This projects studies which early galaxy populations will be visible using the telescopes JWST and Euclid. Galaxy luminosity functions are calculated for different redshifts, with the galaxy number density as a function of apparent magnitude. The apparent magnitude is used to enable easy comparisons with the observational limits of JWST and Euclid. Added to the calculations were also the impact of gravitational lensing and how it may magnify the flux of the galaxies.  Another part of the project studied the impact of a lowest DM halo mass, the limit of the halo mass needed to create a galaxy. The existence of such a limit changes the luminosity function at low luminosities. The aim was to study if this change will be visible using the telescopes. The studies was done using a semi-analytical model of high-redshift galaxies with a Python interface.  The results showed that the visible galaxy populations varies significantly with both redshift and magnification. For lower redshifts and higher magnification more galaxies, mostly for low luminosities, are visible. The lowest DM halo masses needed to be noticeable by the telescopes was between Mmin = 2.0 × 1010 − 1.6 × 1011 M⊙. With a magnification of factor ten or 100 they instead lay between Mmin = 2.2 × 109 − 2.8 × 1010 M⊙. Compared to previous studies the effect from the limiting mass will most probably be visible by JWST with the magnification, while the values without magnification are close to the limit. For Euclid deep field the effects are not predicted to be visible even with a magnification of factor ten, but they will probably be visible with a higher magnification of a factor 100. / I detta projekt studeras vilka tidiga galaxpopulationer som kommer vara synliga med teleskopen JWST och Euclid. Galaxluminositetsfunktioner beräknas för olika rödförskjtningar med galaxtätheten som en funktion av apparent magnitud. Just apparent magnitud används för att jämförelser med de observationella gränserna för JWST och Euclid ska vara enkla att genomföra. Gravitationslinser och hur de kan förstärka galaxers luminositet lades även till beräkningarna.  I projektet studerades även hur en lägsta massa för mörk materia halos kan påverka beräkningarna. Denna massa är då gränsen för halomassan som behövs för att en galax ska kunna skapas. Ifall en sådan begränsning finns så ändras luminositetsfunktionerna för låga luminositeter. I detta projekt undersöktes det ifall denna förändring kommer vara synlig med teleskopen. Fo ̈r att utföra projektet användes en semi-analytisk modell av galaxer med hög rödförskjutning, med ett gränssnitt i Python.  Resultaten visar att de synliga galaxpopulationerna varierade starkt när förstärkning av galaxluminositet eller rödförskjutning ändrades. Med lägre rödförskjutning och högre magnifikation syntes fler galaxer, och de främsta förändringarna skedde för låga lumi- nositeter. De halomassor som behövdes för att vara synliga med teleskopen var mellan Mmin = 2.0 × 1010 − 1.6 × 1011 M⊙ utan förstärkning av luminositeten och mellan Mmin = 2.2 × 109 − 2.8 × 1010 M⊙ med en förstärkning av faktor tio eller 100. Jämfört med tidigare studier så kommer förändringarna troligtvis vara synliga med JWST ifall en förstärkning inkluderas. Utan magnifikation ligger massorna precis på gränsen. För Euclid deep field kommer effekterna ej vara synliga ens med en förstärkning av faktor tio, men de kommer troligtvis vara synliga med en högre magnifikation av faktor 100.

Depuis le milieu des années 1990, la taille des échantillons de galaxies situées à très grande distance (“redshift”) de nous, au tout début de la vie de l'univers a augmenté grace à la sensibilité croissante des télescopes optiques / infrarouge proche. Cependant, les propriétés des poussières continues dans ces galaxies sont mal connues en raison de l'absence d'observations profondes en infrarouge lointain et en sous-mm. Cette thèse explore les propriétés multi-longueur d'onde d'une population de galaxies observées ~ 2Gyr après le Big Bang. Notre échantillon comprend 22000 galaxies, et il a été sélectionné à partir de la discontinuité de Lyman. Nous utilisons une technique statistique, appelée analyse d'empilement, qui combine le signal provenant d'un grand nombre de sources individuellement non détectées dans le but d’ameliorer les limites de détection par rapport aux observations actuelles, à ces longueurs d'onde. Elle nous permet d'obtenir une distribution spectrale d’énergie complete de l’ultraviolet lointain à l’infrarouge lointain, et d'étudier la formation des étoiles et l'atténuation par la poussière sur ces galaxies. / Since the mid-1990s, the sample of galaxies in the early universe has been growing thanks to the increasing sensitivities in the Optical/NIR telescopes. However, their dust properties are poorly known due to the lack of deep far-infrared or sub-mm observations. This thesis explores the multi-wavelength properties of a population of galaxies observed ~2Gyr after the Big Bang. Our sample includes 22000 galaxies, and it has been selected by the classical U-dropout or Lyman Break technique. We use a statistical technique, called stacking analysis, that combines the signal from a large number of sources to lower the detection limits on the current long wavelengths observations. It allows us to obtain data over the full FUV-to-FIR spectral domain, and study the star formation and dust attenuation of these galaxies.

