HerMES: detection of cosmic magnification of sub-mm galaxies using
  angular cross-correlation
  Cosmic magnification is due to the weak gravitational lensing of sources in
the distant Universe by foreground large-scale structure leading to coherent
changes in the observed number density of the background sources. Depending on
the slope of the background source number counts, cosmic magnification causes a
correlation between the background and foreground galaxies, which is unexpected
in the absence of lensing if the two populations are spatially disjoint.
Previous attempts using submillimetre (sub-mm) sources have been hampered by
small number statistics. The large number of sources detected in the {\it
Herschel} Multi-tiered Extra-galactic Survey (HerMES) Lockman-SWIRE field
enables us to carry out the first robust study of the cross-correlation between
sub-mm sources and sources at lower redshifts. Using ancillary data we compile
two low-redshift samples from SDSS and SWIRE with <z> ~ 0.2 and 0.4,
respectively, and cross-correlate with two sub-mm samples based on flux density
and colour criteria, selecting galaxies preferentially at z ~ 2. We detect
cross-correlation on angular scales between ~1 and 50 arcmin and find clear
evidence that this is primarily due to cosmic magnification. A small, but
non-negligible signal from intrinsic clustering is likely to be present due to
the tails of the redshift distribution of the sub-mm sources overlapping with
those of the foreground samples.
