Gravitational lensing and the Sunyaev–Zel’dovich effect in the millimetre/submillimetre waveband

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 350GHz (a wavelength of about 800μm), 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 200GHz (wavelengths shorter than about 1.5mm). 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 dusty star-forming galaxies.