<p>Quasars form one of the most energetic phenomena in the universe, and can be traced out to very large redshifts. By studying the galaxies which host the active nuclei, important insights can be gained into the processes that trigger and maintain the quasar powerhouse. The evolution rate of the quasar population is furthermore similar to that of ordinary galaxies, which implies a connection between black hole accretion and star formation in the host galaxies. While the properties of quasar host galaxies at low redshift have become better constrained in recent years, less is known about hosts at earlier cosmic epochs. In addition, though radio-quiet quasars are by far more common than their radio-loud counterparts their host galaxies have not been studied to the same extent, in particular not at higher redshifts.</p><p>An imaging campaign of a large sample of quasars at intermediate redshift (0.4 < z < 0.8) was carried out at optical wavelengths using the Nordic Optical Telescope, and is studied in this thesis together with two smaller samples. The joint material forms more than half of the total number of observed sources in this redshift interval and increases the number of resolved radio-quiet hosts at z>0.4 considerably. The morphology and mean magnitudes are found to be similar for radio-loud and radio-quiet host galaxies. Both types of host are shown to have optical colours as blue as those of present-day late-type spirals and starburst galaxies, which is likely the result of ongoing star formation.</p><p>With increasing redshift, observations of host galaxies become more difficult. High spatial resolution can be achieved with adaptive optics, but the variation of the point spread function in the near-infrared wavelength band which is most suited for detection is large and rapid. A statistical approach to the problem of characterizing the point spread function has been developed, making use of simulated objects which are matched to the different atmospheric conditions. Bright, compact host galaxies showing signs of merging and interaction were detected in this way for three quasars at z~2.2, which were observed with the ESO 3.6 m telescope. The method is not restricted to host galaxy analysis but can be utilized in other applications as well, provided that the underlying extended source can be described by an analytical model. </p>

<p>Most of the matter in the Universe appears to be in some form which does not emit or absorb light. While evidence for the existence of this dark matter has accumulated over the last seventy years, its nature remains elusive. In this thesis, quasars and low surface brightness galaxies (LSBGs) are used to investigate the properties of the dark matter. </p><p>Quasars are extremely bright light sources which can be seen over vast distances. These cosmic beacons may be used to constrain dark matter in the form of low-mass, compact objects along the line of sight, as such objects are expected to induce brightness fluctuations in quasars through gravitational microlensing effects. Using a numerical microlensing model, we demonstrate that the uncertainty in the typical size of the optical continuum-emitting region in quasars represents the main obstacle in this procedure. We also show that, contrary to claims in the literature, microlensing fails to explain the observed long-term optical variability of quasars. Here, quasar distances are inferred from their redshifts, which are assumed to stem from the expansion of the Universe. Some astronomers do however defend the view that quasar redshifts could have a different origin. A number of potential methods for falsifying claims of such non-cosmological redshifts are proposed. </p><p>As the ratio of dark to luminous matter is known to be unusually high in LSBGs, these objects have become the prime targets for probing dark matter halos around galaxies. Here, we use spectral evolutionary models to constrain the properties of the stellar populations in a class of unusually blue LSBGs. Using rotation curve data obtained at the ESO Very Large Telescope, we also investigate the density profiles of their dark halos. We find our measurements to be inconsistent with the predictions of the currently favoured cold dark matter scenario.</p>

Quasars form one of the most energetic phenomena in the universe, and can be traced out to very large redshifts. By studying the galaxies which host the active nuclei, important insights can be gained into the processes that trigger and maintain the quasar powerhouse. The evolution rate of the quasar population is furthermore similar to that of ordinary galaxies, which implies a connection between black hole accretion and star formation in the host galaxies. While the properties of quasar host galaxies at low redshift have become better constrained in recent years, less is known about hosts at earlier cosmic epochs. In addition, though radio-quiet quasars are by far more common than their radio-loud counterparts their host galaxies have not been studied to the same extent, in particular not at higher redshifts. An imaging campaign of a large sample of quasars at intermediate redshift (0.4 &lt; z &lt; 0.8) was carried out at optical wavelengths using the Nordic Optical Telescope, and is studied in this thesis together with two smaller samples. The joint material forms more than half of the total number of observed sources in this redshift interval and increases the number of resolved radio-quiet hosts at z&gt;0.4 considerably. The morphology and mean magnitudes are found to be similar for radio-loud and radio-quiet host galaxies. Both types of host are shown to have optical colours as blue as those of present-day late-type spirals and starburst galaxies, which is likely the result of ongoing star formation. With increasing redshift, observations of host galaxies become more difficult. High spatial resolution can be achieved with adaptive optics, but the variation of the point spread function in the near-infrared wavelength band which is most suited for detection is large and rapid. A statistical approach to the problem of characterizing the point spread function has been developed, making use of simulated objects which are matched to the different atmospheric conditions. Bright, compact host galaxies showing signs of merging and interaction were detected in this way for three quasars at z~2.2, which were observed with the ESO 3.6 m telescope. The method is not restricted to host galaxy analysis but can be utilized in other applications as well, provided that the underlying extended source can be described by an analytical model.

