Observations of the Lyman-$\alpha$ Universe

TL;DR

This paper reviews the observational and theoretical understanding of hydrogen Lyman-alpha emission at high redshifts, highlighting its significance in probing galaxy formation, the circum-galactic medium, and cosmic reionization.

Contribution

It synthesizes current knowledge on Lyα sources, their properties, and implications for galaxy evolution and reionization, emphasizing the role of upcoming telescopes in advancing this field.

Findings

Lyα emitters at high redshift are similar to local dwarf galaxies.

High-z star-forming galaxies have diffuse Lyα halos extending beyond their virial radius.

Neutral hydrogen during reionization dims Lyα emission, indicating late reionization.

Abstract

Hydrogen Lyman-$\alpha$ (Ly$\alpha$) emission has been one of the major observational probes for the high redshift universe, since the first discoveries of high-$z$ Ly$\alpha$ emitting galaxies in the late 1990s. Due to the strong Ly$\alpha$ emission originated by resonant scattering and recombination of the most-abundant element, Ly$\alpha$ 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 Ly$\alpha$ sources, and present theoretical interpretations reached to date. We conclude that: 1) A typical Ly$\alpha$ emitter (LAE) at $z\gtrsim 2$ with a $L^*$ Ly$\alpha$ 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 $10^{8-9} M_\odot$ ($10^{10-11} M_\odot$) and $1-10 M_\odot$ yr$^{-1}$, respectively; 2) High-$z$ SFGs ubiquitously have a diffuse Ly$\alpha$ 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 Ly$\alpha$ emission for galaxies at $z>6$ that suggest the late reionization history. The next generation large telescope projects will combine Ly$\alpha$ emission data with HI Ly$\alpha$ 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.