The Relationship between IGM Lyα Opacity and Galaxy Density near the End of Reionization

Abstract

Observed scatter in the Lyα opacity of quasar sightlines at z < 6 has motivated measurements of the correlation between Lyα opacity and galaxy density, as models that predict this scatter make strong and sometimes opposite predictions for how they should be related. Our previous work associated two highly opaque Lyα troughs at z ∼ 5.7 with a deficit of Lyα emitting galaxies (LAEs). In this work, we survey two of the most highly transmissive lines of sight at this redshift toward the z = 6.02 quasar SDSS J1306+0356 and the z = 6.17 quasar PSO J359-06. We find that both fields are underdense in LAEs within 10 h −1 Mpc of the quasar sightline, somewhat less extensive than underdensities associated with Lyα troughs. We combine our observations with three additional fields from the literature and find that while fields with extreme opacities are generally underdense, moderate opacities span a wider density range. The results at high opacities are consistent with models that invoke UV background fluctuations and/or late reionization to explain the observed scatter in intergalactic medium (IGM) Lyα opacities. There is tension at low opacities, however, as the models tend to associate lower IGM Lyα opacities with higher densities. Although the number of fields surveyed is still small, the low-opacity results may support a scenario in which the ionizing background in low-density regions increases more rapidly than some models suggest after becoming ionized. Elevated gas temperatures from recent reionization may also be making these regions more transparent.