Large Fluctuations in the High-Redshift Metagalactic Ionizing Background

TL;DR

This paper investigates whether fluctuations in the ionizing background can explain the large opacity scatter in the high-redshift Lyman-alpha forest, suggesting a short mean free path and potential biases in measurements, with implications for reionization.

Contribution

It demonstrates that fluctuations in the ionizing background can account for the observed opacity scatter at z > 5, highlighting the bias in mean free path measurements and their impact on reionization models.

Findings

Short mean free path (<15 Mpc/h) needed to match observed scatter.

Bias in mean free path measurements due to quasar proximity effect.

Enhanced flux power spectrum on large scales supports fluctuating background models.

Abstract

Recent observations have shown that the scatter in opacities among coeval segments of the Lyman-alpha forest increases rapidly at z > 5. In this paper, we assess whether the large scatter can be explained by fluctuations in the ionizing background in the post-reionization intergalactic medium. We find that matching the observed scatter at z ~ 5.5 requires a short spatially averaged mean free path of < 15 comoving Mpc/h, a factor of > 3 shorter than direct measurements at z ~ 5.2. We argue that such rapid evolution in the mean free path is difficult to reconcile with our measurements of the global H I photoionization rate, which stay approximately constant over the interval z ~ 4.8 - 5.5. However, we also show that measurements of the mean free path at z > 5 are likely biased towards higher values by the quasar proximity effect. This bias can reconcile the short values of the mean free path that are required to explain the large scatter in opacities. We discuss the implications of this scenario for cosmological reionization. Finally, we investigate whether other statistics applied to the z > 5 Lyman-alpha forest can shed light on the origin of the scatter. Compared to a model with a uniform ionizing background, models that successfully account for the scatter lead to enhanced power in the line-of-sight flux power spectrum on scales k < 0.1 h/Mpc. We find tentative evidence for this enhancement in observations of the high-redshift Lyman-alpha forest.