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 $S/N>5$ 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 $S/N=4.5$. 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, $\tau_GP^eff$ extending a similar
analysis by Simpson et al. We find $\tau_GP^eff \propto
(1+z)^\xi$ where $= 11.2^+0.4_-0.6$, for $z > 5.5$. The data
nevertheless provide only a weak limit on the volume-weighted hydrogen
intergalactic (IGM) neutral fraction at $z6.5$, $x_HI > 10^-4$,
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.
Description
[1702.03687] Observations of the Lyman series forest towards the redshift 7.1 quasar ULAS J1120+0641
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