ESA Science & Technology - Overdensities of Y-dropout galaxies from the brightest-of-reionizing galaxies survey: a candidate protocluster at redshift z~8

Publication date: 24 January 2012

Authors: Trenti, M., et al.

Journal: ApJ
Volume: 746
Issue: 1
Page: 55
Year: 2012

Copyright: IOP Publishing

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 HST 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 M_AB ~ -20), with detections at e5 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 Y₀₉₈-dropouts at > 99.84% confidence. Field BoRG58, which contains the best bright z ~ 8 candidate (M_AB = -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 M_h ~ (4-7)×10¹¹ M_sun (~5 sigma density peak) and is surrounded by several M_h ~ 10¹¹ M_sun halos which could host the fainter dropouts. In this scenario, we predict that all halos will eventually merge into a M_h > 2×10¹⁴ M_sun galaxy cluster by z = 0.