2004ApJ...611L..69M

A bright quasar residing in a dense and largely neutral intergalactic medium at high redshifts (z≳6) will be surrounded by a large cosmological Strömgren sphere. The quasar's spectrum will then show a sharp increase in resonant Lyman line absorption at wavelengths approaching and shorter than that corresponding to the Strömgren sphere's boundary along the line of sight. We show here that simultaneously considering the measured absorption in two or more hydrogen Lyman lines can provide the dynamical range required to detect this feature. We model broad and robust features of the Lyα and Lyβ regions of the spectrum of the z=6.28 quasar SDSS J1030+0524, using a hydrodynamical simulation. From the steep wavelength dependence of the inferred absorption opacity, we detect the boundary of the Strömgren sphere at a proper distance of 6.0±0.2 Mpc away from the source redshift. From the spectrum alone, we also find that beyond this distance, cosmic hydrogen turns nearly neutral, with a neutral fraction of x_HI_≳0.2, and that the ionizing luminosity of this quasar is in the range of (5.2±2.5)x10⁵⁶ photons/s. The method presented here, when applied to future quasars, can probe the complex topology of overlapping ionized regions and can be used to study the details of the reionization process.