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Insight from JWST/NIRCam into galaxy overdensities around bright Lyα emitters during reionization: implications for ionized bubbles at z \sim 9 Published: 5/26/2023 9:37:53 AM Updated: 5/26/2023 9:37:53 AM

Paper abstract: Several studies have detected Ly\alpha from bright (M_{UV}<~-21.5) galaxies during the early stages of reionization despite the significantly neutral IGM. To explain these detections, it has been suggested that z>7 Ly\alpha emitters (LAEs) inhabit large, physical Mpc (pMpc)-scale ionized regions. However, systematic searches for the overdensities of faint galaxies expected to be powering these ionized bubbles around LAEs have been challenging. Here, we use CEERS NIRCam imaging to investigate the possibility of galaxy overdensities associated with two very UV-bright LAEs at z=8.7 in the EGS field. We design a color selection to identify objects at z=8.4-9.1, selecting 28 candidates (including the one LAE in the footprint, EGSY8p7). We model the SEDs of these objects and infer that all are moderately faint (-21.2<~ M_{UV}<~-19.1) with stellar masses of M_* ~ 10^{7.5 - 8.8} M_\odot. All are efficient ionizing agents (\xi_{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. Of the 27 objects besides EGSY8p7, 13 lie within 5' of EGSY8p7, leading to a 4\times overdensity in projection at separations <5' (1.4 pMpc in projection at z=8.7). Separations of 10'-15' (2.7-4.1 projected pMpc) are consistent with an average field. The spatial distribution of our sample may qualitatively suggest a large (R>=2 pMpc) ionized bubble encompassing both LAEs in the field, which is theoretically unexpected but may be possible for a galaxy population four times more numerous than the average to create given moderate escape fractions (f_{esc}\gtrsim0.15) over long times (\gtrsim200 Myr). Upcoming spectroscopic follow up will enable characterization of the size of any ionized bubble that may exist and the properties of the galaxies powering such a bubble.