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

Journal Article · Mon Sep 19 00:00:00 EDT 2022 · The Astrophysical Journal

DOI:https://doi.org/10.3847/1538-4357/ac8d0a· OSTI ID:1864205

Gnedin, Nickolay Y. ^[1]

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1. Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)

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. The CROC simulations in the 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. Nevertheless, the run that matches the dark gap distributions serves as a counterexample 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 and 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, although it may do so in combination with the distribution of mean opacities.

• Research Organization: Fermi National Accelerator Laboratory (FNAL), Batavia, IL (United States)
• Sponsoring Organization: National Science Foundation (NSF); USDOE Office of Science (SC), High Energy Physics (HEP)
• Grant/Contract Number: AC02-06CH11357; AC02-07CH11359
• OSTI ID: 1864205
• Report Number(s): FERMILAB-PUB-22-253-T; arXiv:2204.05338; oai:inspirehep.net:2066116
• Journal Information: The Astrophysical Journal, Journal Name: The Astrophysical Journal Journal Issue: 1 Vol. 937; ISSN 0004-637X
• Publisher: IOP Publishing
• Country of Publication: United States
• Language: English

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