment around them. Because of the huge distance between us and the quasar and due to the expansion of the universe they have their light emissions shifted towards longer wavelengths. Thus, to study the young age of our universe, we need to observe objects in the infrared. These quasars are rare objects, we need to search over a large area of the sky to hope to get a few. There are many projects focused on observational cosmology, a future major project named Euclid will be a big step for the study of reionization. This future ESA satellite (launch planned in 2020) will be particularly effective in the infrared, thus allowing to detect quasars at distances currently unfathomed. In order to prepare this mission we must establish and predict the expected performance of Euclid. To do this, we must conduct a similar study on existing data and extrapolate the performance of Euclid. To do this, in my thesis, I used datas from the terrestrial telescope CFHT (Canada France Hawaii Telescope) located in hawaii at Mauna Kea.

No / The reactions of ionised pyridazine, aminopyrazine and aminopyridine and the corresponding ¿-distonic ions are examined by a combination of tandem mass spectrometric techniques, including analysis of metastable ion (MI), collision induced dissociation and neutralisation¿reionisation mass spectra (NRMS). Further insight into the relative stability and energy barriers towards tautomerism of each ionised heterocycle with its ¿-distonic isomer is obtained by computational methods. In all these systems, both the conventional radical-cation and the ¿-distonic tautomer are stable species which exist in discrete energy wells, with a significant barrier towards their interconversion. Although each ¿-distonic ion is sufficiently stable to survive neutralisation¿reionisation, the conventional ionised heterocycle is more stable in each case. The possibility of investigating proton-transport catalysis in the tautomerism of these ionic systems is discussed.

The first billion years from the beginning of the Universe is the focus of multiple astronomical facilities in the upcoming decade. This unique era is marked by the formation of the first stars and galaxies, which release ionising radiation into the intergalactic medium(IGM). As a result, these sources initiate a period during which the cold and dense IGM, primarily consisting of neutral hydrogen (HI ), is heated and ionised. We refer to this era as the Epoch of Reionisation (EoR). How the EoR transpired hence depends on the properties of these ionising sources, and this forms a vital piece to the puzzle of understanding the early Universe. This licentiate thesis aims to educate the reader on the power spectrum (PS) statistic of the 21-cm signal from HI during the EoR. The PS is a prospective observable by radio interferometers, such as the Low-Frequency Array (LOFAR) and the future Square Kilometre Array (SKA). It is an essential stepping stone in comprehending the dominant physical processes affecting the IGM at a given length scale during the EoR. In Paper I, we analyse the decomposition of the 21-cm PS from Lidz et al. (2007) (eq. 2)and study the evolution of its terms. We conduct our investigation for a set of C 2 -Ray and 21cmFAST simulations with volumes of (714 cMpc)3, concentrating on large-scales (k &lt; 0.3 Mpc−1 ) as the signal-to-noise ratio of observing these scales will be high. We find that the 21-cm PS tracks the PS of neutral hydrogen fluctuations, which itself past a certain scale tracks the matter PS after a global ionisation fraction of x̄HII∼ 0.1. Hence, the 21-cm PS possesses a two-regime form for which the large-scale PS is a biased version of the cosmological density field and the small-scale PS depends on the astrophysics of the EoR. We construct a bias parameter to explore whether the 21-cm PS can be used as a probe of cosmology on large k-scales. We discover a transition feature for both simulations, following the ktrans ≈ 2/λMFP empirical formula. The transition scale between the scale-independent and scale-dependent bias regimes is directly related to the value of the mean free path of ionising photons (λMFP ).

