abstract: Galaxies with strong Lyman-alpha (Lya) emission line (also called Lya galaxies or emitters) offer an unique probe of the epoch of reionization - one of the important phases when most of the neutral hydrogen in the universe was ionized. In addition, Lya galaxies at high redshifts are a powerful tool to study low-mass galaxy formation. Since current observations suggest that the reionization is complete by redshift z~ 6, it is therefore necessary to discover galaxies at z > 6, to use their luminosity function (LF) as a probe of reionization. I found five z = 7.7 candidate Lya galaxies with line fluxes > 7x10-18 erg/s/cm/2 , from three different deep near-infrared (IR) narrowband (NB) imaging surveys in a volume > 4x104Mpc3. From the spectroscopic followup of four candidate galaxies, and with the current spectroscopic sensitivity, the detection of only the brightest candidate galaxy can be ruled out at 5 sigma level. Moreover, these observations successfully demonstrate that the sensitivity necessary for both, the NB imaging as well as the spectroscopic followup of z~ 8 Lya galaxies can be reached with the current instrumentation. While future, more sensitive spectroscopic observations are necessary, the observed Lya LF at z = 7.7 is consistent with z = 6.6 LF, suggesting that the intergalactic medium (IGM) is relatively ionized even at z = 7.7, with neutral fraction xHI≤ 30%. On the theoretical front, while several models of Lya emitters have been developed, the physical nature of Lya emitters is not yet completely known. Moreover, multi-parameter models and their complexities necessitates a simpler model. I have developed a simple, single-parameter model to populate dark mater halos with Lya emitters. The central tenet of this model, different from many of the earlier models, is that the star-formation rate (SFR), and hence the Lya luminosity, is proportional to the mass accretion rate rather than the total halo mass. This simple model is successful in reproducing many observable including LFs, stellar masses, SFRs, and clustering of Lya emitters from z~ 3 to z~ 7. Finally, using this model, I find that the mass accretion, and hence the star-formation in > 30% of Lya emitters at z~ 3 occur through major mergers, and this fraction increases to ~ 50% at z~7. / Dissertation/Thesis / Ph.D. Astrophysics 2011

Briser la frontière observationnelle de l’Univers lointain suscite de nombreux axes de recherches dans la cosmologie moderne. Exigeantes par la sensibilité demandée, ces observations constituent aussi bien un défi technologique pour les prochaines générations d’instruments qu’une étape nécessaire à la compréhension des différentes étapes d’évolution de l’Univers. Une des étapes importantes est la réionisation du milieu intergalactique, époque charnière marquant la fin des “Âges Sombres”. Déterminer quand et comment ce processus s’est déroulé et quelles ont été les sources l’ayant déclenché et entretenu sont les principales questions motivant ces recherches. Les travaux présentés dans cette thèse participent à l’effort de la communauté scientifique internationale dans la caractérisation de l’histoire de l’époque de la réionisation en recherchant des galaxies à grand décalage spectral vers le rouge (redshift). Après un panorama complet des contraintes observationnelles actuelles sur l’époque de la réionisation, sont ensuite présentés les deux projets ayant fournis les données constituant la matière première de cette analyse. En utilisant une même technique d’imagerie en bande étroite dans le domaine de longueurs d’onde proche-infrarouge, des observations avec l’instrument WIRCAM (CFHT) et d’autres avec l’instrument HAWK-I (VLT) ont permis d’établir des contraintes robustes sur la fonction deluminosité des Emetteurs Lyman-α au redshift z = 7.7. A l’occasion de la présentation du second projet, la problématique de la stratégie observationnelle est abordée, notamment via l’utilisation des amas de galaxies comme lentilles gravitationnelles amplifiant le flux des sources d’arrière-plan. L’absence de confirmation spectroscopique des Emetteurs Lyman-– sélectionnés durant l’analyse des données WIRCAM et l’absence de candidats dans le sondage HAWK-I suggèrent une évolution de la fonction de luminosité des Emetteurs Lyman-– entre les redshifts z = 6.5, telle quedéterminée dans d’autres études, et z = 7.7. La tendance de cette évolution, en luminosité ou en densité de galaxies, ne peut pas encore être déterminée compte tenu de la variabilité des résultats obtenus à plus bas redshift. Si cette évolutions’effectuait en luminosité, ce pourrait être le signe d’une augmentation de l’opacité du milieu intergalactique au rayonnement Lyman-α causée par une plus importante fraction d’hydrogène neutre, ce qui est une signature attendue de la réionisation. / Breaking through the observational frontier of the distant universe motivates numerous scientific cases in modern cosmology. Demanding by the required sensitivity, high-redshift observations provide both a technological challenge for next generation instruments and a necessary step toward the understanding of cosmic evolution. One critical epoch is the reionization of the intergalactic medium bringing to light the end of the so-called “Dark Ages”. Determine when and how this process took place and what were the sources that started and maintained the reionization are the mainquestions motivating this research. The work presented in this thesis contributes to the effort of the internationalscientific community in characterizing the reionization epoch history by searching for galaxies at high redshift. First, I describe the current observational constraints in a comprehensive chapter. Then the two datasets used in that work are presented. From narrow-band observations in the near-infrared with WIRCAM at the CFHT and HAWK-I at the VLT, I put strong constraints on the luminosity function of Lyman-α Emitters at z = 7.7. The best strategy for this type of survey is discussed, in particular with the use of massive galaxy clusters acting as gravitational lenses that amplify the flux from background sources. The lack of spectroscopic confirmation of candidates found with WIRCAM and the lack of candidates found with HAWK-I suggest an evolution of the luminosity function from z = 6.5. This evolution could be either in luminosity or in number density but somewhat discrepant results at lower redshift prevent any definitive answer to this question. If the evolution was mostly in luminosity, it could be interpreted as an increase in the opacity of the intergalactic medium caused by an increased fraction of neutral hydrogen, which is an expected signature of reionization.