Re-examining the case for neutral gas near the redshift 7 quasar ULAS J1120+0641

TL;DR

This study re-examines the evidence for neutral intergalactic gas near the z~7 quasar ULAS J1120+0641 by analyzing Lyman-alpha emission and its relation to C IV properties, questioning previous interpretations of damping wing absorption.

Contribution

It demonstrates that the Lyman-alpha flux of ULAS J1120+0641 is consistent with lower-redshift quasars with similar C IV properties, challenging prior claims of significant neutral gas presence.

Findings

Lyman-alpha flux shows significant scatter for quasars with similar C IV properties.

ULAS J1120+0641's Lyman-alpha emission is within the distribution of comparable lower-redshift quasars.

The evidence for damping wing absorption and a neutral IGM around ULAS J1120+0641 may be weaker than previously thought.

Abstract

Signs of damping wing absorption attenuating the Lyman-$\alpha$ emission line of the first known $z \sim 7$ quasar, ULAS J1120+0641, recently provided exciting evidence of a significantly neutral IGM. This long-awaited signature of reionization was inferred, in part, from a deficit of flux in the quasar's Lyman-$\alpha$ emission line based on predictions from a composite of lower-redshift quasars. The composite sample was chosen based on its C IV emission line properties; however, as the original study by Mortlock et al. noted, the composite contained a slight velocity offset in C IV compared to ULAS J1120+0641. Here we test whether this offset may be related to the predicted strength of the Lyman-$\alpha$ emission line. We confirm the significant ($\sim 10$ per cent at r.m.s.) scatter in Lyman-$\alpha$ flux for quasars of a given C IV velocity and equivalent width found by Mortlock et al. We further find that among lower-redshift objects chosen to more closely match the C IV properties of ULAS J1120+0641, its Lyman-$\alpha$ emission falls within the observed distribution of fluxes. Among lower-redshift quasars chosen to more closely match in C IV velocity and equivalent width, we find that ULAS J1120+0641 falls within the observed distribution of Lyman-$\alpha$ emission line strengths. This suggests that damping wing absorption may not be present, potentially weakening the case for neutral gas around this object. Larger samples of z$>$7 quasars may therefore be needed to establish a clearer picture of the IGM neutral fraction at these redshifts.