OVERDENSITIES OFY-DROPOUT GALAXIES FROM THE BRIGHTEST-OF-REIONIZING GALAXIES SURVEY: A CANDIDATE PROTOCLUSTER AT REDSHIFTz≈ 8

Abstract

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 Y 098 -dropout sources in four of the 23 non-contiguous Wide Field Camera 3 fields observed. We extend here the search for Y 098 -dropouts to fainter luminosities (M * galaxies with M AB ∼ −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 Y 098 -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 Y 098 -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 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 ≈ (4-7) × 10 11 M (∼5σ density peak) and is surrounded by several M h ≈ 10 11 M halos which could host the fainter dropouts. In this scenario, we predict that all halos will eventually merge into a M h > 2 × 10 14 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.

A spectroscopically confirmedz = 1.327 galaxy-scale deflector magnifying az ∼ 8 Lyman-break galaxy in the Brightest of Reionizing Galaxies survey

“…The core of this earlier dataset is based on WFC3 imaging in four filters (V600, Y098, J125, H160) obtained as part of HST programs #11700 and #12572 (PI Trenti), complemented by archival data with a similar observational design and the same infrared filters, but with F600LP substituting F606W (HST program # 11702, PI Yan, Yan et al 2011). A full description of the survey and of its findings is reported by Trenti et al (2011Trenti et al ( , 2012, Bradley et al (2012) and Schmidt et al (2014). To summarize, the most recent comprehensive data release of the BoRG survey yielded n = 38 z ∼ 8 LBG candidates identified as Y098-dropouts with J125 magnitudes between 25.5 and 27.6 mag (Schmidt et al 2014).…”

“…Based on dark matter (DM) clustering, we expect additional dropouts in the field if the primary image is not strongly magnified. Abundance matching predicts that the dropout is hosted in a rare dark matter halo with MDM 2 × 10 11 M (Muñoz & Loeb 2008;Trenti et al 2012). However, the connection between DM halo mass and galaxy luminosity seems unclear at high redshift, as highlighted by the recent measurement of the two-point correlation function at z 7 from CANDELS and XDF catalogs (Barone-Nugent et al 2014) and by the lack of strong clustering around a more luminous and isolated galaxy at z = 7.7 .…”

A SYSTEMATIC SURVEY OF PROTOCLUSTERS AT z ∼ 3–6 IN THE CFHTLS DEEP FIELDS

“…The highest-redshift protocluster discovered to date is at z = 6.01 (Toshikawa et al 2012) and is composed of at least 10 galaxies (Toshikawa et al 2014). Some candidates without spectroscopic confirmation beyond z = 6 have also been found (Trenti et al 2012;Ishigaki et al 2016). Although the majority of protocluster members are young and star-forming galaxies, a red sequence, composed of bright and red galaxies, is found to appear in protoclusters around z ∼ 2-3 (e.g., Kurk et al 2004;Kodama et al 2007;Zirm et al 2008;Kubo et al 2013;Lemaux et al 2014).…”

THE CANDIDATE CLUSTER AND PROTOCLUSTER CATALOG (CCPC) OF SPECTROSCOPICALLY IDENTIFIED STRUCTURES SPANNING 2.74 < z < 3.71

“…At these higher redshifts, there have been a number of amazing discoveries of groups of galaxies found at z 4 (Venemans et al 2002;Lee et al 2014), z 6 > (Utsumi et al 2010;Toshikawa et al 2014), and up to z 8 (Trenti et al 2012). The emergence of these protoclusters offer a test of ΛCDM, as simulations provide constraints on the number of clusters and on structure mass as a function of redshift and the dark energy equation of state (Vikhlinin et al 2009;Mortonson et al 2011), with greater leverage provided at larger redshifts.…”