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.

We present the rationale for and the observational description of ASPECS: the ALMA SPECtroscopic Survey in the Hubble Ultra-Deep Field (UDF), the cosmological deep field that has the deepest multi-wavelength data available. Our overarching goal is to obtain an unbiased census of molecular gas and dust continuum emission in high-redshift (z > 0.5) galaxies. The similar to 1' region covered within the UDF was chosen to overlap with the deepest available imaging from the Hubble Space Telescope. Our ALMA observations consist of full frequency scans in band. 3 (84-115 GHz) and band. 6 (212-272 GHz) at approximately uniform line sensitivity (L'(CO) similar to 2 x 10(9) K km s(-1) pc(2)), and continuum noise levels of 3.8 mu Jy beam(-1) and 12.7 ae Jy beam(-1), respectively. The molecular surveys cover the different rotational transitions of the CO molecule, leading to essentially full redshift coverage. The [C II] emission line is also covered at redshifts 6.0 < z < 8.0. We present a customized algorithm to identify line candidates in the molecular line scans. and quantify our ability to recover artificial sources from our data. Based on whether multiple CO lines are detected, and whether optical spectroscopic redshifts as well as optical counterparts exist, we constrain the most likely line identification. We report 10 (11) CO line candidates in the 3mm (1mm) band, and our statistical analysis shows that < 4 of these (in each band) are likely spurious. Less than one-third. of the total CO flux in the low-J CO line candidates are from sources that are not associated with an optical/NIR counterpart. We also present continuum maps of both the band. 3 and band. 6 observations. The data presented here form the basis of a number of dedicated studies that are presented in subsequent papers.

We study the galaxy populations in 74 Sunyaev-Zeldovich effect selected clusters from the South Pole Telescope survey, which have been imaged in the science verification phase of the Dark Energy Survey. The sample extends up to z similar to 1.1 with 4 x 10(14)M(circle dot) <= M-200 <= 3 x 10(15)M(circle dot). Using the band containing the 4000 angstrom break and its redward neighbour, we study the colour-magnitude distributions of cluster galaxies to similar to m(*) + 2, finding that: (1) The intrinsic rest frame g - r colour width of the red sequence (RS) population is similar to 0.03 out to z similar to 0.85 with a preference for an increase to similar to 0.07 at z = 1, and (2) the prominence of the RS declines beyond z similar to 0.6. The spatial distribution of cluster galaxies is well described by the NFW profile out to 4R(200) with a concentration of c(g) = 3.59(-0.18)(+0.20), 5.37(-0.24)(+0.27) and 1.38(-0.19)(+0.21) for the full, the RS and the blue non-RS populations, respectively, but with similar to 40 per cent to 55 per cent cluster to cluster variation and no statistically significant redshift or mass trends. The number of galaxies within the virial region N-200 exhibits a mass trend indicating that the number of galaxies per unit total mass is lower in the most massive clusters, and shows no significant redshift trend. The RS fraction within R-200 is (68 +/- 3) per cent at z = 0.46, varies from similar to 55 per cent at z = 1 to similar to 80 per cent at z = 0.1 and exhibits intrinsic variation among

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.