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 \sim 8 candidates as Y _ { 098 } -dropout sources in four of the 23 non-contiguous WFC3 fields observed .
We extend here the search for Y _ { 098 } -dropouts to fainter luminosities ( M _ { * } galaxies with M _ { AB } \sim - 20 ) , with detections at \geqslant 5 \sigma confidence ( compared to the 8 \sigma confidence threshold adopted earlier ) identifying 17 new candidates .
We demonstrate that there is a correlation between number counts of faint and bright Y _ { 098 } -dropouts at \geqslant 99.84 \% confidence .
Field BoRG58 , which contains the best bright z \sim 8 candidate ( M _ { AB } = -21.3 ) , has the most significant overdensity of faint Y _ { 098 } -dropouts .
Four new sources are located within 70 ^ { \prime \prime } ( corresponding to 3.1 comoving \mathrm { Mpc } at z = 8 ) from the previously known brighter z \sim 8 candidate .
The overdensity of Y _ { 098 } -dropouts in this field has a physical origin to very high confidence ( p > 99.975 \% ) , independent of completeness and contamination rate of the Y _ { 098 } -dropout selection .
We modeled the overdensity by means of cosmological simulations and estimate that the principal dark matter halo has mass M _ { h } \approx ( 4 - 7 ) \times 10 ^ { 11 } ~ { } \mathrm { M _ { \sun } } ( \sim 5 \sigma density peak ) and is surrounded by several M _ { h } \approx 10 ^ { 11 } ~ { } \mathrm { M _ { \sun } } halos which could host the fainter dropouts .
In this scenario , we predict that all halos will eventually merge into a M _ { h } > 2 \times 10 ^ { 14 } ~ { } \mathrm { M _ { \sun } } galaxy cluster by z = 0 .
Follow-up observations with ground and space based telescopes are required to secure the z \sim 8 nature of the overdensity , discover new members , and measure their precise redshift .