Most of the matter in the Universe appears to be in some form which does not emit or absorb light. While evidence for the existence of this dark matter has accumulated over the last seventy years, its nature remains elusive. In this thesis, quasars and low surface brightness galaxies (LSBGs) are used to investigate the properties of the dark matter. Quasars are extremely bright light sources which can be seen over vast distances. These cosmic beacons may be used to constrain dark matter in the form of low-mass, compact objects along the line of sight, as such objects are expected to induce brightness fluctuations in quasars through gravitational microlensing effects. Using a numerical microlensing model, we demonstrate that the uncertainty in the typical size of the optical continuum-emitting region in quasars represents the main obstacle in this procedure. We also show that, contrary to claims in the literature, microlensing fails to explain the observed long-term optical variability of quasars. Here, quasar distances are inferred from their redshifts, which are assumed to stem from the expansion of the Universe. Some astronomers do however defend the view that quasar redshifts could have a different origin. A number of potential methods for falsifying claims of such non-cosmological redshifts are proposed. As the ratio of dark to luminous matter is known to be unusually high in LSBGs, these objects have become the prime targets for probing dark matter halos around galaxies. Here, we use spectral evolutionary models to constrain the properties of the stellar populations in a class of unusually blue LSBGs. Using rotation curve data obtained at the ESO Very Large Telescope, we also investigate the density profiles of their dark halos. We find our measurements to be inconsistent with the predictions of the currently favoured cold dark matter scenario.

During the quasar era (redshifts between 1 and 3) Radio Galaxies (RGs) have been claimed to have substantially influenced the growth and evolution of large scale structures in the universe. In this dissertation I test the robustness of these exciting claims. In order to probe the impacts in more detail, good theoretical models for such RG systems are required. With this motivation, I seek to develop an essentially analytical model for the evolution of Fanaroff-Riley Class II radio galaxies both as they age individually and as their numbers vary with cosmological epoch. To do so, I first compare three sophisticated semi-analytical models for the dynamical and radio lobe power evolution of FR II galaxies, those given by Kaiser, Dennett-Thorpe & Alexander (1997, KDA), Blundell, Rawlings, & Willott (1999, BRW) and Manolakou & Kirk (2002, MK). I perform multi-dimensional Monte Carlo simulations leading to virtual radio surveys. The predictions of each model for redshift, radio power (at 151 MHz), linear size and spectral index are then compared with data. The observational samples are the low frequency radio surveys, 3CRR, 6CE and 7CRS, which are flux-limited and redshift complete. I next perform extensive statistical tests to compare the distributions of model radio source parameters and those of the observational samples. The statistics used are the 1-Dimensional and 2-Dimensional Kolmogorov-Smirnov (K-S) tests and the 4-variable Spearman partial rank correlation coefficient. I search for and describe the "best" parameters for each model. I then produced modifications to each of the three original models, and extensively compare the original and the modified model performances in fitting the data. The key result of my dissertation is that using the Radio Luminosity Function of Willott et al. (2001) as the redshift birth function of radio sources, the KDA and MK models perform better than the BRW models in fitting the 3CRR, 6CE and 7CRS survey data when using K-S based statistical tests, and the KDA model provides the best fits to the correlation coefficients. However, no pre-existing or modified model can provide adequate fits for the spectral indices. I also calculate the volume fraction of the relevant universe filled by the generations of radio galaxies over the quasar era. This volume filling factor is not as large as estimated earlier. Nonetheless, the allowed ranges of various model parameters produce a rather wide range of astrophysically interesting relevant volume fraction values. I conclude that the expanding RGs born during the quasar era may still play significant roles in the cosmological history of the universe.

