Interpreting the transmission windows of distant quasars

We propose the apparent shrinking criterion (ASC) to interpret the spatial extent, Rw, of transmitted flux windows in the absorption spectra of high-z quasars. The ASC can discriminate between the two regimes in which Rw corresponds either to the physical size, RHII, of the quasar HII region or to the distance, Rmaxw, at which the transmitted flux drops to =0.1 and a Gunn-Peterson (GP) trough appears. In the first case [HII region (HR) regime], one can determine the intergalactic medium mean HI fraction, xHI in the second [proximity region (PR) regime], the value of Rw allows one to measure the local photoionization rate and the local enhancement of the photoionization rate, ΓG, due to nearby/intervening galaxies. The ASC has been tested against radiative transfer+smoothed particle hydrodynamics numerical simulations, and applied to a sample of 15 high-z (z > 5.8) quasar spectra. All sample quasars are found to be in the PR regime; hence, their observed spectral properties (inner flux profile, extent of transmission window) cannot reliably constrain the value of xHI. Four sample quasars show evidence for a local enhancement (up to 50 per cent) in the local photoionization rate possibly produced by a galaxy overdensity. We discuss the possible interpretations and uncertainties of this result.