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

We present a catalogue of 3000 submillimetre sources detected (3.5 ) at 850 m over 5 deg 2 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 850 m, increasing the sample size of 850 m selected submillimetre galaxies by an order of magnitude. The wide 850 m survey component of S2CLS covers the extragalactic fields: UKIDSS-UDS, COS-MOS, Akari-NEP, Extended Groth Strip, Lockman Hole North, SSA22 and GOODS-North. The average 1 depth of S2CLS is 1.2 mJy beam -1 , approaching the SCUBA-2 850 m confusion limit, which we determine to be c 0.8 mJy beam -1 . We measure the 850 m number counts, reducing the Poisson errors on the differential counts to approximately 4 per cent at S 850 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 S 850 > 3 mJy, with scatter consistent with the Poisson and estimated cosmic variance uncertainties, although there is a marginal (2 ) 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 (S 850 < 3 mJy) remains a key test. The large solid angle of S2CLS allows us to measure the bright-end counts: at S 850 > 10 mJy there are approximately 10 sources per square degree, and we detect the distinctive up-turn in the number counts indicative of the detection of local sources of 850 m emission,