Determining the Nature of Late Gunn-Peterson Troughs with Galaxy Surveys

TL;DR

This paper investigates the origins of long Gunn-Peterson troughs at z ~ 5.5 by analyzing galaxy distributions around these features, using simulations to differentiate between models of voids and temperature variations.

Contribution

It introduces a method to distinguish between void and temperature fluctuation models of Gunn-Peterson troughs through galaxy surveys, supported by large-scale simulations.

Findings

Galaxy deficits indicate voids with weak ionizing backgrounds.

Galaxy overdensities suggest residual temperature variations from reionization.

Narrowband surveys can effectively differentiate models with ~90% likelihood.

Abstract

Recent observations have discovered long (up to ~110 Mpc/h), opaque Gunn-Peterson troughs in the z ~ 5.5 Lyman-alpha forest, which are challenging to explain with conventional models of the post-reionization intergalactic medium. Here we demonstrate that observations of the galaxy populations in the vicinity of the deepest troughs can distinguish two competing models for these features: deep voids where the ionizing background is weak due to fluctuations in the mean free path of ionizing photons would show a deficit of galaxies, while residual temperature variations from extended, inhomogeneous reionization would show an overdensity of galaxies. We use large (~550 Mpc/h) semi-numerical simulations of these competing explanations to predict the galaxy populations in the largest of the known troughs at z ~ 5.7. We quantify the strong correlation of Lyman-alpha effective optical depth and galaxy surface density in both models and estimate the degree to which realistic surveys can measure such a correlation. While a spectroscopic galaxy survey is ideal, we also show that a relatively inexpensive narrowband survey of Lyman-alpha-emitting galaxies is ~90% likely to distinguish between the competing models.