Mapping neutral hydrogen during reionization with the Lyα emission from quasar ionization fronts

We present a new method to directly map the neutral-hydrogen distribution during the reionization epoch and constrain the emission properties of the highest redshift quasars (QSOs). As a tracer of H I, we propose to use the Lyα radiation produced by quasar ionization fronts (I-fronts) that expand in the partially ionized intergalactic medium (IGM) before reionization is complete. These Lyα photons are mainly generated by H I collisional excitations. Combining two radiative transfer models (one for the QSO ionizing radiation and one for the Lyα photons), we estimate the expected Lyα spectral shape and surface brightness (SBLyα) for a large number of configurations where we varied both the properties of the ionizing QSO and of the surrounding medium. We find that the expected signal is observable as a single (broad) emission line with a characteristic width of 100-200 km s_1. The expected SBLyα produced at redshift z ≃ 6.5 within a fully neutral region (at mean density) by a typical QSO I-front lies in the range 10-21 to 10-20 ergs s -1 cm-2 arcsec-2 and decreases proportionally to (1 + z)2 for a given QSO age. QSOs with harder spectra may produce a significantly brighter emission at early phases. The signal may cover up to a few hundred square arcminutes on the sky and should be already detectable with current facilities by means of moderate- to high-resolution spectroscopy. The detection of this Lyα emission can shed new light on the reionization history, the age and the emission properties of the highest redshift QSOs. © 2008. The American Astronomical Society. All rights reserved.