This thesis presents spectroscopic observations of z ≥ 7 galaxy candidates in the Hubble Ultra Deep Field, which were selected with HST WFC3 imaging, using the Lyman-Break technique. Four z-band (z ≈ 7) dropout galaxies were targeted with Gemini/GNIRS, one z-band dropout galaxy and three Y -band (z ≈ 8 − 9) dropout galaxies with VLT/XSHOOTER, and 22 z-band dropouts with VLT/FORS2, where 15 of the latter are strong candidates. No evidence of Lyman-α emission is found, and the upper limits on the Lyman-α flux and the broad-band magnitudes are used to constrain the rest-frame equivalent widths for this line emission. Amongst the targeted objects, observations were made of HUDF.YD3, a relatively bright Y -band dropout galaxy likely to be at z ≈ 8 − 9 on the basis of its colours in the HST ACS and WFC3 images. Lehnert et al. (2010) observed this galaxy using the VLT/SINFONI integral field spectrograph and claim that it exhibits Lyman-α emission at z = 8.55. In observations of this object described in this thesis, which were made with VLT/XSHOOTER and Subaru/MOIRCS, this line was not reproduced despite the expected signal in the combined MOIRCS & XSHOOTER data being 5σ. Hence it appears unlikely that the reported Lyman-α line emission at z > 8 is real. Accounting for incomplete spectral coverage, in total (across all spectro- graphs) 9.63 z-band dropouts and 1.15 Y -band dropouts are surveyed to a Lyman-α rest-frame Equivalent Width better than 75 ̊A. A model where the fraction of high rest-frame equivalent width emitters follows the trend seen at z = 3−6.5 is inconsistent with these non-detections at z = 7−9 at a confidence level of ∼ 91%, which may indicate that a significant neutral HI fraction (χHI) in the intergalactic medium suppresses the Lyman-α line at z > 7. In particular, the lack of detection of Lyman-α emission in this spectroscopy is compared with results at lower redshift by Stark et al. (2010), who derive a mapping between Lyman-α fractions and χHI based on radiative transfer simulations by McQuinn et al. (2007). These results suggest a lower limit of χHI ~ 0.5.

L'époque de la réionisation, qui s'étend pendant le premier milliard d'années de l'Univers, correspond à la période où les premières étoiles et galaxies apparaissent. Dans ce contexte, l'enjeu majeur de cette thèse est d'étudier la formation de ces premières structures et leur rétroaction radiative sur leur environnement. Pour cela, j'utilise différentes méthodes de simulations numériques permettant de modéliser le transfert radiatif dans les galaxies, dans un cadre cosmologique.En particulier, les plus petites galaxies seraient celles qui apportent la majorité des photons nécessaires à ioniser l'Univers. J'explore cette hypothèse dans une première partie à l'aide de simulations radio-hydrodynamiques avec le code RAMSES-RT, ciblant trois petites galaxies avec une très haute résolution spatiale et temporelle. Je me suis d'abord intéressé aux mécanismes régulant la production et le transfert de photons ionisants dans les galaxies, et j'ai montré que les explosions de supernovae en sont un facteur essentiel. Ensuite, j'ai exploré les propriétés observables dérivées à partir de ces simulations.Dans un second temps, j'ai développé une extension au code Monte Carlo de transfert radiatif MCLya prenant en compte la polarisation du rayonnement et l'émission diffuse dans une simulation. J'ai utilisé ce code pour post-traiter une simulation d'un blob Lyman-alpha, une source étendue d'émission Lyman-alpha, et étudier ses propriétés de polarisation à l'aide de pseudo-observations. Contrairement à ce qui était proposé précédemment, j'ai pu montrer que la polarisation n'était pas un indicateur aussi utile qu'espéré pour tracer l'origine des photons Lyman-alpha / The Epoch of Reionisation, which spans during the first billion year of te Universe, corresponds to the period during which the first stars and galaxy form. In this context, the main topic of this thesis is to study the formation of those early structures and their radiative feedback to their environment. For this purpose, I use various numerical simulations tools designed to model the radiative transfer in galaxies in a cosmological framework.More specifically, I look at very small galaxies, which are believed to contribute the bulk of the photons required to reionise the Universe. I explore this idea using radiative hydrodynamics simulations performed with RAMSES-RT, focusing on three small galaxies with a very high spatial and temporal resolution. I first detail the mechanism that regulate te production and escape of ionising photons in galaxies, and I show that supernovae explosions are a crucial element for this regulation. I then started to investigate the observable properties of those galaxies.In a second part of my thesis, I developped an extension to the Monte Carlo radiative transfer code MCLya to take light polarisation into account and to model the diffuse emission. I applied this code to post-process the simulation of a Lyman-alpha blob (an extended Lyman-alpha source), and to study its polarisation properties with mock observations. Contrary to what was suggested before, I showed that polarisation is not a strong tracer of the origin of Lyman-alpha photons