title: A massive protocluster of galaxies at a redshift of z ≈ 5.3.
abstract: Massive clusters of galaxies have been found that date from as early as 3.9 billion years (3.9 Gyr; z = 1.62) after the Big Bang, containing stars that formed at even earlier epochs. Cosmological simulations using the current cold dark matter model predict that these systems should descend from 'protoclusters'-early overdensities of massive galaxies that merge hierarchically to form a cluster. These protocluster regions themselves are built up hierarchically and so are expected to contain extremely massive galaxies that can be observed as luminous quasars and starbursts. Observational evidence for this picture, however, is sparse because high-redshift protoclusters are rare and difficult to observe. Here we report a protocluster region that dates from 1 Gyr (z = 5.3) after the Big Bang. This cluster of massive galaxies extends over more than 13 megaparsecs and contains a luminous quasar as well as a system rich in molecular gas. These massive galaxies place a lower limit of more than 4 × 10(11) solar masses of dark and luminous matter in this region, consistent with that expected from cosmological simulations for the earliest galaxy clusters.
authors.affiliation: Spitzer Science Centre, 314-6 California Institute of Technology, 1200 East California Boulevard, Pasadena, California 91125, USA. capak@astro.caltech.edu
authors.forename: Peter L
authors.initials: PL
authors.lastname: Capak
authors.forename: Dominik
authors.initials: D
authors.lastname: Riechers
authors.forename: Nick Z
authors.initials: NZ
authors.lastname: Scoville
authors.forename: Chris
authors.initials: C
authors.lastname: Carilli
authors.forename: Pierre
authors.initials: P
authors.lastname: Cox
authors.forename: Roberto
authors.initials: R
authors.lastname: Neri
authors.forename: Brant
authors.initials: B
authors.lastname: Robertson
authors.forename: Mara
authors.initials: M
authors.lastname: Salvato
authors.forename: Eva
authors.initials: E
authors.lastname: Schinnerer
authors.forename: Lin
authors.initials: L
authors.lastname: Yan
authors.forename: Grant W
authors.initials: GW
authors.lastname: Wilson
authors.forename: Min
authors.initials: M
authors.lastname: Yun
authors.forename: Francesca
authors.initials: F
authors.lastname: Civano
authors.forename: Martin
authors.initials: M
authors.lastname: Elvis
authors.forename: Alexander
authors.initials: A
authors.lastname: Karim
authors.forename: Bahram
authors.initials: B
authors.lastname: Mobasher
authors.forename: Johannes G
authors.initials: JG
authors.lastname: Staguhn