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We present a catalogue of nearly 3,000 submillimetre sources detected at 850um over ~5 square degrees surveyed as part of the James Clerk Maxwell Telescope (JCMT) SCUBA-2 Cosmology Legacy Survey (S2CLS). This is the largest survey of its kind at 850um, probing a meaningful cosmic volume at the peak of star formation activity and increasing the sample size of submillimetre galaxies selected at 850um by an order of magnitude. We describe the wide 850um survey component of S2CLS, which covers the key extragalactic survey fields: UKIDSS-UDS, COSMOS, Akari-NEP, Extended Groth Strip, Lockman Hole North, SSA22 and GOODS-North. The average 1-sigma depth of S2CLS is 1.2 mJy/beam, approaching the SCUBA-2 850um confusion limit, which we determine to be ~0.8 mJy/beam. We measure the single dish 850um number counts to unprecedented accuracy, reducing the Poisson errors on the differential counts to approximately 4% at S_850~3mJy. With several independent fields, we investigate field-to-field variance, finding that the number counts on 0.5-1 degree scales are generally within 50% of the S2CLS mean for S_850>3mJy, with scatter consistent with the Poisson and estimated cosmic variance uncertainties, although there is a marginal (2-sigma) density enhancement in the GOODS-North field. The observed number counts are in reasonable agreement with recent phenomenological and semi-analytic models. Finally, the large solid angle of S2CLS allows us to measure the bright-end counts: at S_850>10mJy there are approximately ten sources per square degree, and we detect the distinctive up-turn in the number counts indicative of the detection of local sources of 850um emission and strongly lensed high-redshift galaxies. Here we describe the data collection and reduction procedures and present calibrated maps and a catalogue of sources; these are made publicly available., Comment: Submitted to MNRAS, comments welcome. Catalogue and maps at http://dx.doi.org/10.5281/zenodo.57792

We report the results of a deep SCUBA-2 850- and 450-$\mu$m survey for dust-obscured ultra-luminous infrared galaxies (U/LIRGs) in the field of the z=1.46 cluster XCS J2215.9-1738. We detect a striking overdensity of sub-millimeter sources coincident with the core of this cluster: $\sim 3-4 \times$ higher than expected in a blank field. We use the likely radio and mid-infrared counterparts to show that the bulk of these sub-millimeter sources have spectroscopic or photometric redshifts which place them in the cluster and that their multi-wavelength properties are consistent with this association. The average far-infrared luminosities of these galaxies are $(1.0\pm0.1) \times 10^{12} L_{\odot}$, placing them on the U/LIRG boundary. Using the total star formation occurring in the obscured U/LIRG population within the cluster we show that the resulting mass-normalized star-formation rate for this system supports previous claims of a rapid increase in star-formation activity in cluster cores out to $z\sim1.5$, which must be associated with the on-going formation of the early-type galaxies which reside in massive clusters today., Comment: accepted by ApJ, 10 pages, 6 figures. 2 tables

We present new, high-angular resolution interferometric observations with the Karl G. Jansky Very Large Array of $^{12}$CO $J=1-0$ line emission and 4-8 GHz continuum emission in the strongly lensed, $z=2.3$ submillimetre galaxy, SMM J21352-0102. Using these data, we identify and probe the conditions in $\sim 100$pc clumps within this galaxy, which we consider to be potential giant molecular cloud complexes, containing up to half of the total molecular gas in this system. In combination with far-infrared and submillimetre data, we investigate the far-infrared/radio correlation, measuring $q_{IR} = 2.39 \pm 0.17$ across SMM J21352. We search for variations in the properties of the interstellar medium throughout the galaxy by measuring the spatially-resolved $q_{IR}$ and radio spectral index, ${\alpha}_{\rm radio}$, finding ranges $q_{IR} = [2.1, 2.6]$ and ${\alpha}_{\rm radio} = [-1.5, -0.7]$. We argue that these ranges in ${\alpha}_{\rm radio}$ and $q_{IR}$ may reflect variations in the age of the ISM material. Using multi-$J$ $^{12}$CO data, we quantitatively test a recent theoretical model relating the star-formation rate surface density to the excitation of $^{12}$CO, finding good agreement between the model and the data. Lastly, we study the Schmidt-Kennicutt relation, both integrated across the system and within the individual clumps. We find small offsets between SMM J21352 and its clumps relative to other star-forming galaxy populations on the Schmidt-Kennicutt plot - such offsets have previously been interpreted as evidence for a bi-modal star-formation law, but we argue that they can be equally-well explained as arising due to a combination of observational uncertainties and systematic biases in the choice of model used to interpret the data., Comment: 15 pages, 5 colour figures; accepted for publication in MNRAS

