A tale of seven narrow spikes and a long trough: constraining the timing of the percolation of HII bubbles at the tail-end of reionization with ULAS J1120+0641

TL;DR

This study uses radiative transfer simulations to analyze high-quality Lyα forest data from a z=7.085 quasar, revealing how narrow transmission spikes and a long trough inform the timing of cosmic reionization's end.

Contribution

It demonstrates that the occurrence of transmission spikes is highly sensitive to reionization timing and provides a detailed match between simulations and observed spectral features.

Findings

Seven narrow spikes are linked to fully ionized underdense regions.

The long trough indicates a rapid decrease in photo-ionization rate at z~6.1.

Simulations can reproduce observed spectral features, constraining reionization history.

Abstract

High-signal to noise observations of the Ly$\alpha$ forest transmissivity in the z = 7.085 QSO ULAS J1120+0641 show seven narrow transmission spikes followed by a long 240 cMpc/h trough. Here we use radiative transfer simulations of cosmic reionization previously calibrated to match a wider range of Ly$\alpha$ forest data to show that the occurrence of seven transmission spikes in the narrow redshift range z = 5.85 - 6.1 is very sensitive to the exact timing of reionization. Occurrence of the spikes requires the most under dense regions of the IGM to be already fully ionised. The rapid onset of a long trough at z = 6.12 requires a strong decrease of the photo-ionisation rate at z$\sim$6.1 in this line-of-sight, consistent with the end of percolation at this redshift. The narrow range of reionisation histories that we previously found to be consistent with a wider range of Ly$\alpha$ forest data have a reasonable probability of showing seven spikes and the mock absorption spectra provide an excellent match to the spikes and the trough in the observed spectrum of ULAS J1120+0641. Despite the large overall opacity of Ly$\alpha$ at z > 5.8, larger samples of high signal-to-noise observations of rare transmission spikes should therefore provide important further insights into the exact timing of the percolation of HII bubbles at the tail-end of reionization