Lyman-alpha damping wings in z~6-13 galaxies

We observe a decline in the median and variance of flux around the Ly{alpha} break with increasing redshift, consistent with an increasingly neutral IGM, as ionized regions become smaller and rarer. At z>~9 the spectra become consistent with an almost fully neutral IGM. We find S/N > 15 per pixel is required to robustly estimate IGM properties from prism spectra.

We fit a sub-sample of high S/N spectra and infer mean IGM neutral fractions of X{bar}HI=0.33_-0.27_^+0.18^, 0.64_-0.23_^+0.17^ (>0.70 excluding GNz11) at z~=6.5, 9.3. We also investigate local HI absorption, finding a median column density of log10NHI~=10^20.8^cm^-2^, comparable to z~3 Lyman-break galaxies, with no significant redshift evolution z>~5.5. We find galaxies showing the highest column density absorption are more likely to be in close associations of sources (<~500pkpc), implying absorption is enhanced in massive dark matter halos.

Future deep prism and grating spectroscopy of z>9 sources will provide tighter constraints on the earliest stages of reionisation, key for understanding the onset of star formation.