Islands of Reionization: Evidence for an Overdensity at z=8.7 in the CANDELS EGS Field

While the end of reionization is a frequent target of observations, how this process begins and what causes it are presently unconstrained. One scenario allows reionization to start late and complete rapidly, where moderately luminous galaxies dominate the ionizing photon budget (Robertson et al. 2015). Another scenario allows a smooth temporal evolution of reionization, where very faint galaxies, common at early times, dominate the photon budget (Finkelstein et al. 2019). These models differ drastically in their predictions for the ionized fraction at z~9 (~20% vs. ~60%), and our recent MOSFIRE program has sought Lyα emission at this epoch to distinguish between these two possibilities. Through improved data reduction and stacking of spectra from our 2018A and 2019A semesters, accounting for the drift of objects in our slits throughout the night and unknown offsets of our objects in the slits, we have found a tentative Lyα detection (5σ) at z = 8.661 in the EGS field in 9.53 hours of integration. If confirmed (through additional integration time and multiple line detections with upcoming NIRES observations) this galaxy would lie physically close (3.5 pMpc) to another confirmed galaxy in this field, with Lyα detected at z = 8.684 (Zitrin et al. 2015). This nearby galaxy also has a tenuous NV emission line (Mainali et al. 2018) indicating the presence of an AGN. Confirmation of our source might suggest the presence of the highest redshift overdensity in the reionization era, indicate the existence of ≥1Mpc ionized bubbles as early as 500Myr after the Big Bang, and shed light on galaxy growth in the early universe.