Kavli Affiliate: Robert A. Simcoe
| First 5 Authors: , , , ,
| Summary:
The spectra of high-redshift ($zgtrsim 6$) quasars contain valuable
information on the progression of the Epoch of Reionization (EoR). At redshifts
$z<6$, the observed Lyman-series forest shows that the intergalactic medium
(IGM) is nearly ionized, while at $z>7$ the observed quasar damping wings
indicate high neutral gas fractions. However, there remains a gap in neutral
gas fraction constraints at $6lesssim z lesssim 7$ where the Lyman series
forest becomes saturated but damping wings have yet to fully emerge. In this
work, we use a sample of 18 quasar spectra at redshifts $6.0<z<7.1$ to close
this gap. We apply neural networks to reconstruct the quasars’ continuum
emission around the partially absorbed Lyman $alpha$ line to normalize their
spectra, and stack these continuum-normalized spectra in three redshift bins.
To increase the robustness of our results, we compare the stacks to a grid of
models from two hydrodynamical simulations, ATON and CROC, and we measure the
volume-averaged neutral gas fraction, $barx_rm HI$, while jointly fitting
for the mean quasar lifetime, $t_rm Q$, for each stacked spectrum. We
chronicle the evolution of neutral gas fraction using the ATON (CROC) models as
follows: $barx_rm HI = 0.21_-0.07^+0.17$ ($barx_rm HI =
0.10_<10^-4^+0.73$) at $langle z rangle =6.10$, $barx_rm HI =
0.21_-0.07^+0.33$ ($barx_rm HI =0.57_-0.47^+0.26$) at $langle z
rangle =6.46$, and $barx_rm HI = 0.37_-0.17^+0.17$ ($barx_rm
HI =0.57_-0.21^+0.26$) at $langle z rangle =6.87$. At the same time we
constrain the average quasar lifetime to be $t_rm Q lesssim 7 rm Myr$
across all redshift bins, in good agreement with previous studies.
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