We present the catalogue and properties of sources in AS2UDS, an 870-µm continuum survey with the Atacama Large Millimetre/sub-millimetre Array (ALMA) of 716 single-dish sub-millimetre sources detected in the UKIDSS/UDS field by the SCUBA-2 Cosmology Legacy Survey. In our sensitive ALMA follow-up observations we detect 708 sub-millimetre galaxies (SMGs) at > 4.3σ significance across the ∼ 1-degree diameter field. We combine our precise ALMA positions with the extensive multi-wavelength coverage in the UDS field to fit the spectral energy distributions of our SMGs to derive a median redshift of z phot = 2.61±0.09. This large sample reveals a statistically significant trend of increasing sub-millimetre flux with redshift suggestive of galaxy downsizing. 101 ALMA maps do not show a > 4.3σ SMG, but we demonstrate from stacking Herschel SPIRE observations at these positions, that the vast majority of these blank maps correspond to real single-dish sub-millimetre sources. We further show that these blank maps contain an excess of galaxies at z phot = 1.5-4 compared to random fields, similar to the redshift range of the ALMA-detected SMGs. In addition, we combine X-ray and mid-infrared active galaxy nuclei activity (AGN) indicators to yield a likely range for the AGN fraction of 8-28 % in our sample. Finally, we compare the redshifts of this population of high-redshift, strongly star-forming galaxies with the inferred formation redshifts of massive, passive galaxies being found out to z ∼ 2, finding reasonable agreement -in support of an evolutionary connection between these two classes of massive galaxy.

We explore the structures of protoclusters and their relationship with high redshift clusters using the Millennium Simulation combined with a semi-analytic model. We find that protoclusters are very extended, with 90 per cent of their mass spread across ∼ 35 h −1 Mpc comoving at z = 2 (∼ 30 arcmin). The 'main halo', which can manifest as a high redshift cluster or group, is only a minor feature of the protocluster, containing less than 20 per cent of all protocluster galaxies at z = 2. Furthermore, many protoclusters do not contain a main halo that is massive enough to be identified as a high redshift cluster. Protoclusters exist in a range of evolutionary states at high redshift, independent of the mass they will evolve to at z = 0. We show that the evolutionary state of a protocluster can be approximated by the mass ratio of the first and second most massive haloes within the protocluster, and the z = 0 mass of a protocluster can be estimated to within 0.2 dex accuracy if both the mass of the main halo and the evolutionary state is known. We also investigate the biases introduced by only observing star-forming protocluster members within small fields. The star formation rate required for line-emitting galaxies to be detected is typically high, which leads to the artificial loss of low mass galaxies from the protocluster sample. This effect is stronger for observations of the centre of the protocluster, where the quenched galaxy fraction is higher. This loss of low mass galaxies, relative to the field, distorts the size of the galaxy overdensity, which in turn can contribute to errors in predicting the z = 0 evolved mass.

We present an analysis of the morphology and profiles of the dust continuum emission in 153 bright sub-millimetre galaxies (SMGs) detected with ALMA at signal to noise ratios of > 8 in high-resolution 0. 18 (∼ 1 kpc) 870 µm maps. We measure sizes, shapes and light profiles for the rest-frame far-infrared emission from these luminous starforming systems and derive a median effective radius (R e ) of 0. 10 ± 0. 04 for our sample with a median flux of S 870 = 5.6 ± 0.2 mJy. We find that the apparent axial ratio (b/a) distribution of the SMGs peaks at b/a ∼ 0.63 ± 0.02 and is best described by triaxial morphologies, while their emission profiles are best fit by a Sérsic model with n 1.0 ± 0.1, similar to exponential discs. This combination of triaxiality and n ∼ 1 Sérsic index are characteristic of bars and we suggest that the bulk of the 870 µm dust continuum emission in the central ∼ 2 kpc of these galaxies arises from bar-like structures. As such we caution against using the orientation of shape of the bright dust continuum emission at resolution to assess either the orientation of any disc on the sky or tits inclination. By stacking our 870 µm maps we recover faint extended dust continuum emission on ∼ 4 kpc scales which contributes 13 ± 1% of the total 870 µm emission. The scale of this extended emission is similar to that seen for the molecular gas and rest-frame optical light in these systems, suggesting that it represents an extended dust and gas disc at radii larger than the more active bar component. Including this component in our estimated size of the sources we derive a typical effective radius of 0. 15 ± 0. 05 or 1.2 ± 0.4 kpc. Our results suggest that kpc-scale bars are ubiquitous features of high star-formation rate systems at z 1, while these systems also contain fainter and more extended gas and stellar envelopes. We suggest that these features, seen some 10-12 Gyrs ago, represent the formation phase of the earliest galactic-scale components: stellar bulges.

