On the correlation between Lyα forest opacity and galaxy density in late reionization models

Observations of quasar absorption spectra provide strong evidence that reionization extended below z = 6. The relationship between Lyα forest opacity and local galaxy density (the opacity-density relation) is a key observational test of this scenario. Using narrow-band surveys of z ≈ 5.7 Lyα emitters (LAEs) centered on quasar sight lines, ref. [1] showed that two of the most transmissive Lyα forest segments at this redshift intersect under-densities in the galaxy distribution. This result is in tension with models of a strongly fluctuating ionizing background, including some models of late reionization, which predict that the vast majority of these segments should intersect over-densities where the ionizing intensity is strongest. In this paper, we use radiative transfer simulations to explore in more detail the opacity-density relation predicted by late reionization models. We find that fields like the one toward quasar PSO J359-06 — the more under-dense of the two transmissive sight lines in ref. [1] — are typically associated with recently reionized gas inside of cosmic voids where the hotter temperatures and rarefied densities enhance Lyα transmission. The opacity-density relation's transmissive end is sensitive to the amount of neutral gas in the voids, as well as its morphology, set by the clustering of reionization sources. These effects are, however, largely degenerate with each other. We demonstrate that models with very different source clustering can nonetheless yield nearly identical opacity-density relations when their reionization histories are calibrated to match Lyα forest mean flux measurements at z < 6. In models with fixed source clustering, a lower neutral fraction increases the likelihood of intersecting hot, recently reionized gas in the voids, increasing the likelihood of observing fields like PSO J359-06. For instance, the probability of observing this field is 15% in a model with neutral fraction xHI = 5% at z = 5.7, three times more likely than in a model with xHI = 15%. The opacity-density relation may thus provide a complementary probe of reionization's tail end.