Overdensities of Y-dropout Galaxies from the Brightest-of-Reionizing
  Galaxies Survey: A Candidate Protocluster at Redshift z~8
  Theoretical and numerical modeling of dark-matter halo assembly predicts that
the most luminous galaxies at high redshift are surrounded by overdensities of
fainter companions. We test this prediction with HST observations acquired by
our Brightest of Reionizing Galaxies (BoRG) survey, which identified four very
bright z~8 candidates as Y-dropout sources in four of the 23 non-contiguous
WFC3 fields observed. We extend here the search for Y-dropouts to fainter
luminosities (M_* galaxies with M_AB\sim-20), with detections at >5sigma
confidence (compared to >8sigma confidence adopted earlier) identifying 17 new
candidates. We demonstrate that there is a correlation between number counts of
faint and bright Y-dropouts at >99.84% confidence. Field BoRG58, which contains
the best bright z\sim8 candidate (M_AB=-21.3), has the most significant
overdensity of faint Y-dropouts. Four new sources are located within 70arcsec
(corresponding to 3.1 comoving Mpc at z=8) from the previously known brighter
z\sim8 candidate. The overdensity of Y-dropouts in this field has a physical
origin to high confidence (p>99.975%), independent of completeness and
contamination rate of the Y-dropout selection. We modeled the overdensity by
means of cosmological simulations and estimate that the principal dark matter
halo has mass M_h\sim(4-7)x10^11Msun (\sim5sigma density peak) and is
surrounded by several M_h\sim10^11Msun halos which could host the fainter
dropouts. In this scenario, we predict that all halos will eventually merge
into a M_h>2x10^14Msun galaxy cluster by z=0. Follow-up observations with
ground and space based telescopes are required to secure the z\sim8 nature of
the overdensity, discover new members, and measure their precise redshift.
