Cosmic Reionization on Computers: Physical Origin of Long Dark Gaps in Quasar Absorption Spectra

TL;DR

This study uses large-scale cosmological simulations to analyze the properties of dark gaps in quasar spectra, revealing that their distribution is primarily influenced by ionization levels in voids rather than the timing of reionization.

Contribution

It demonstrates that dark gap statistics alone do not constrain reionization timing and highlights the importance of void ionization levels in shaping gap distributions.

Findings

Simulations match observed dark gap distributions but not mean opacities.

Dark gap distribution correlates with ionization in voids.

Reionization ending at z=6.7 does not necessarily produce the observed dark gaps.

Abstract

I explore the properties of "dark gaps" - regions in quasar absorption spectra without significant transmission - with several simulations from the Cosmic Reionization On Computers (CROC) project. CROC simulations in largest available boxes (120 cMpc) come close to matching both the distribution of mean opacities and the frequency of dark gaps, but alas not in the same model: the run that matches the mean opacities fails to contain enough dark gaps and vice versa.:( Never-the-less, the run that matches the dark gap distributions serves as a counter-example to claims in the literature that the dark gap statistics requires a late end to reionization - in that run reionization ends at z=6.7 (likely too early). While multiple factors contribute to the frequency of large dark gaps in the simulations, the primary factor that controls the overall shape of the dark gap distribution is the ionization level in voids - the lowest density regions produce the highest transmission spikes that terminate long gaps. As the result, the dark gap distribution correlates strongly with the fraction of the spectrum above the gap detection threshold, the observed distribution is matched by the simulation in which this fraction is 2%. Hence, the gap distribution by itself does not constrain the timing of reionization.