Seeing Through the Trough: Outflows and the Detectability of Lyman Alpha Emission from the First Galaxies

Abstract

The next generation of telescopes aim to directly observe the first generation of galaxies that initiated the reionization process in our Universe. The Lyman Alpha (Lya) emission line is robustly predicted to be the most prominent intrinsic spectral feature of these galaxies, making it an ideal target to search for and study high redshift galaxies. Unfortunately the large Gunn-Peterson optical depth of the surrounding neutral intergalactic medium (IGM) is thought to render this line extremely difficult to detect prior to reionization. In this paper we demonstrate that the radiative transfer effects in the interstellar medium (ISM), which cause Lya flux to emerge from galaxies at frequencies where the Gunn-Peterson optical depth is reduced, can substantially enhance the prospects for detection of the Lya line at high redshift. In particular, scattering off outflows of interstellar HI gas can modify the Lya spectral line shape such that >5% of the emitted Lya radiation is transmitted directly to the observer, even through a fully neutral IGM. It may therefore be possible to directly observe `strong' Lya emission lines (EW > 50 Angstrom rest frame) from the highest redshift galaxies that reside in the smallest HII `bubbles' early in the reionization era with JWST. In addition, we show that outflows can boost the fraction of Lya radiation that is transmitted through the IGM during the latter stages of reionization, and even post-reionization. Coupled with the fact that the first generation of galaxies are thought to have very large intrinsic equivalent Lya equivalent widths, our results suggest that the search for galaxies in their redshifted Lya emission line can be competitive with the drop-out technique out to the highest redshifts that can be probed in the JWST era.