Mapping neutral hydrogen during reionization with the Ly alpha 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 Ly alpha radiation produced by quasar ionization fronts (I-fronts) that expand in the partially ionized intergalactic medium (IGM) before reionization is complete. These Ly alpha photons are mainLygenerated by H I collisional excitations. Combining two radiative transfer models (one for the QSO ionizing radiation and one for the Ly alpha photons), we estimate the expected Ly alpha spectral shape and surface brightness (SBLy alpha) 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 SBLy alpha produced at redshift z similar or equal to 6.5 within a fullyneutral 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 proportionalLyto (1 + z)(2) for a given QSO age. QSOs with harder spectra may produce a significantLybrighter emission at earLyphases. 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 Ly alpha emission can shed new light on the reionization history, the age and the emission properties of the highest redshift QSOs.