Lyman-Alpha Emitting Galaxies as a Probe of Reionization: Large-Scale Bubble Morphology and Small-Scale Absorbers

TL;DR

This paper investigates how large-scale bubble morphology and small-scale absorbers in the intergalactic medium affect the visibility of LyA emitting galaxies during reionization, using combined analytic and numerical models.

Contribution

It introduces a novel combined modeling approach to distinguish the effects of different IGM structures on LyA signals and proposes methods to break degeneracies in reionization studies.

Findings

Degeneracy exists between IGM ionization structure and HI fraction in LyA observations.

Joint analysis of LyA luminosity function and equivalent width distribution can constrain reionization topology.

UV-faint galaxies show larger LyA fraction drops, but this is not exclusive evidence of patchy reionization.

Abstract

The visibility of LyA emitting galaxies during the Epoch of Reionization is controlled by both diffuse HI patches in large-scale bubble morphology and small-scale absorbers. To investigate the impact on LyA photons, we apply a novel combination of analytic and numerical calculations to three scenarios: (i) the `bubble' model, where only diffuse HI outside ionized bubbles is present; (ii) the `web' model, where HI exists only in overdense self-shielded gas; and (iii) the more realistic 'web-bubble' model, which contains both. Our analysis confirms that there is a degeneracy between the ionization structure of the intergalactic medium (IGM) and the HI fraction inferred from LyA surveys, as the three models suppress LyA flux equally with very different HI fractions. We argue that a joint analysis of the LyA luminosity function and the rest-frame equivalent width distribution/LyA fraction can break this degeneracy and provide constraints on the reionization history and its topology. We further show that constraints can improve if we consider the full shape of the M_UV-dependent redshift evolution of the LyA fraction of Lyman break galaxies. Contrary to conventional wisdom, we find that (i) a drop of LyA fraction larger for UV-faint than for UV-bright galaxies can be reproduced with web and web-bubble models and therefore does not provide exclusive evidence of patchy reionization, and (ii) the IGM-transmission PDF is unimodal for bubble models and bimodal in web models. We further highlight the importance of galaxy-absorber cross-correlation. Comparing our models to observations, the neutral fraction at z~7 is likely to be of order of tens of per cent when interpreted with bubble or web-bubble models. Alternatively, we obtain a conservative lower limit ~1% in the web models, if we allow for a drop in the photoionization rate by a factor of ~100 from the post-reionized universe. [abridged]