We analyse HST WFC3/$H_{160}$-band observations of a sample of 48 ALMA-detected submillimeter galaxies (SMGs) in the Extended Chandra Deep Field South field, to study their stellar morphologies and sizes. We detect 79$\pm$17% of the SMGs in the $H_{160}$-band imaging with a median sensitivity of 27.8 mag, and most (80%) of the non-detections are SMGs with 870$\mu$m fluxes of $S_{870} < $3 mJy. With a surface brightness limit of $\mu_H \sim $26 mag arcsec$^{-2}$, we find that 82$\pm$9% of the $H_{160}$-band detected SMGs at $z =$ 1-3 appear to have disturbed morphologies, meaning they are visually classified as either irregulars or interacting systems, or both. By determining a S\'ersic fit to the $H_{160}$ surface-brightness profiles we derive a median S\'ersic index of $n = $1.2$\pm$0.3 and a median half-light radius of $r_e = $4.4$^{+1.1}_{-0.5}$ kpc for our SMGs at $z = $1-3. We also find significant displacements between the positions of the $H_{160}$-component and 870$\mu$m emission in these systems, suggesting that the dusty star-burst regions and less-obscured stellar distribution are not co-located. We find significant differences in the sizes and the S\'ersic index between our $z = $2-3 SMGs and $z \sim $2 quiescent galaxies, suggesting a major transformation of the stellar light profile is needed in the quenching processes if SMGs are progenitors of the red-and-dead $z\sim$2 galaxies. Given the short-lived nature of SMGs, we postulate that the majority of the $z = $2-3 SMGs with $S_{870} \gtrsim $2 mJy are early/mid-stage major mergers., Comment: ApJ in press, 32 pages, 13 figures

We present high-resolution (0.3'') ALMA 870um imaging of 52 sub-millimeter galaxies (SMGs) in the Ultra Deep Survey (UDS) field and investigate the size and morphology of the sub-millimeter (sub-mm) emission on 2-10kpc scales. We derive a median intrinsic angular size of FWHM=0.30$\pm$0.04'' for the 23 SMGs in the sample detected at a signal-to-noise ratio (SNR) >10. Using the photometric redshifts of the SMGs we show that this corresponds to a median physical half-light diameter of 2.4$\pm$0.2kpc. A stacking analysis of the SMGs detected at an SNR <10 shows they have sizes consistent with the 870um-bright SMGs in the sample. We compare our results to the sizes of SMGs derived from other multi-wavelength studies, and show that the rest-frame ~250um sizes of SMGs are consistent with studies of resolved 12CO (J=3-2 to 7-6) emission lines, but that sizes derived from 1.4GHz imaging appear to be approximately two times larger on average, which we attribute to cosmic ray diffusion. The rest-frame optical sizes of SMGs are around four times larger than the sub-millimeter sizes, indicating that the star formation in these galaxies is compact relative to the pre-existing stellar distribution. The size of the starburst region in SMGs is consistent with the majority of the star formation occurring in a central region, a few kpc in extent, with a median star formation rate surface density of 90$\pm$30Msol/yr/kpc$^2$, which may suggest that we are witnessing an intense period of bulge growth in these galaxies., Comment: 16 pages, 7 figures, accepted for publication in ApJ