  We study lensing magnification of source galaxies by intervening galaxy
groups and clusters using a halo model. Halos are modeled with truncated NFW
profiles with ellipticity added to their lensing potential and propagated to
observable lensing statistics. We present the formalism to calculate observable
effects due to a distribution of halos of different masses at different
redshifts along the l ine of sight. We calculate the effects of magnification
on the number counts of high-redshift galaxies. Using BLAST survey data for
submillimeter galaxies (SMGs), we find that magnification affects the steep,
high flux par t of the counts by about 60%. The effect becomes much stronger if
the intrinsic distribution is signi ficantly steeper than observed. We also
consider the effect of this high-redshift galaxy population on contaminating
the Sunyaev-Zel'dovich (SZ) signal of massive clusters using the halo model
approach. We find that for the majority of clusters expected to be detected
with ongoing SZ surveys, there is significant contamination from the Poisson
noise due to background SMGs. This contr ibution can be comparable to the SZ
increment for typical clusters and can also contaminate the SZ decrement of low
mass clusters. Thus SZ observations, especially for the increment part of the
SZ spectrum, need to include careful modeling of this irreducible contamination
for mass estimation. Lensing further enhances the contamination, especially
close to the cores of massive clusters and for very disturbed clusters with
large magnification cross-section.
