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# Large Lyman-$\alpha$ opacity fluctuations and low CMB $\tau$ in models of late reionization with large islands of neutral hydrogen extending to z<5.5

Abstract : High-redshift quasi-stellar object (QSO) spectra show large spatial fluctuations in the Ly α opacity of the intergalactic medium on surprisingly large scales at |$z$| ≳ 5.5. We present a radiative transfer simulation of cosmic reionization driven by galaxies that reproduces this large scatter and the rapid evolution of the Ly α opacity distribution at 5 < |$z$| < 6. The simulation also reproduces the low Thomson scattering optical depth reported by the latest cosmic microwave background (CMB) measurement and is consistent with the observed short near-zones and strong red damping wings in the highest redshift QSOs. It also matches the rapid disappearance of observed Ly α emission by galaxies at |$z$| ≳ 6. Reionization is complete at |$z$| = 5.3 in our model, and 50 per cent of the volume of the Universe is ionized at |$z$| = 7. Agreement with the Ly α forest data in such a late reionization model requires a rapid evolution of the ionizing emissivity of galaxies that peaks at |$z$| ∼ 6.8. The late end of reionization results in a large scatter in the photoionization rate and the neutral hydrogen fraction at redshifts as low as |$z$| ≲ 5.5 with large residual neutral ‘islands’ that can produce very long Gunn–Peterson troughs resembling those seen in the data.
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https://hal.archives-ouvertes.fr/hal-01891216
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Submitted on : Tuesday, October 9, 2018 - 1:34:29 PM
Last modification on : Wednesday, November 3, 2021 - 9:29:33 AM

### Citation

Girish Kulkarni, Laura C. Keating, Martin G. Haehnelt, Sarah E.I. Bosman, Ewald Puchwein, et al.. Large Lyman-$\alpha$ opacity fluctuations and low CMB $\tau$ in models of late reionization with large islands of neutral hydrogen extending to z<5.5. Monthly Notices of the Royal Astronomical Society, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2019, 485 (1), pp.L24-L28. ⟨10.1093/mnrasl/slz025⟩. ⟨hal-01891216⟩

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