Lyman-alpha emission at the end of reionization: Line strengths and profiles from MMT and JWST observations at <i>z</i> ∼ 5–6

Aims. With JWST, it is now possible to use Lyman-Alpha (Ly α ) emission from galaxies beyond z > 8 to trace neutral hydrogen in the intergalactic medium (IGM) as the Universe became reionized. However, observed Ly α emission lines are known to be affected by scattering by neutral hydrogen in the IGM and the interstellar and circumgalactic medium (CGM), necessitating “baseline” models of Ly α properties in the ionized IGM to disentangle their impacts. In this work, we characterize Ly α properties at the end of of the epoch of reionization (EoR) at z ∼ 5 − 6, providing a baseline that can be applied to z > 6 observations. Methods. We targeted GOODS-N with MMT/Binospec, obtaining R ∼ 4360 rest-frame UV spectra of 236 galaxies at z ∼ 5 − 6, selected primarily from HST/CANDELS, finding 62 Ly α detections. We used JWST observations from JADES and FRESCO for a subset of our sources to characterize Ly α properties as a function of the UV continuum and H α emission. We present the first statistical measurements of the Ly α FWHM distribution at z ∼ 5 − 6, along with empirical baseline models for the Ly α equivalent width (EW Ly α ) and escape fraction ( f esc Ly α ) that are conditional on the UV magnitude and slope. Results. We found our EW Ly α and f esc Ly α models strongly depend on UV magnitude, inferring that 45 ± 5% and < 62 ± 8% of M UV = −19.5 galaxies have EW Ly α > 25 Å and f esc Ly α > 0.2, respectively. We also found a mean FWHM Ly α of 245 km s −1 and median Ly α velocity offset of 258 km s −1 , both displaying a correlation with increasing UV luminosity. Our median observed Ly α line profile is broader and has higher velocity offset compared to pre-JWST models based on z ∼ 2 lines, which might reflect both resonant scattering by residual neutral hydrogen in the IGM at z ∼ 5 − 6 and increasing ISM and CGM densities. Our median line profile thus predicts higher Ly α transmission in a fully neutral IGM, providing insights into recent detections of Ly α at z > 10.