Insight from JWST/NIRCam into galaxy overdensities around bright Lyα emitters during reionization: implications for ionized bubbles at z ∼ 9

Several studies have detected Lyman-alpha (Lyα) from bright (M_UV≲-21.5) galaxies during the early stages of reionization despite the significantly neutral intergalactic medium. To explain these detections, it has been suggested that z>7 Lyα emitters (LAEs) inhabit physical Mpc (pMpc)-scale ionized regions powered by overdensities of faint galaxies, but systematic searches for these overdensities near LAEs have been challenging. Here, we use CEERS JWST/NIRCam imaging to search for large-scale galaxy overdensities near two very UV-bright, z=8.7 LAEs in the EGS field. We colour select 27 z=8.4-9.1 candidates, including the one LAE in the footprint (EGSY8p7). From SED models, we infer moderately faint UV luminosities (-21.2≲M_UV≲ -19.1) and stellar masses of M_*≈10^7.5-8.8 M_⊙. All are efficient ionizing agents (ξ_ion^*≈10^25.5-26.0 Hz erg^-1) and are generally morphologically simple with only one compact (r_e≲140 to ∼650 pc) star-forming component. 13 candidates lie within 5 arcmin of EGSY8p7, leading to a factor-of-four galaxy overdensity at ≲ 5 arcmin (∼ 1.4 projected pMpc at z∼8.7) separations from EGSY8p7. Separations of 10-15 arcmin (∼2.7-4.1 projected pMpc) are consistent with an average field. The spatial distribution of our sample may qualitatively suggest an R≥2 pMpc ionized bubble encompassing both LAEs in EGS, which is theoretically unexpected but may be possible for a galaxy population 4× more numerous than the average to create with moderate escape fractions (f_esc≳0.15) over long times (≳200 Myr). Upcoming spectroscopic follow-up will characterize the size of any ionized bubble that may exist and the properties of the galaxies powering such a bubble.