Interpreting the Transmission Windows of Distant Quasars

TL;DR

The paper introduces the Apparent Shrinking Criterion (ASC) to interpret quasar transmission windows, distinguishing between physical HII regions and flux drop-off regimes, enabling better understanding of the intergalactic medium at high redshift.

Contribution

The paper presents the ASC method for analyzing quasar spectra, tested with simulations, and applies it to high-z quasars to infer properties of the IGM and local environments.

Findings

All analyzed quasars are in the PR regime, limiting x_HI constraints.

Four quasars show evidence of local photoionization enhancement.

The ASC effectively differentiates between physical and flux drop-off regimes.

Abstract

We propose the Apparent Shrinking Criterion (ASC) to interpret the spatial extent, R_w, of transmitted flux windows in the absorption spectra of high-z quasars. The ASC can discriminate between the two regimes in which R_w corresponds either to the physical size, R_HII, of the quasar HII region, or to the distance, R^{max}_w, 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, x_HI; in the second (PR regime), the value of R_w allows to measure the local photoionization rate and the local enhancement of the photoionization rate, Gamma_G, 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 x_HI. 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.