Insight from JWST/Near Infrared Camera into galaxy overdensities around bright Lyman-alpha emitters during reionization: implications for ionized bubbles at z ∼ 9

ABSTRACT Several studies have detected Lyman-alpha (Ly α) from bright (M ₔₕ -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 Ly α emitters (LAEs) inhabit physical Mpc (pMpc) -scale ionized regions powered by overdensities of faint galaxies; however, systematic searches for these overdensities near LAEs have been challenging. Here, we use Cosmic Evolution Early Release Science JWST/Near Infrared Camera imaging to search for large-scale galaxy overdensities near two very ultraviolet (UV) -bright, z = 8. 7 LAEs in the Extended Groth Strip (EGS) field. We colour select 27 z = 8. 4–9. 1 candidates, including the one LAE in the footprint (EGSY8p7). From spectral energy distribution models, we infer moderately faint UV luminosities (-21. 2 M ₔₕ -19. 1) and stellar masses of M* ≈ 107. 5–8. 8 M⊙. All are efficient ionizing agents (₈₎₍^* 10^25. 5-26. 0 Hz erg−1) and are generally morphologically simple with only one compact (re ≲ 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 four times more numerous than the average to create with moderate escape fractions (fesc ≳ 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.