Lyman-alpha emission at the end of reionization: line strengths and profiles from MMT and JWST observations at z 5-6

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 is scattered by neutral hydrogen in the IGM and the interstellar and circum-galactic medium, 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 reionization, z 5-6, providing a baseline that can be applied to z>6 observations. We targeted GOODS-N with MMT/Binospec, obtaining R 4360 rest-frame UV spectra of 236 galaxies at z 5-6, selected from HST/CANDELS, finding 62 Ly α detections. We use JWST observations from JADES and FRESCO for a subset of our sources to characterize Ly α properties as a function of UV continuum and H α emission. We present the first statistical measurements of the Ly α FWHM distribution at z 5-6, and produce empirical baseline models of Ly α equivalent width (EWLy α) and escape fraction (f esc Lyα) conditional on UV magnitude and slope. We find our EWLy α and f esc Lyα models depend on UV magnitude, and infer 45 plus or minus 5 % and <62 plus or minus 8 % of MUV=-19.5 galaxies have EWLy α >25 Å and f esc Lyα >0.2, respectively. We find a mean Ly α FWHM of 245km/s and median Ly α velocity offset of 258km/s, both correlating with higher 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 may reflect resonant scattering by residual neutral hydrogen in the IGM at z 5-6 and increasing ISM/CGM densities. Our median line profile predicts higher Ly α transmission in a fully neutral IGM, providing insight into recent z>10 Ly α detections.