Over-density of SMGs in fields containing z~0.3 galaxies: magnification
  bias and the implications for studies of galaxy evolution
  We report a remarkable over-density of high-redshift submillimetre galaxies
(SMG), 4-7 times the background, around a statistically complete sample of
twelve 250-micron selected galaxies at z=0.35, which were targeted by ALMA in a
study of gas tracers. This over-density is consistent with the effect of
lensing by the halos hosting the target z=0.35 galaxies. The angular
cross-correlation in this sample is consistent with statistical measures of
this effect made using larger sub-mm samples. The magnitude of the over-density
as a function of radial separation is consistent with intermediate scale
lensing by halos of order 7x 10^{13} M_o, which should host one or possibly two
bright galaxies and several smaller satellites. This is supported by
observational evidence of interaction with satellites in four out of the six
fields with SMG, and membership of a spectroscopically defined group for a
fifth. We also investigate the impact of these SMG on the reported Herschel
fluxes of the z=0.35 galaxies, as they produce significant contamination in the
350 and 500-micron Herschel bands. The higher than random incidence of these
boosting events implies a significantly larger bias in the sub-mm colours of
Herschel sources associated with z<0.7 galaxies than has previously been
assumed, with f_boost = 1.13, 1.26, 1.44 at 250, 350 and 500-microns. This
could have implications for studies of spectral energy distributions, source
counts and luminosity functions based on Herschel samples at z=0.2-0.7.