We calculate the angular power spectrum of the cosmic microwave background temperature fluctuations induced by the kinetic Sunyaev-Zel'dovich (kSZ) effect from the epoch of reionization (EOR). We use detailed N-body+radiative-transfer simulations to follow inhomogeneous reionization of the intergalactic medium. For the first time, we take into account the "self-regulation" of reionization: star formation in low-mass dwarf galaxies or minihalos is suppressed if these halos form in the regions that were already ionized or Lyman-Werner dissociated. Some previous work suggested that the amplitude of the kSZ power spectrum from the EOR can be described by a two-parameter family: the epoch of half-ionization and the duration of reionization. However, we argue that this picture applies only to simple forms of the reionization history which are roughly symmetric about the half-ionization epoch. In self-regulated reionization, the universe begins to be ionized early, maintains a low level of ionization for an extended period, and then finishes reionization as soon as high-mass atomically cooling halos dominate. While inclusion of self-regulation affects the amplitude of the kSZ power spectrum only modestly (~10%), it can change the duration of reionization by a factor of more than two. We conclude that the simple two-parameter family does not capture the effect of a physical, yet complex, reionization history caused by self-regulation. When added to the post-reionization kSZ contribution, our prediction for the total kSZ power spectrum is below the current upper bound from the South Pole Telescope. Therefore, the current upper bound on the kSZ effect from the EOR is consistent with our understanding of the physics of reionization. / text

Depuis une quinzaine d'années, de nombreuses galaxies sont détectées grâce à leur raie d'émission Lyman-alpha à des décalages spectraux supérieurs à 3. Ces objets, dits Émetteurs Lyman-alpha, nous permettent ainsi de sonder les galaxies dans l'Univers lointain, alors âgé de moins de deux milliards d'années. Bien qu'un grand nombre d'Émetteurs Lyman-alpha ait été détecté jusqu'à présent, leurs propriétés statistiques et physiques sont encore mal contraintes. En effet, les observations sont difficiles à interpréter du fait des effets de transfert résonnant des photons Lyman-alpha dans le milieu interstellaire et de la cinématique du gaz. En plus des observations, des efforts théoriques sont donc nécessaires pour mieux comprendre les caractéristiques physiques de ces objets, leur rôle dans le scénario de formation hiérarchique des galaxies, et leur lien avec l'autre grande populations de galaxies lointaines, les galaxies à discontinuité de Lyman. Cette thèse a pour but de proposer une modélisation originale de cette population d'Émetteurs Lyman-alpha dans le contexte cosmologique. La formation et l'évolution hiérarchique des galaxies est décrite grâce au modèle hybride GALICS, associant une simulation à N corps de matière noire, à des prescriptions semi-analytiques pour modéliser la physique des galaxies. GALICS prédit les propriétés physiques d'un large échantillon de galaxies entre z~3 et 7. Les propriétés de la raie Lyman-alpha sont obtenues grâce au couplage de GALICS avec une librairie de modèles numériques, réalisés avec le code MCLya. Avec cette approche, la fraction d'évasion des photons Lyman-alpha et le profil de la raie émergent des galaxies peuvent ainsi être prédits, en prenant en compte les effets de transfert résonnant des photons Lyman-alpha et la cinématique du milieu. Le modèle prévoit une forte dispersion de la fraction d'évasion des photons Lyman-alpha fesc en fonction de leur taux de formation stellaire. Les galaxies peu massives, formant peu d'étoiles ont une fraction d'évasion Lyman-alpha de l'ordre de l'unité. En revanche, fesc est distribué entre 0 et 1 pour les objets plus massifs, formant intensément des étoiles, selon les propriétés physiques des galaxies. Les résultats du modèle donnent un accord satisfaisant avec la plupart des observations, en particulier les fonctions de luminosité Lyman-alpha et UV entre z~3 et 7. Le modèle parvient à reproduire conjointement les propriétés UV des galaxies sélectionnées en UV (galaxies à discontinuité de Lyman) et celles des Émetteurs Lyman-alpha. Nous trouvons que les Émetteurs Lyman-alpha sont des galaxies de masse modérée présentant des profils de raie asymétriques, en bon accord avec les données observationnelles. Le modèle prévoit notamment une forte abondance de galaxies de faible luminosité Lyman-alpha. Ces objets peu lumineux seront une des cibles privilégiées du spectrographe intégral de champ MUSE, qui sera prochainement installé au VLT. Dans le but d'aider à la préparation des futurs