OVERDENSITIES OF Y-DROPOUT GALAXIES FROM THE BRIGHTEST-OF-REIONIZING GALAXIES SURVEY: A CANDIDATE PROTOCLUSTER AT REDSHIFT z ≈ 8

Theoretical and numerical modeling of the assembly of dark-matter halos predicts that the most massive and luminous galaxies at high redshift are surrounded by overdensities of fainter companions. We test this prediction with Hubble Space Telescope observations acquired by our Brightest-of-Reionizing Galaxies (BoRG) survey, which identified four very bright z ∼ 8 candidates as Y098-dropout sources in four of the 23 non-contiguous Wide Field Camera 3 fields observed. We extend here the search for Y098-dropouts to fainter luminosities (M* galaxies with MAB ∼ –20), with detections at ≥5σ confidence (compared to the 8σ confidence threshold adopted earlier) identifying 17 new candidates. We demonstrate that there is a correlation between number counts of faint and bright Y098-dropouts at ≥99.84% confidence. Field BoRG58, which contains the best bright z ∼ 8 candidate (MAB = –21.3), has the most significant overdensity of faint Y098-dropouts. Four new sources are located within 70'' (corresponding to 3.1 comoving Mpc at z = 8) from the previously known brighter z ∼ 8 candidate. The overdensity of Y098-dropouts in this field has a physical origin to very high confidence (p > 99.975%), independent of completeness and contamination rate of the Y098-dropout selection. We modeled the overdensity by means of cosmological simulations and estimate that the principal dark-matter halo has mass Mh ≈ (4-7) × 1011 M☉ (∼5σ density peak) and is surrounded by several Mh ≈ 1011 M☉ halos which could host the fainter dropouts. In this scenario, we predict that all halos will eventually merge into a Mh > 2 × 1014 M☉ galaxy cluster by z = 0. Follow-up observations with ground- and space-based telescopes are required to secure the z ∼ 8 nature of the overdensity, discover new members, and measure their precise redshift.