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

Abstract

With JWST, it is now possible to use Lyman-Alpha (Ly$\alpha$) emission from galaxies beyond z>8 to trace neutral hydrogen in the intergalactic medium (IGM) as the Universe became reionized. However, observed Ly$\alpha$ emission is scattered by neutral hydrogen in the IGM and the interstellar and circum-galactic medium, necessitating `baseline' models of Ly$\alpha$ properties in the ionized IGM to disentangle their impacts. In this work, we characterize Ly$\alpha$ 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$\alpha$ detections. We use JWST observations from JADES and FRESCO for a subset of our sources to characterize Ly$\alpha$ properties as a function of UV continuum and H$\alpha$ emission. We present the first statistical measurements of the Ly$\alpha$ FWHM distribution at z~5-6, and produce empirical baseline models of Ly$\alpha$ equivalent width (EWLy$\alpha$) and escape fraction (f$_{esc}^{Ly\alpha}$) conditional on UV magnitude and slope. We find our EWLy$\alpha$ and f$_{esc}^{Ly\alpha}$ models depend on UV magnitude, and infer 45$\pm$5$\%$ and <62$\pm$8$\%$ of MUV=-19.5 galaxies have EWLy$\alpha$>25$\r{A}$ and f$_{esc}^{Ly\alpha}$>0.2, respectively. We find a mean Ly$\alpha$ FWHM of 245km/s and median Ly$\alpha$ velocity offset of 258km/s, both correlating with higher UV luminosity. Our median observed Ly$\alpha$ 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$\alpha$ transmission in a fully neutral IGM, providing insight into recent z>10 Ly$\alpha$ detections.