relevés que MUSE effectuera, des champs fictifs d'Émetteurs Lyman-alpha ont été construits grâce au modèle pour fournir des prédictions, notamment en terme de comptages et de variance cosmique. / Many galaxies have been detected at high redshift since the late nineties thanks to their strong Lyman-alpha emission line. These objects, knows as Lyman-alpha Emitters, allow us to probe galaxies in the first two Gigayears of the Universe. Although a large amount of detections, their statistical and physical properties are still poorly constrained. Indeed, observations are difficult to interpret, mainly due to line transfer effects in the interstellar mediaum and gas kinematics. In addition to observations, theoretical efforts are needed to reach a better understanding of the properties of these objects, their role in the scenario of hierarchical formation of galaxies, and their link with the other main high redshift galaxy population, the so-called Lyman Break galaxies. In this thesis, we model Lyman-alpha Emitters in the cosmological context with an original method. The hierarchical galaxy formation process is described with the GALICS model, which couples a N body simulation of Dark Matter, with semi-analytic prescriptions to model galaxy physics. GALICS can predict the physical properties of a large sample of mock galaxies between z~3 and 7. Lyman-alpha properties are computed thanks to the coupling of GALICS with a library of numerical tranfser models, generated with the MCLya code. With this approach, Lyman-alpha photons escape fractions and line profiles can be predicted, taking into account resonnant scattering effects and gas kinematics. We find a strong dispersion of the escape fraction with respect to the star formation rate of the galaxies. The model predictions are in good agreement with most of the observationnal data, especially the Lyman-alpha luminosity functions between z~3 and 7. The model is able to reproduce UV properties of UV and Lyman-alpha selected galaxies. We find that Lyman-alpha Emitters have moderate mass on average and display asymetric line profiles, as it is shown by the observations. In particular, we predict a strong abundance of faint Lyman-alpha Emitters. These objects will be a main target of the forthcoming MUSE instrument that will be installed at VLT. In order to help preparing future surveys with MUSE, mock fields of Lyman-alpha Emitters have been created with our model to make predictions, especially in terms of number counts and cosmic variance.

Le milieu circum-galactique (CGM pour "Circum-Galactic Medium" en anglais) constitue l'interface entre les galaxies et les grandes structures au sein desquelles elles évoluent. Le milieu inter-galactique est principalement composé de gaz d'hydrogène froid, dit primordial, qui en s'accretant sur les galaxies constitue le carburant de la formation stellaire. La formation stellaire apparait alors régulée par les échanges de matière entre la galaxie et l'extérieur. En ce sens, l'étude de l'environnement des galaxies se révèle cruciale pour comprendre les mécanismes qui régissent leur formation et leur évolution. L'observation directe du CGM est toutefois assez délicate en raison de la chute de brillance des galaxies dans leurs régions externes. Sa détection est d'autant plus difficile pour les galaxies de l'Univers lointain. Quelques techniques existent pour contrecarrer cette difficulté : l'observation du CGM en absorption dans le spectre d'un quasar brillant situe sur la ligne de visée de la galaxie, ou sa détection statistique en combinant de nombreuses images de galaxies. Ces techniques ont toutefois de sévères limitations car elles ne donnent que des informations parcellaires sur le CGM. Je rapporte dans cette thèse la détection de gaz d'hydrogène froid autour de 145 galaxies (soit 80% des galaxies testees) peu massives, peu lumineuses et très distantes, émettant de l'émission Lyα. Longtemps utilisée pour son pouvoir de détection des galaxies lointaines, l'émission Lyα est maintenant utilisée comme un traceur du gaz froid du CGM, alors observable sous forme de "halos" Lyα. Notre échantillon constitue le plus grand échantillon de halos Lyα détectés individuellement autour de galaxies de faible masse et ce, à une époque pendant laquelle l'Univers est en pleine construction. Ces avancées ont été rendues possible grâce à l'incomparable sensibilité de l'instrument MUSE installé sur le "Very Large Telescope" au Chili il y a bientôt 4 ans. Seule une centaine d'heures de télescope dans la région du champ ultra profond de Hubble ont été nécessaires pour permettre la détection de halos Lyα. Nos résultats confirment la présence de grande quantité de gaz froid dans l'environnement immédiat des galaxies distantes. Ces observations