Anomaly in the Opacity of the Post-Reionization Intergalactic Medium in the Ly$\alpha$ and Ly$\beta$ Forest

TL;DR

This study measures the IGM opacity in Lyα and Lyβ forests at high redshift to understand reionization, revealing that models with temperature-density inversion best fit the data but still have discrepancies.

Contribution

It provides new measurements of IGM opacity at z~5.5-6.1 and compares them with advanced inhomogeneous reionization models, highlighting the importance of spectral noise and temperature-density relations.

Findings

Spectral noise reduces sensitivity to high opacity regions.

Models with temperature-density inversion best match Lyβ data.

Discrepancies remain at z > 5.8 despite improved models.

Abstract

We measure the intergalactic medium (IGM) opacity in the Ly$\alpha$ as well as in the Ly$\beta$ forest along $19$ quasar sightlines between $5.5\lesssim z_{\rm abs}\lesssim 6.1$, probing the end stages of the reionization epoch. Owing to its lower oscillator strength the Ly$\beta$ transition is sensitive to different gas temperatures and densities than Ly$\alpha$, providing additional constraints on the ionization and thermal state of the IGM. A comparison of our measurements to different inhomogeneous reionization models, derived from post-processing the Nyx cosmological hydrodynamical simulation to include spatial fluctuations in the ultraviolet background (UVB) or the gas temperature field, as well as to a uniform reionization model with varying thermal states of the IGM, leads to two primary conclusions: First, we find that including the effects of spectral noise is key for a proper data to model comparison. Noise effectively reduces the sensitivity to high opacity regions, and thus even stronger spatial inhomogeneities are required to match the observed scatter in the observations than previously inferred. Second, we find that models which come close to reproducing the distribution of Ly$\alpha$ effective optical depths nevertheless underpredict the Ly$\beta$ opacity at the same spatial locations. The origin of this disagreement is not entirely clear but models with an inversion in the temperature-density relation of the IGM just after reionization is completed match our measurements best, although they still do not fully capture the observations at $z\gtrsim 5.8$.

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