How neutral is the intergalactic medium surrounding the redshift z=7.085 quasar ULAS J1120+0641?

TL;DR

This study investigates the ionisation state of the intergalactic medium around a high-redshift quasar using radiative transfer simulations, revealing partial ionisation and implications for quasar age and black hole growth.

Contribution

It provides the first detailed simulation-based analysis of the IGM around a z=7.085 quasar, linking observed spectral features to IGM ionisation levels and quasar lifetime.

Findings

IGM near the quasar has a volume-averaged HI fraction >0.1.

The quasar has been active for 1-10 million years.

A highly neutral IGM with f_HI ~ 10^-3 to 10^-4 is possible if a damped Lya system is nearby.

Abstract

The quasar ULAS J1120+0641 at redshift z=7.085 has a highly ionised near zone which is smaller than those around quasars of similar luminosity at z~6. The spectrum also exhibits evidence for a damping wing extending redward of the systemic Lya redshift. We use radiative transfer simulations in a cosmological context to investigate the implications for the ionisation state of the inhomogeneous IGM surrounding this quasar. Our simulations show that the transmission profile is consistent with an IGM in the vicinity of the quasar with a volume averaged HI fraction of f_HI>0.1 and that ULAS J1120+0641 has been bright for 10^6--10^7 yr. The observed spectrum is also consistent with smaller IGM neutral fractions, f_HI ~ 10^-3--10-4, if a damped Lya system in an otherwise highly ionised IGM lies within 5 proper Mpc of the quasar. This is, however, predicted to occur in only ~5 per cent of our simulated sight-lines for a bright phase of 10^6--10^7 yr. Unless ULAS J1120+0641 grows during a previous optically obscured phase, the low age inferred for the quasar adds to the theoretical challenge of forming a 2x10^9 M_sol black hole at this high redshift.