  The intensity of the cosmic microwave background radiation in the fields of
clusters of galaxies is modified by inverse Compton scattering in the hot
intracluster gas - the Sunyaev-Zel'dovich (SZ) effect. The effect is expected
to be most pronounced at a frequency of about 350 GHz (a wavelength of about
800 microns), and has been detected in the centimetre and millimetre wavebands.
In the millimetre/submillimetre waveband, the gravitationally-lensed images of
distant dusty star-forming galaxies in the background of the cluster are
predicted to dominate the appearance of clusters on scales of several
arcseconds, and could confuse observations of the SZ effect at frequencies
greater than about 200 GHz (wavelengths shorter than about 1.5 mm). Recent
observations by Smail, Ivison & Blain (1997) confirm that a significant
population of confusing sources are present in this waveband. Previous
estimates of source confusion in observations of the
millimetre/submillimetre-wave SZ effect did not include the effects of lensing
by the cluster, and so the accuracy of such measurements could be lower than
expected. Source subtraction may be required in order to measure the SZ effect
accurately, and a careful analysis of the results of an ensemble of SZ
measurements could be used to impose limits to the form of evolution of distant
star-forming galaxies.
