UC Riverside

Cosmic reionization, the transition of the intergalactic medium (IGM) from neutral to ionized, was long believed to have concluded by $z=6$. Recent quasar studies, however, have revealed a large scatter in the IGM \lya\ opacity measured at $z<6$ that cannot be explained by density fluctuations alone. There are several proposed explanations, including large-scale fluctuations in the ionizing ultraviolet background (UVB) or temperature of the IGM, or that reionization is still ongoing at $z<6$. We note that while fluctuations in the temperature and UVB are expected as reionization ends, temperature fluctuations enhance the opacity of dense regions and reduce the opacity of underdense regions, while UVB fluctuations have the opposite effect. Models that predict this scatter therefore make strong predictions for how \lya\ opacity is correlated to density. This work begins to characterize the relationship between IGM \lya\ opacity and density, using \lya\ emitting galaxies (LAEs) to probe density. We survey LAEs in the fields surrounding four quasar sightlines with extreme \lya\ opacity: two highly opaque sightlines, which display long \lya\ troughs, and two highly transmissive sightlines. We find that all four fields are underdense in LAEs within 10 $h^{-1}$ Mpc of the quasar sightline, and that the opaque fields are underdense to a larger lateral extent ($\gtrsim$ 20 $h^{-1}$ Mpc). The high-opacity sightlines strongly disfavor temperature models, but are consistent with the other models. There is tension between the UVB and late reionization models and the low-opacity sightlines, however, as the models associate low opacity with high density. We further consider three recently observed fields from the literature, and find that there is scatter in the density associated with moderate opacities that is not well-predicted by the models. Altogether, these observations are not clearly consistent with any of the models we consider. The observed association between low opacity and low density regions may suggest that the ionizing background in low density regions increases rapidly after they have been reionized. If these results are supported by future observations, it would suggest that current reionization models do not fully capture the physical conditions leading to extreme opacity.