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Theoretical and numerical modeling of dark-matter halo assembly predicts thatthe most luminous galaxies at high redshift are surrounded by overdensities offainter companions. We test this prediction with HST observations acquired byour Brightest of Reionizing Galaxies (BoRG) survey, which identified four verybright z~8 candidates as Y-dropout sources in four of the 23 non-contiguousWFC3 fields observed. We extend here the search for Y-dropouts to fainterluminosities (M_* galaxies with M_AB\sim-20), with detections at >5sigmaconfidence (compared to >8sigma confidence adopted earlier) identifying 17 newcandidates. We demonstrate that there is a correlation between number counts offaint and bright Y-dropouts at >99.84% confidence. Field BoRG58, which containsthe best bright z\sim8 candidate (M_AB=-21.3), has the most significantoverdensity of faint Y-dropouts. Four new sources are located within 70arcsec(corresponding to 3.1 comoving Mpc at z=8) from the previously known brighterz\sim8 candidate. The overdensity of Y-dropouts in this field has a physicalorigin to high confidence (p>99.975%), independent of completeness andcontamination rate of the Y-dropout selection. We modeled the overdensity bymeans of cosmological simulations and estimate that the principal dark matterhalo has mass M_h\sim(4-7)x10^11Msun (\sim5sigma density peak) and issurrounded by several M_h\sim10^11Msun halos which could host the fainterdropouts. In this scenario, we predict that all halos will eventually mergeinto a M_h>2x10^14Msun galaxy cluster by z=0. Follow-up observations withground and space based telescopes are required to secure the z\sim8 nature ofthe overdensity, discover new members, and measure their precise redshift.

Overdensities of Y-dropout Galaxies from the Brightest-of-Reionizing Galaxies Survey: A Candidate Protocluster at Redshift z~8