The SCUBA-2 Cosmology Legacy Survey: 850 μm maps, catalogues and number counts

We present a catalogue of ˜3000 submillimetre sources detected(≥3.5σ) at 850 μm over ˜5 deg2 surveyed aspart of the James Clerk Maxwell Telescope (JCMT) SCUBA-2 Cosmology Legacy Survey (S2CLS). This is the largest survey of its kind at 850μm, increasing the sample size of 850 μm selected submillimetre galaxies by an order of magnitude. The wide 850 μm survey component of S2CLS covers the extragalactic fields: UKIDSS-UDS, COSMOS, Akari-NEP,Extended Groth Strip, Lockman Hole North, SSA22 and GOODS-North. Theaverage 1σ depth of S2CLS is 1.2 mJy beam-1,approaching the SCUBA-2 850 μm confusion limit, which we determine tobe σc ≈ 0.8 mJy beam-1. We measure the850 μm number counts, reducing the Poisson errors on the differential counts to approximately 4 per cent at S850 ≈ 3 mJy. With several independent fields, we investigate field-to-field variance, finding that the number counts on 0.5°-1° scales are generally within 50 per cent of the S2CLS mean for S850 > 3 mJy, with scatter consistent with the Poisson and estimated cosmic variance uncertainties, although there is a marginal (2σ) density enhancement in GOODS-North. The observed counts are in reasonable agreement with recent phenomenological and semi-analytic models, although determining the shape of the faint-end slope (S850<3 mJy) remains a key test. The large solid angle of S2CLS allows usto measure the bright-end counts: at S850 > 10 mJy thereare approximately 10 sources per square degree, and we detect the distinctive up-turn in the number counts indicative of the detection oflocal sources of 850 μm emission, and strongly lensed high-redshift galaxies. All calibrated maps and the catalogue are made publicly available.