AzTEC half square degree survey of the SHADES fields - II.
  Identifications, redshifts, and evidence for large-scale structure
  The AzTEC 1.1 mm survey of the SCUBA HAlf Degree Extragalactic Survey
(SHADES) fields is the largest (0.7 deg2) blank-field millimetre-wavelength
survey undertaken to date at a resolution of ~18" and a depth of ~1 mJy. We
have used the deep optical-to-radio multi-wavelength data in the SHADES Lockman
Hole East and SXDF/UDS fields to obtain galaxy identifications for ~64% (~80%
with tentative identifications) of the 148 AzTEC-SHADES 1.1 mm sources,
exploiting deep radio and 24 um data complemented by methods based on 8 um
flux-density and red optical-infrared (i-K) colour. This unusually high
identification rate can be attributed to the relatively bright
millimetre-wavelength flux-density threshold, combined with the relatively deep
supporting multi-frequency data. We have further exploited the
optical-mid-infrared-radio data to derive a ~60% (~75% with tentative
identifications) complete redshift distribution for the AzTEC-SHADES sources,
yielding a median redshift of z~2.2, with a high-redshift tail extending to at
least z~4. Despite the larger area probed by the AzTEC survey relative to the
original SCUBA SHADES imaging, the redshift distribution of the AzTEC sources
is consistent with that displayed by the SCUBA sources, and reinforces
tentative evidence that the redshift distribution of mm/sub-mm sources in the
Lockman Hole field is significantly different from that found in the SXDF/UDS
field. Comparison with simulated surveys of similar scale extracted from
semi-analytic models based on the Millennium simulation indicates that this is
as expected if the mm/sub-mm sources are massive (M>10^11 Mo) star-forming
galaxies tracing large-scale structures over scales of 10-20 Mpc. This confirms
the importance of surveys covering several square degrees (as now underway with
SCUBA2) to obtain representative samples of bright (sub)mm-selected galaxies.
