* Astronomy

Hubble Pinpoints Farthest Protocluster of Galaxies Ever Seen



Using NASA's Hubble Space Telescope, astronomers have uncovered a cluster of galaxies in the initial stages of development. It is the most distant such grouping ever observed in the early universe.
In a random sky survey made in near-infrared light, Hubble found five tiny galaxies clustered together 13.1 billion light-years away. They are among the brightest galaxies at that epoch and very young -- existing just 600 million years after the big bang.
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Title: Overdensities of Y-dropout Galaxies from the Brightest-of-Reionizing Galaxies Survey: A Candidate Protocluster at Redshift z~8
Authors: M. Trenti, L. D. Bradley, M. Stiavelli, J. M. Shull, P. Oesch, R. J. Bouwens, J. A. Munoz, E. Romano-Diaz, T. Treu, I. Shlosman, C. M. Carollo
(Version v2)

Theoretical and numerical modelling 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 Reionising 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~-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 ~8 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 modelled the overdensity by means of cosmological simulations and estimate that the principal dark matter halo has mass M_h ~(4-7)x10^11solar mass (~5sigma density peak) and is surrounded by several M_h ~10^11solar mass halos which could host the fainter dropouts. In this scenario, we predict that all halos will eventually merge into a M_h>2x10^14 solar mass 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.

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