We report the first results of AS2UDS: an 870 µm continuum survey with the Atacama Large Millimeter/Submillimeter Array (ALMA) of a total area of ∼ 50 arcmin 2 comprising a complete sample of 716 submillimeter sources drawn from the SCUBA-2 Cosmology Legacy Survey (S2CLS) map of the UKIDSS/UDS field. The S2CLS parent sample covers a 0.96 degree 2 field at σ 850 = 0.90 ± 0.05 mJy beam −1 . Our deep, high-resolution ALMA observations with σ 870 ∼ 0.25 mJy and a 0. 15-0. 30 FWHM synthesized beam, provide precise locations for 695 submillimeter galaxies (SMGs) responsible for the submillimeter emission corresponding to 606 sources in the low resolution, singledish map. We measure the number counts of SMGs brighter than S 870 ≥ 4 mJy, free from the effects of blending and show that the normalisation of the counts falls by 28 ± 2 % in comparison to the SCUBA-2 parent sample, but that the shape remains unchanged. We determine that 44 +16 −14 % of the brighter single-dish sources with S 850 ≥ 9 mJy consist of a blend of two or more ALMA-detectable SMGs brighter than S 870 ∼ 1 mJy (corresponding to a galaxy with a totalinfrared luminosity of L IR > ∼ 10 12 L ), in comparison to 28 ± 2 % for the single-dish sources at S 850 ≥ 5 mJy. Using the 46 single-dish submillimeter sources that contain two or more ALMA-detected SMGs with photometric redshifts, we show that there is a significant statistical excess of pairs of SMGs with similar redshifts (< 1 % probability of occurring by chance), suggesting that at least 30 % of these blends arise from physically associated pairs of SMGs.

We report spectroscopic results from our 40-orbit Hubble Space Telescope slitless grism spectroscopy program observing the 20 densest Clusters Around Radio-Loud AGN (CARLA) candidate galaxy clusters at 1.4<z<2.8. These candidate rich structures, among the richest and most distant known, were identified on the basis of [3.6]-[4.5] color from a 408hr multi-cycle Spitzer program targeting 420 distant radio-loud AGN. We report the spectroscopic confirmation of 16 distant structures at 1.4<z<2.8 associated with the targeted powerful highredshift radio-loud AGN. We also report the serendipitous discovery and spectroscopic confirmation of seven additional structures at 0.87<z<2.12 not associated with the targeted radio-loud AGN. We find that 10 10 -10 11 M e member galaxies of our confirmed CARLA structures form significantly fewer stars than their field counterparts at all redshifts within 1.4z2. We also observe higher star-forming activity in the structure cores up to z=2, finding similar trends as cluster surveys at slightly lower redshifts (1.0 < z < 1.5). By design, our efficient strategy of obtaining just two grism orbits per field only obtains spectroscopic confirmation of emission line galaxies. Deeper spectroscopy will be required to study the population of evolved, massive galaxies in these (forming) clusters. Lacking multi-band coverage of the fields, we adopt a very conservative approach of calling all confirmations "structures," although we note that a number of features are consistent with some of them being bona fide galaxy clusters. Together this survey represents a unique and large homogenous sample of spectroscopically confirmed structures at high redshifts, potentially more than doubling the census of confirmed, massive clusters at z>1.4.