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

Abstract

Observed scatter in the Ly α opacity of quasar sightlines at z < 6 has motivated measurements of the correlation between Ly α 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 Ly α troughs at z ∼ 5.7 with a deficit of Ly α 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 Ly α 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) Ly α opacities. There is tension at low opacities, however, as the models tend to associate lower IGM Ly α 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.

Keywords

• Intergalactic medium
• Reionization
• High-redshift galaxies
• Lyman-alpha galaxies