The SCUBA-2 Cosmology Legacy Survey: The EGS deep field I - Deep number
  counts and the redshift distribution of the recovered Cosmic Infrared
  Background at 450 and 850 um
  We present deep observations at 450 um and 850 um in the Extended Groth Strip
field taken with the SCUBA-2 camera mounted on the James Clerk Maxwell
Telescope as part of the deep SCUBA-2 Cosmology Legacy Survey (S2CLS),
achieving a central instrumental depth of $\sigma_{450}=1.2$ mJy/beam and
$\sigma_{850}=0.2$ mJy/beam. We detect 57 sources at 450 um and 90 at 850 um
with S/N > 3.5 over ~70 sq. arcmin. From these detections we derive the number
counts at flux densities $S_{450}>4.0$ mJy and $S_{850}>0.9$ mJy, which
represent the deepest number counts at these wavelengths derived using directly
extracted sources from only blank-field observations with a single-dish
telescope. Our measurements smoothly connect the gap between previous shallower
blank-field single-dish observations and deep interferometric ALMA results. We
estimate the contribution of our SCUBA-2 detected galaxies to the cosmic
infrared background (CIB), as well as the contribution of 24 um-selected
galaxies through a stacking technique, which add a total of $0.26\pm0.03$ and
$0.07\pm0.01$ MJy/sr, at 450 um and 850 um, respectively. These surface
brightnesses correspond to $60\pm20$ and $50\pm20$ per cent of the total CIB
measurements, where the errors are dominated by those of the total CIB. Using
the photometric redshifts of the 24 um-selected sample and the redshift
distributions of the submillimetre galaxies, we find that the redshift
distribution of the recovered CIB is different at each wavelength, with a peak
at $z\sim1$ for 450 um and at $z\sim2$ for 850um, consistent with previous
observations and theoretical models.
