Probing Reionization with Quasar Spectra: the Impact of the Intrinsic Lyman-alpha Emission Line Shape Uncertainty

TL;DR

This study assesses how uncertainties in the intrinsic Lyman-alpha emission line shape of quasars impact measurements of the intergalactic medium's reionization state at high redshift, highlighting potential biases and robustness of current methods.

Contribution

It quantifies the effect of intrinsic line shape uncertainties on reionization constraints using high-resolution quasar spectra and simulated IGM absorption.

Findings

Systematic blueshift of Lyman-alpha lines biases neutral fraction estimates low.

Biases may reinforce claims of high neutral hydrogen at z~6.

Few bright quasars can robustly distinguish ionization states.

Abstract

Arguably the best hope of understanding the tail end of the reionization of the intergalactic medium (IGM) at redshift z > 6 is through the detection and characterization of the Gunn-Peterson (GP) damping wing absorption of the IGM in bright quasar spectra. However, the use of quasar spectra to measure the IGM damping wing requires a model of the quasar's intrinsic Lyman-alpha emission line. Here we quantify the uncertainties in the intrinsic line shapes, and how those uncertainties affect the determination of the IGM neutral fraction. We have assembled a catalog of high-resolution HST spectra of the emission lines of unobscured low-redshift quasars, and have characterized the variance in the shapes of their lines. We then add simulated absorption from the high-redshift IGM to these quasar spectra in order to determine the corresponding uncertainties in reionization constraints using current and future samples of z > 6 quasar spectra. We find that, if the redshift of the Lyman-alpha emission line is presumed to coincide with the systemic redshift determined from metal lines, the inferred IGM neutral fraction is systematically biased to low values due to a systematic blueshift of the Lyman-alpha line relative to the metal lines. If a similar blueshift persists in quasars at z > 6, this bias strengthens previous claims of a significant neutral hydrogen fraction at z ~ 6. This technique is capable of making a robust distinction between a highly ionized (x_IGM ~ 10^-3) and a neutral (x_IGM = 1) IGM with even a few bright quasars.