Introduction Lyman-$\alpha$ spectra can provide insights into the small-scale structure and kinematics of neutral hydrogen (HI) within galaxies as well as the ionization state of the intergalactic medium (IGM). The former defines the intrinsic spectrum of a galaxy, which is modified by the latter. These two effects are degenerate: While at low redshifts, double peaked Lyman-$\alpha$ spectra are the norm, we find more and more spectra only showing a red peak at high redshifts.
Lyman-alpha emitters (LAEs) show a rich variety of spectral shapes due to the emission line’s resonant nature and typically high optical depths. While there is a large body of literature exploring how small-scale density and velocity distributions can explain this variety of features in spectra, the intergalactic medium (IGM) has often been neglecting as a contributing factor for such features.
Above sketch helps visualizing how the IGM density and velocity structure along a line-of-sight give rise to an attenuation profile possibly shaping the arising spectrum.