Cosmic Reionization On Computers: Biases and Uncertainties in the Measured Mean Free Path at the End Stage of Reionization

TL;DR

This study uses simulations to analyze biases in measuring the mean free path during cosmic reionization, revealing that quasar lifetime significantly impacts inferred values at late stages.

Contribution

The paper demonstrates how quasar lifetime and small-scale structures bias mean free path measurements during reionization, providing insights into modeling systematics.

Findings

Inferred MFP can be biased by a factor of a few depending on quasar lifetime.

Minimal bias in MFP measurements at earlier reionization stages for t_Q > 1 Myr.

Modeling systematics are crucial for accurate MFP inference in small true MFP regimes.

Abstract

Recent observations and analyses of absorption in quasar spectra suggest a rapid drop in the mean free path (MFP) at the late stage of reionization at $z\sim6$. We use the Cosmic Reionization on Computers simulation to examine potential biases in observed measurements of the MFP at the late stage of reionization, particularly in the presence of a quasar. We analyze three snapshots surrounding the `ankle' point of reionization history, when extended neutral patches of the intergalactic medium disappeared in the simulation box. Specifically, these are $z=6.8$ (true MFP $\approx 0.4$ pMpc), in addition to $z=6.1$ (true MFP $\approx 2$ pMpc) and $z=5.4$ (true MFP $\approx 6$ pMpc). We compare the inferred MFP $\lambda_{\rm mfp}$ from synthetic spectra fits to the true MFP. We find that the mean Lyman continuum (LyC) profile at $z=6.8$ changes significantly with quasar lifetime $t_Q$. We attribute this sensitivity to $t_Q$ to a combination of extended neutral IGM patches and the prevalence of small-scale dense clumps. Consequently, the inferred MFP can be biased by a factor of few depending on $t_Q$. On the other hand, for the $z=6.1$ and $z=5.4$ snapshots, the mean LyC profile shows minimal sensitivity to variation in $t_Q\gtrsim 1$ Myr. The inferred MFP in these two cases is accurate to the $\lesssim 30\%$ level. Our results highlight how modeling systematics can affect the inferred MFP, particularly in the regime of small true MFP ($\lesssim 0.5$ pMpc). We also discuss the potential of this regime to provide a testing ground for constraining quasar lifetimes from LyC profiles.