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 analyse the multi-frequency radio spectral properties of 41 6 GHz-detected ALMA-identified, submillimetre galaxies (SMGs), observed at 610 MHz, 1.4 GHz, and 6 GHz with GMRT and the VLA. Combining high-resolution (∼ 0.5 ′′ ) 6 GHz radio and ALMA 870 µm imaging (tracing rest-frame ∼ 20 GHz, and ∼ 250 µm dust continuum), we study the far-infrared/radio correlation via the logarithmic flux ratio q IR , measuring q IR = 2.20 ± 0.06 for our sample. We show that the high-frequency radio sizes of SMGs are ∼ 1.9 ± 0.4× (∼ 2-3 kpc) larger than those of the cool dust emission, and find evidence for a subset of our sources being extended on ∼ 10 kpc scales at 1.4 GHz. By combining radio flux densities measured at three frequencies, we can move beyond simple linear fits to the radio spectra of high-redshift star-forming galaxies, and search for spectral curvature, which has been observed in local starburst galaxies. At least a quarter (10/41) of our sample show evidence of a spectral break, with a median α 1.4 GHz 610 GHz = −0.60 ± 0.06, but α 6 GHz 1.4 GHz = −1.06 ± 0.04 obtained via stacking -a high-frequency flux deficit relative to simple extrapolations from the low-frequency data. We explore this result within this subset of sources in the context of age-related synchrotron losses, showing that a combination of weak magnetic fields (B ∼ 35 µG) and young ages (t SB ∼ 40-80 Myr) for the central starburst can reproduce the observed spectral break. Assuming these represent evolved (but ongoing) starbursts and we are observing these systems roughly half-way through their current episode of star formation, this implies starburst durations of 100 Myr, in reasonable agreement with estimates derived via gas depletion timescales.

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We report the discovery of two new giant radio galaxies (GRGs) using the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE) survey. Both GRGs were found within a ${\sim}1\,$ deg2 region inside the COSMOS field. They have redshifts of z = 0.1656 and z = 0.3363 and physical sizes of 2.4 and 2.0 Mpc, respectively. Only the cores of these GRGs were clearly visible in previous high-resolution Very Large Array observations, since the diffuse emission of the lobes was resolved out. However, the excellent sensitivity and uv coverage of the new MeerKAT telescope allowed this diffuse emission to be detected. The GRGs occupy an unpopulated region of radio power – size parameter space. Based on a recent estimate of the GRG number density, the probability of finding two or more GRGs with such large sizes at z &lt; 0.4 in a ${\sim}1\,$ deg2 field is only 2.7 × 10−6, assuming Poisson statistics. This supports the hypothesis that the prevalence of GRGs has been significantly underestimated in the past due to limited sensitivity to low surface brightness emission. The two GRGs presented here may be the first of a new population to be revealed through surveys like MIGHTEE that provide exquisite sensitivity to diffuse, extended emission.