Searching for Islands of Reionization: A Potential Ionized Bubble
  Powered by a Spectroscopic Overdensity at z = 8.7
  We present the results from a spectroscopic survey using the MOSFIRE
near-infrared spectrograph on the 10m Keck telescope to search for Ly$\alpha$
emission from candidate galaxies at z ~9-10 in four of the CANDELS fields
(GOODS-N, EGS, UDS, and COSMOS). We observed 11 target galaxies, detecting
Ly$\alpha$ from one object in ~8.1 hours of integration, at z = 8.665 +/- 0.001
with an integrated signal-to-noise > 7. This galaxy is in the CANDELS Extended
Groth Strip (EGS) field, and lies physically close (3.5 physical Mpc [pMpc]) to
another confirmed galaxy in this field with Ly$\alpha$ detected at z=8.683
(Zitrin et al. 2015). The detection of Ly$\alpha$ suggests the existence of
large (~1 pMpc) ionized bubbles fairly early in the reionization process. We
explore the ionizing output needed to create bubbles of this size at this epoch
and find that such a bubble requires more than the ionizing power provided by
the full expected population of galaxies (by integrating the UV Luminosity
Function down to M_UV = -13). The Ly$\alpha$ we detect would be able to escape
the predominantly-neutral intergalactic medium at this epoch if our galaxy is
inhabiting an overdensity, which would be consistent with the photometric
overdensity previously identified in this region by Finkelstein et al. 2021.
This implies that the CANDELS EGS field is hosting an overdensity at z = 8.7
which is powering one (or more) ionized bubbles, a hypothesis that will be
imminently testable with forthcoming James Webb Space Telescope observations in
this field.
