Observations of Lyα Emitters at High Redshift

In this series of lectures, I review our observational understanding of high-z Lyα emitters (LAEs) and relevant scientific topics. Since the discovery of LAEs in the late 1990s, significant progresses in LAE studies have been made over the past two decades by deep multi-wavelength observations. More than ten thousands of LAEs have been identified photometrically, and more than one thousand spectroscopically, in optical and near-infrared data, and the redshifts of these LAEs range from z∼ 0 to z∼ 10. These large samples of LAEs are useful to address two major astrophysical issues, galaxy formation and cosmic reionization. Statistical studies have revealed the general picture of LAEs' physical properties: young stellar populations, remarkable luminosity function evolutions, compact morphologies, highly ionized inter-stellar media (ISM) with low metal/dust contents, low masses of dark-matter halos. Typical LAEs represent low-mass high-z galaxies, high-z analogs of dwarf galaxies, some of which are thought to be candidates of population III galaxies. These observational studies have also pinpointed rare bright Lyα sources extended over ∼ 10-100 kpc, dubbed Lyα blobs, whose physical origins are under debate. LAEs are used as probes of cosmic reionization history through the Lyα damping wing absorption given by the neutral hydrogen of the inter-galactic medium (IGM), which complement the cosmic microwave background radiation and 21 cm observations targeting the epoch of reionization. The low-mass and highly-ionized population of LAEs can be major sources of cosmic reionization, and physical parameters including the ionizing photon escape fraction have been extensively investigated. The budget of ionizing photons for cosmic reionization has been constrained, although there remain large observational uncertainties in the parameters. Beyond these two established topics of LAEs, galaxy formation and cosmic reionization, several new usages of LAEs for science frontiers have been suggested such as the distribution of Hi gas in the circum-galactic medium and filaments of large-scale structures. On-going 10 m-class optical telescope programs and future telescope projects, such as JWST, ELTs, and SKA, will address the remaining open questions related to LAEs, and push the horizons of the science frontiers.