# Observations of the Lyman series forest towards the redshift 7.1 quasar ULAS J1120+0641

## TL;DR

This study presents a detailed spectrum of a z=7.084 quasar, revealing limited transmission in the Lyman series forest, a long Gunn-Peterson trough, and constraining the evolution of the IGM's neutral hydrogen fraction at high redshift.

## Contribution

First detailed spectrum of a z=7.084 quasar's Lyman forest, revealing transmission spikes, a long Gunn-Peterson trough, and constraints on IGM neutral fraction evolution.

## Key findings

- Transmission confined to seven narrow spikes in Lyα forest
- Gunn-Peterson trough extends from z=6.122 to 7.04, over 240 h^{-1} Mpc
- Effective optical depth increases steeply with redshift, τ_GP^eff ∝ (1+z)^{11.2}

## Abstract

We present a 30h integration Very Large Telescope X-shooter spectrum of the Lyman series forest towards the $z = 7.084$ quasar ULAS J1120+0641. The only detected transmission at ${\rm S/N}>5$ is confined to seven narrow spikes in the Ly$\alpha$ forest, over the redshift range $5.858<z<6.122$, just longward of the wavelength of the onset of the Ly$\beta$ forest. There is also a possible detection of one further unresolved spike in the Ly$\beta$ forest at $z=6.854$, with ${\rm S/N}=4.5$. We also present revised Hubble Space Telescope F814W photometry of the source. The summed flux from the transmission spikes is in agreement with the F814W photometry, so all the transmission in the Lyman series forest may have been detected. There is a Gunn-Peterson (GP) trough in the Ly$\alpha$ forest from $z=6.122$ all the way to the quasar near zone at $z=7.04$. The trough, of comoving length $240\,h^{-1}$Mpc, is over twice as long as the next longest known GP trough. We combine the spectroscopic and photometric results to constrain the evolution of the Ly$\alpha$ effective optical depth with redshift, $\tau_{\rm GP}^{\rm eff}$ extending a similar analysis by Simpson et al. We find $\tau_{\rm GP}^{\rm eff} \propto (1+z)^{\xi}$ where $\xi = 11.2^{+0.4}_{-0.6}$, for $z > 5.5$. The data nevertheless provide only a weak limit on the volume-weighted hydrogen intergalactic (IGM) neutral fraction at $z\sim 6.5$, $x_{{\rm HI}} > 10^{-4}$, similar to limits at redshift $z\sim6$ from less distant quasars. The new observations cannot extend measurements of the neutral fraction of the IGM to higher values because absorption in the Ly$\alpha$ forest is already saturated near $z\sim6$. For higher neutral fractions, other methods such as measuring the red damping wing of the IGM will be required.