S20A0093abst
S20A0093
We aim to understand the origin of the observed large-scale fluctuations in the effective H i optical depth (τeff) of the intergalactic medium (IGM) at the tail end of reionization. The observed τeff variations at z>∼5.5 cannot be explained by the density fluctuations alone, and thus additional spatial fluctuations either in the UV background or IGM temperature appear to be required. Elucidation of the origin of the τeff variations will provide us with new insights about the detailed processes of reionization. Four possible scenarios have been proposed to account for the observed τeff variation. The models predict quite different relationships between density and τeff, and thus can in principle be tested by observations. We are currently carrying out galaxy surveys in multiple quasar fields, where Lyα forest measurements are available, to achieve this goal through approved observing time in S18B-S19B. The first observations have proven that our strategy works well and are already giving us a striking indication of an association between a high-τeff region and an underdensity of galaxies, as detailed in SJ. In S20A, we request additional observing time to complete observations of the target fields that are visible in the A term, which are essential for more conclusive statements.
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