Weak lensing mass estimates of galaxy groups and the line-of-sight
  contamination
  Weak lensing is an important technique to determine the masses of galaxy
groups. However, the distortion imprint on the shape of the background galaxies
is affected by all the mass content along the line-of-sight. Using COSMOS shear
mock data we study the shear profile of 165 groups and investigate the level at
which the neighbouring groups can enhance or suppress the shear signal from the
main halo. Our mock data are based on CFHT and Subaru observations and the
information on the galaxy groups is taken from the COSMOS X-ray catalogue of
extended sources. The expected gravitational shear field of these groups is
calculated assuming that the haloes follow NFW density profiles. We conclude
that, on average, the signal-to-noise for a detection of the main halo is
affected by ~15%x\sqrt{ngal/30} with respect to the signal-to-noise the same
halo would have if it was isolated in the sky. Groups with neighbours that are
close in projected distance (<1') are the most affected, but haloes located at
larger angular distances also cause a measurable shear signal, which can be
interpreted as uncorrelated large-scale structure. The average bias in the mass
excess estimate of individual groups that is introduced by the external haloes
is zero with an rms of ~6-72%, depending on the aperture size used. The shear
signal introduced by large-scale structure acts as an external source of noise.
The averaged uncertainty introduced is \sigma_{\gamma_t}^{LSS}~0.006 per
component for an aperture size of ~5', which corresponds to 1.8% of the
one-component intrinsic ellipticity value. This large-scale structure noise
error becomes equal to intrinsic ellipticity noise if there are measurements
for ~3000 galaxies within a certain aperture, a number that is already achieved
by current deep surveys such as COSMOS and, therefore, should not be ignored.
