Observations of the Lyman-α Universe

Hydrogen Lyman-α (Lyα) emission has been one of the major observational probes for the high-redshift Universe since the first discoveries of high-z Lyα-emitting galaxies in the late 1990s. Due to the strong Lyα emission originated by resonant scattering and recombination of the most abundant element, Lyα observations witness not only Hii regions of star formation and active galactic nuclei (AGNs) but also diffuse Hi gas in the circumgalactic medium (CGM) and the intergalactic medium (IGM). Here, we review Lyα sources and present theoretical interpretations reached to date. We conclude the following: A typical Lyα emitter (LAE) at z ≳ 2 with a L^* Lyα luminosity is a high-z counterpart of a local dwarf galaxy, a compact metal-poor star-forming galaxy (SFG) with an approximate stellar (dark matter halo) mass and star-formation rate of 10^8-9M_⊙ (10^10-11M_⊙) and 1-10 M_⊙ year^-1, respectively. High-z SFGs ubiquitously have a diffuse Lyα-emitting halo in the CGM extending to the halo virial radius and beyond. Remaining neutral hydrogen at the epoch of cosmic reionization makes a strong dimming of Lyα emission for galaxies at z > 6 that suggests the late reionization history. The next-generation large-telescope projects will combine Lyα emission data with Hi Lyα absorptions and 21-cm radio data that map out the majority of hydrogen (Hi+Hii) gas, uncovering the exchanges of (a) matter by outflow and inflow and (b) radiation, relevant to cosmic reionization, between galaxies and the CGM/IGM.