Distant galaxies are also lensed by clusters, and their lensed images are expected to be bright sources of submillimetre-wave radiation (Blain (1997c)). An example of the effects of gravitational lensing by a cluster on the appearance of background galaxies in the submillimetre waveband as compared with the optical waveband is shown in Fig.5. Individual images could not be detected at the 4-arcmin resolution of Planck, however their presence in the observing beam is expected to increase the level of source confusion noise by a factor of about 3 as compared with observations in the field (Blain (1997b), Blain (1997d)). If a sufficiently large sample of clusters were observed, then this increased confusion noise could be measured, and so the form of evolution of star-forming galaxies with flux densities fainter than the sensitivity of Planck could be investigated. A large sample of about distant clusters will be detected in a Planck survey (Bersanelli et al. (1996)) due to the flux density of their Sunyaev--Zel'dovich effects (Rephaeli (1995)), and so this type of investigation will be practical using Planck.
Figure 5: Simulated surface brightness distributions of a rich cluster of galaxies, such as Abell 2218 at a redshift , in the optical R-band (left) and at in the sub-mm waveband (right). Both the emission from galaxies within the cluster and from lensed images is shown, coloured red and blue respectively. Two models of the population of distant lensed galaxies are shown: one described by an evolving IRAS luminosity function (top), and the other by a model in which galaxies form by hierarchical clustering (bottom) (Blain & Longair (1996), Blain (1997c)). The blue lensed images are relatively much brighter in the submillimetre waveband as compared with the optical waveband. The flux densities of the brightest submillimetre-wave sources in the right-hand fields are several mJy. The angular resolution of all the frames is 1arcsec, and the axes are graduated at 20arcsec intervals.