An ALMA survey of the brightest sub-millimetre sources in the SCUBA-2
  COSMOS field
  We present an ALMA study of the ~180 brightest sources in the SCUBA-2 map of
the COSMOS field from the S2COSMOS survey, as a pilot study for AS2COSMOS - a
full survey of the ~1,000 sources in this field. In this pilot we have obtained
870-um continuum maps of an essentially complete sample of the brightest 182
sub-millimetre sources (S_850um=6.2mJy) in COSMOS. Our ALMA maps detect 260
sub-millimetre galaxies (SMGs) spanning a range in flux density of
S_870um=0.7-19.2mJy. We detect more than one SMG counterpart in 34+/-2 per cent
of sub-millimetre sources, increasing to 53+/-8 per cent for SCUBA-2 sources
brighter than S_850um>12mJy. We estimate that approximately one-third of these
SMG-SMG pairs are physically associated (with a higher rate for the brighter
secondary SMGs, S_870um>3mJy), and illustrate this with the serendipitous
detection of bright [CII] 157.74um line emission in two SMGs, AS2COS0001.1 &
0001.2 at z=4.63, associated with the highest significance single-dish source.
Using our source catalogue we construct the interferometric 870um number counts
at S_870um>6.2mJy. We use the extensive archival data of this field to
construct the multiwavelength spectral energy distribution of each AS2COSMOS
SMG, and subsequently model this emission with MAGPHYS to estimate their
photometric redshifts. We find a median photometric redshift for the
S_870um>6.2mJy AS2COSMOS sample of z=2.87+/-0.08, and clear evidence for an
increase in the median redshift with 870-um flux density suggesting strong
evolution in the bright-end of the 870um luminosity function.
