We report the identification of an extreme proto-cluster of galaxies in the early Universe whose core (nicknamed Dusty Red Core, DRC, because of its very red color in the Herschel SPIRE 250-, 350and 500-µm bands) is formed by at least ten dusty star-forming galaxies (DSFGs), spectroscopically confirmed to lie at z spec = 4.002 via detection of [C i](1-0), 12 CO(6-5), 12 CO(4-3), 12 CO(2-1) and H 2 O(2 11 − 2 02 ) emission lines, detected using ALMA and ATCA. The spectroscopically-confirmed components of the proto-cluster are distributed over a 260 kpc × 310 kpc region and have a collective obscured star-formation rate (SFR) of ∼ 6500 M ⊙ yr −1 , considerably higher than has been seen before in any proto-cluster of galaxies or over-densities of DSFGs at z 4. Most of the star formation is taking place in luminous DSFGs since no Lyα emitters are detected in the proto-cluster core, apart from a Lyα blob located next to one of the DRC dusty components, extending over 60 kpc. The total obscured SFR of the proto-cluster could rise to SFR ∼ 14, 400 M ⊙ yr −1 if all the members of an over-density of bright DSFGs discovered around DRC in a wide-field LABOCA 870-µm image are part of the same structure. [C i](1-0) emission reveals that DRC has a total molecular gas mass of at least M H2 ∼ 6.6 × 10 11 M ⊙ , and its total halo mass could be as high as ∼ 4.4 × 10 13 M ⊙ , indicating that it is the likely progenitor of a cluster at least as massive as Coma at z = 0. The relatively short gas-depletion times of the DRC components suggest either the presence of a mechanism able to trigger extreme star formation simultaneously in galaxies spread over a few hundred kpc or the presence of gas flows from the cosmic web able to sustain star formation over several hundred million years.

Citation for published item:svisonD FtF nd vewisD eFtFF nd eissD eF nd erumugmD F nd impsonD tFwF nd rollndD FF nd wddoxD F nd hunneD vF nd linteD iF nd vn der erfD F nd ymontD eF nd hnneruerD rF nd milD sF nd fertoldiD pF nd fremerD wF nd fussmnnD FF nd giD FEF nd glementsD hFvF nd gooryD eF nd he ottiD qF nd ilesD FeF nd pullerD gF nd qonzlezExuevoD tF nd srD iF nd xegrelloD wF nd yteoD sF nd ¡ erezEpournonD sF nd iehersD hF nd tevensD tFeF nd winnkD eFwF nd rdlowD tF @PHITA 9he spe density of luminous dusty strEforming glxies t z b R X gfeEP nd vefyge imging of ultrred glxies from rershelEeveF9D estrophysil journlFD VQP @IAF pF UVF Further information on publisher's website: Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. ABSTRACT Until recently, only a handful of dusty, star-forming galaxies (DSFGs) were known at z>4, most of them significantly amplified by gravitational lensing. Here, we have increased the number of such DSFGs substantially, selecting galaxies from the uniquely wide 250, 350, and 500 μm Herschel-ATLAS imaging survey on the basis of their extremely red far-infrared colors and faint 350 and 500 μm flux densities, based on which, they are expected to be largely unlensed, luminous, rare, and very distant. The addition of ground-based continuum photometry at longer wavelengths from the James Clerk Maxwell Telescope and the Atacama Pathfinder Experiment allows us to identify the dust peak in their spectral energy distributions (SEDs), with which we can better constrain their redshifts. We select the SED templates that are best able to determine photometric redshifts using a sample of 69 high-redshift, lensed DSFGs, then perform checks to assess the impact of the CMB on our technique, and to quantify the systematic uncertainty associated with our photometric redshifts, σ=0.14 (1 + z), using a sample of 25 galaxies with spectroscopic redshifts, each consistent with our color selection. For Herschel-selected ultrared galaxies with typical colors of S 500 /S 250 ∼2.2 and S 500 /S 350 ∼1.3 and flux densities, S 500 ∼50 mJy, we determine a median redshift, = z 3.66 phot , an interquartile redshift range, 3.30-4.27, with a median rest-frame 8-1000 μm luminosity, L IR , of 1.3× 1013 L e . A third of the galaxies lie at z>4, suggesting a space density, ρ z>4 , of ≈6×10 −7 Mpc −3 . Our sample contains the most luminous known star-forming galaxies, and the most overdense cluster of starbursting proto-ellipticals found to date.

We present sensitive 850 µm imaging of the COSMOS field using 640 hr of new and archival observations taken with SCUBA-2 at the East Asian Observatory's James Clerk Maxwell Telescope. The SCUBA-2 COSMOS survey (S2COSMOS) achieves a median noise level of σ 850µm = 1.2 mJy beam −1 over an area of 1.6 sq. degree (main; Hubble Space Telescope / Advanced Camera for Surveys footprint), and σ 850µm = 1.7 mJy beam −1 over an additional 1 sq. degree of supplementary (supp) coverage. We present a catalogue of 1020 and 127 sources detected at a significance level of > 4 σ and > 4.3 σ in the main and supp regions, respectively, corresponding to a uniform 2 % false-detection rate. We construct the single-dish 850 µm number counts at S 850 > 2 mJy and show that these S2COSMOS counts are in agreement with previous single-dish surveys, demonstrating that degree-scale fields are sufficient to overcome the effects of cosmic variance in the S 850 = 2-10 mJy population. To investigate the properties of the galaxies identified by S2COSMOS sources we measure the surface density of nearinfrared-selected galaxies around their positions and identify an average excess of 2.0 ± 0.2 galaxies within a 13 radius (∼ 100 kpc at z ∼ 2). The bulk of these galaxies represent near-infrared-selected SMGs and / or spatially-correlated sources and lie at a median photometric redshift of z = 2.0 ± 0.1. Finally, we perform a stacking analysis at sub-millimeter and far-infrared wavelengths of stellar-massselected galaxies (M = 10 10 -10 12 M ) from z = 0-4, obtaining high-significance detections at 850 µm in all subsets (signal-to-noise ratio, SNR = 4-30), and investigate the relation between far-infrared luminosity, stellar mass, and the peak wavelength of the dust SED. The publication of this survey adds a new deep, uniform sub-millimeter layer to the wavelength coverage of this well-studied COSMOS field.