(Sub)millimetre interferometric imaging of a sample of COSMOS/AzTEC
  submillimetre galaxies III. Environments
  We study the environment of 23 submillimetre galaxies (SMGs) drawn from the
JCMT/AzTEC 1.1mm S/N-limited sample in the COSMOS field, as well as 4 COSMOS
SMGs at z_spec>4.5, and 1 at z_spec=2.49, yielding a sample of 28 SMGs. We
search for overdensities using the COSMOS photometric redshifts based on over
30 UV-NIR photometric bands, reaching an accuracy of sigma(Delta
z/(1+z))=0.0067 (0.0155) at z<3.5 (>3.5). To identify overdensities we apply
the Voronoi tessellation analysis, and estimate the overdensity estimator
delta_g as a function of distance from the SMG and/or overdensity center. We
test and validate our approach via simulations, X-ray detected groups, and
spectroscopic verifications using VUDS and zCOSMOS catalogues showing that even
with photometric redshifts in COSMOS we can efficiently retrieve overdensities
out to z~5. Our results yield that 11/23 (48%) JCMT/AzTEC 1.1mm SMGs occupy
overdense environments. Considering the entire JCMT/AzTEC 1.1mm S/N>4 sample,
and accounting for the expected fraction of spurious detections, yields that
35-61% of the SMGs in the S/N-limited sample occupy overdense environments. We
perform an X-ray stacking analysis in the 0.5-2keV band using a 32" aperture
and our SMG positions, and find statistically significant detections. For our
z<2 [z>2] subsample we find an average flux of (4.0+/-0.8)x10^{-16}
[(1.3+/-0.5)x10^{-16}] erg/s/cm^2 and a corresponding total mass of M_200 =
2.8x10^{13} [2x10^{13}] MSol. Our results suggest a higher occurrence of SMGs
occupying overdense environments at z>3, than at z<3. This may be understood if
highly star forming galaxies can only be formed in the highest peaks of the
density field tracing the most massive dark matter haloes at early cosmic
epochs, while at later times cosmic structure may have matured sufficiently
that more modest overdensities correspond to sufficiently massive haloes to
form SMGs.
