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Abstract

Recent observations have discovered long (up to ̃110 Mpc/h), opaque Gunn-Peterson troughs in the z ̃ 5.5 Lyα 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 Lyα 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 Lyα-emitting galaxies is ̃90% likely to distinguish between the competing models.