We discuss new Keck/MOSFIRE spectroscopic observations of four luminous galaxies at z similar or equal to 7-9 selected to have intense optical line emission by Roberts-Borsani et al. Previous follow-up has revealed Ly alpha in two of the four galaxies. Our new MOSFIRE observations confirm that Lya is present in the entire sample. We detect Lya emission in the galaxy COS-zs7-1, confirming its redshift as z.(Ly alpha) = 7.154, and we detect Lya in EGS-zs8-2 at z(Ly alpha) = 7.477, verifying an earlier tentative detection. The ubiquity of Lya in this sample is puzzling given that the IGM is expected to be significantly neutral over 7 < z < 9. To investigate this result in more detail, we have initiated a campaign to target UV metal lines in the four Lya emitters as a probe of both the ionizing field and the Lya velocity offset at early times. Here we present the detection of C III] emission in the z = 7.73 galaxy EGS-zs8-1, requiring an intense radiation field and moderately low metallicity. We argue that the radiation field is likely to affect the local environment, increasing the transmission of Lya through the galaxy. Moreover, the centroid of C III] indicates that Lya is redshifted by 340 km s (1). This velocity offset is larger than that seen in less luminous systems, providing an explanation for the transmission of Lya emission through the IGM. Since the transmission is further enhanced by the likelihood that such systems are also situated in large ionized bubbles, the visibility of Lya at z > 7 is expected to be strongly luminosity-dependent, with transmission accelerated in systems with intense star formation.

Understanding the formation and evolution of galaxies is a crucially important task in modern astronomy. It is well known that galaxy formation is strongly affected by the environments they reside in. Galaxy clusters, as the densest large-scale structures in the Universe, thus serve as ideal laboratories to study how galaxy formation proceeds in dense environments. Clusters already began to form at $z>2$, therefore to directly witness the early stage of galaxy formation in dense environments, it is necessary to identify progenitors of clusters (`protoclusters') and study their galaxy constituents within. In this thesis, I present two observational studies on high-redshift protoclusters at $z>3$. Utilizing multiwavelength data and different galaxy selection techniques, significant galaxy overdensities are found in the two protoclusters, which are predicted to evolve into Coma-like clusters by present day. Various types of galaxies are identified in the protocluster, such as normal star-forming galaxies, massive quiescent galaxies and post-starburst galaxies. Together with extreme and rare sources such as giant Lyman-alpha nebulae and brighest cluster galaxy, they paint a picture of how different galaxy populations trace the underlying dark matter halos. Finally, the environmental impact on galaxy properties appears to be a subtle one for these protoclusters, which might depend on the galaxy population one chooses to study.