Evidence of a cosmological Stromgren surface and of significant neutral hydrogen surrounding the quasar SDSS J1030+0524

A bright quasar residing in a dense and largely neutral intergalactic medium at high redshifts (zgreater than or similar to6) will be surrounded by a large cosmological Stromgren 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 Stromgren 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 Lyalpha and Lybeta 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 Stromgren 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)greater than or similar to0.2, and that the ionizing luminosity of this quasar is in the range of (5.2+/-2.5)x10(56) photons s(-1). 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.