Quasar Stromgren spheres before cosmological reionization

Ionizing sources embedded in the neutral intergalactic medium (IGM) before cosmological reionization generate discrete H II regions. We show that a sufficiently bright quasar (for example, 1/10 as luminous as that recently discovered by Fan et al.) can ionize a large volume, allowing the transmission of a substantial fraction of the flux of its intrinsic Ly alpha emission line on both sides of the Ly alpha wavelength. The observed line profile is richly informative. We show that a sufficiently accurate, high spectral resolution (R approximate to 10(4)) measurement of the line profile of a bright quasar is feasible using the Next Generation Space Telescope as well as large ground-based telescopes. Such a measurement has two potentially important applications. First, the red side of the Ly alpha emission line provides a way to measure the quasar lifetime. Second, the blue side provides a direct measure of the density fluctuations of the IGM at the quasar redshift. The estimate of the absorption of the red side is limited by the accuracy to which the intrinsic profile of the Ly alpha emission line is known. The blue side, however, does not sensitively depend on the intrinsic profile because the former is much narrower than the latter.