Interpreting the Transmission Windows of Distant Quasars
Abstract
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 (HR regime), one can determine the IGM mean HI fraction, xHI; in the second (PR regime), the value of Rw allows to measure the local photoionization rate and the local enhancement of the photoionization rate, GammaG, due to nearby/intervening galaxies. The ASC has been tested against radiative transfer+SPH numerical simulations, and applied to 15 high-z (z>5.8) quasars sample from Fan et al. (2006). 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%) in the local photoionization rate possibly produced by a galaxy overdensity. We discuss the possible interpretations and uncertainties of this result.