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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.

We present new Atacama Large Millimeter/Submillimeter Array (ALMA) 850 μm continuum observations of the original Lyα Blob (LAB) in the SSA22 field at z=3.1 (SSA22-LAB01). The ALMA map resolves the previously identified submillimeter source into three components with a total flux density ofS 850 =1.68±0.06 mJy, corresponding to a star-formation rate of ∼150 M e yr −1 . The submillimeter sources are associated with several faint (m ≈ 27 mag) rest-frame ultraviolet sources identified in Hubble Space Telescope Imaging Spectrograph (STIS) clear filter imaging (λ ≈ 5850 Å). One of these companions is spectroscopically confirmed with the Keck Multi-Object Spectrometer For Infra-Red Exploration to lie within 20 projected kpc and 250 km s −1 of one of the ALMA components. We postulate that some of these STIS sources represent a population of lowmass star-forming satellites surrounding the central submillimeter sources, potentially contributing to their growth and activity through accretion. Using a high-resolution cosmological zoom simulation of a 10 13 M e halo at z=3, including stellar, dust,and Lyα radiative transfer, we can model the ALMA+STIS observations and demonstrate that Lyα photons escaping from the central submillimeter sources are expected to resonantly scatter in neutral hydrogen, the majority of which is predicted to be associated with halo substructure. We show how this process gives rise to extended Lyα emission with similar surface brightness and morphology to observed giant LABs.

The overdense environments of protoclusters of galaxies in the early Universe (z > 2) are expected to accelerate the evolution of galaxies, with an increased rate of stellar mass assembly and black hole accretion compared to co-eval galaxies in the average density 'field'. These galaxies are destined to form the passive population of massive galaxies that dominate the cores of rich clusters today. While signatures of the accelerated growth of galaxies in the SSA22 protocluster (z = 3.1) have been observed, the mechanism driving this remains unclear. Here we show an enhanced rate of galaxy-galaxy mergers could be responsible. We morphologically classify Lyman-break Galaxies (LBGs) in the SSA22 protocluster and compare these to those of galaxies in the field at z = 3.1 as either active mergers or non-merging using Hubble Space Telescope ACS/F814W imaging, probing the rest-frame ultraviolet stellar light. We measure a merger fraction of 48±10 per cent for LBGs in the protocluster compared to 30±6 per cent in the field. Although the excess is marginal, an enhanced rate of mergers in SSA22 hints that galaxy-galaxy mergers are one of the key channels driving accelerated star formation and AGN growth in protocluster environments.

We study the heating mechanisms and Lyα escape fractions of 35 Lyα blobs (LABs) at z ≈ 3.1 in the SSA22 field. Dust continuum sources have been identified in 11 of the 35 LABs, all with star formation rates (SFRs) above 100M /yr. Likely radio counterparts are detected in 9 out of 29 investigated LABs. The detection of submm dust emission is more linked to the physical size of the Lyα emission than to the Lyα luminosities of the LABs. A radio excess in the submm/radio detected LABs is common, hinting at the presence of active galactic nuclei. Most radio sources without X-ray counterparts are located at the centers of the LABs. However, all X-ray counterparts avoid the central regions. This may be explained by absorption due to exceptionally large column densities along the line-of-sight or by LAB morphologies, which are highly orientation dependent. The median Lyα escape fraction is about 3% among the submm-detected LABs, which is lower than a lower limit of 11% for the submm-undetected LABs. We suspect that the large difference is due to the high dust attenuation supported by the large SFRs, the dense large-scale environment as well as large uncertainties in the extinction corrections required to apply when interpreting optical data.