Observations of the Lyman-α Universe
Abstract
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 AGN but also diffuse HI gas in the circum-galactic medium (CGM) and the inter-galactic medium (IGM). Here we review Lyα sources, and present theoretical interpretations reached to date. We conclude that: 1) 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 (halo) mass and star-formation rate of 108-9 M (1010-11 M) and 1-10 M yr-1, respectively; 2) High-z SFGs ubiquitously have a diffuse Lyα emitting halo in the CGM extending to the halo virial radius and beyond; 3) Remaining neutral hydrogen at the epoch of reionization makes a strong dimming of Lyα emission for galaxies at z>6 that suggest the late reionization history. The next generation large telescope projects will combine Lyα emission data with HI Lyα absorptions and 21cm radio data that map out the majority of hydrogen (HI+HII) gas, uncovering the exchanges of i) matter by outflow/inflow and ii) radiation, relevant to cosmic reionization, between galaxies and the CGM/IGM.