étaient en effet prédites par les modèles théoriques et les simulations numériques. En plus d'être quasi-omniprésents autour des galaxies, les halos Lyα observés montrent une diversité (taille, flux, forme, profil de la raie d'émission, etc) particulièrement remarquable dans une région du ciel si restreinte (9_×9_). De plus, la possibilité d'analyser le CGM galaxie par galaxie et en trois dimensions permet maintenant d'étudier de manière directe l'impact de l'environnement sur la galaxie mais aussi l'évolution des propriétés du CGM avec les époques cosmiques. Notre grand échantillon de galaxies nous a permis de réaliser un traitement statistique robuste et de mettre en évidence que les propriétés stellaires des galaxies étudiées ne sont pas systématiquement liées à celles de l'émission Lyα. Enfin, d'après les modèles théoriques, nos observations (spectroscopiques) indiquent la présence de matière en expansion dans et/ou autour des galaxies. La présence d'accrétion de matière est, quant à elle, moins bien contrainte par nos données. Finalement, l'analyse décrite dans ce manuscrit rapporte des informations importantes et inédites sur les propriétés du CGM d'une population de galaxies relativement peu lumineuses et très abondantes dans l'Univers lointain / The circum-galactic medium (CGM) serves as the interface between galaxies and the larger structures within which they evolve. Composed primarily of cold hydrogen gas (also called primordial gas), the CGM is a major fuel source for star formation as material falls onto a galaxy from its surrounding halo. This suggests that star formation is in fact regulated by gas exchange between a galaxy and its vicinity. Thus, studying the surrounding environment of galaxies represents a crucial step in understanding the mechanisms governing their formation and evolution. Unfortunately, direct observation of the CGM is often quite difficult, since these regions are very faint. This task becomes even more challenging for galaxies in the distant Universe, though some techniques have been developed for this purpose. The CGM can be detected through absorption features in the spectrum of a more-distant quasar located along a galaxy’s line of sight or statistically, by stacking many images of galaxies together, in order to increase the overall S/N ratio of the sample. However, these methods are not ideal : both have severe limitations and only provide partial information about the CGM. In this thesis, I report the detection of cold hydrogen gas surrounding 145 low-mass, faint and very distant galaxies emitting Lyα photons (forming 80% of the total galaxy sample used in this work). While historically, Lyα emission was seen simply as a powerful tool for detecting distant galaxies, it is now possible to use it as a tracer of cold CGM gas in the form of Lyα halos. The sample presented here represents the largest collection ever compiled of individually-detected Lyα halos around normal star forming galaxies, observed in an epoch when the Universe was still forming. This achievement is possible thanks to the unrivaled sensitivity of the Multi-Unit Spectroscopic Explorer (MUSE), a next-generation instrument installed on the Very Large Telescope (VLT). In particular, we need only 100 hours of telescope time to detect the presence of Lyα halos, a significant improvement over previous efforts. My results confirm the presence of large amounts of cold gas in the immediate vicinity of distant galaxies. While such results have been predicted by theoretical models and numerical simulations, this work provides some of the first direct observational evidence of this fact. Besides being quasi-ubiquitous around galaxies, the observed Lyα halos show a large diversity in physical properties which is particularly remarkable for such a small region of the sky (9_×9_). Moreover, the 3D galaxy-by-galaxy nature of my analysis allows me to study the direct impact of environment on galaxies, as well as the evolution of the CGM with cosmic time. With such a large sample, I am also able to perform a robust statistical analysis, highlighting the fact that the stellar properties of galaxies are not systematically linked to the Lyα ones. Finally, based on theoretical models, my (spectroscopic) observations indicate the presence of expanding materials inside and/or around the galaxies. However, the presence of galactic inflows are less constrained by the data. Taken as a whole, the analysis described in this thesis represents important, new information about the CGM properties of the relatively faint galaxies which make up the bulk of the galaxy population in the distant Universe. Therefore, this work should serve as a useful reference point as research into the CGM continues to advance