1.2 Surveys in other wavelength regions

The first survey conducted outside the optical region of the spectrum resulted in the Cambridge Catalogue of radio sources published by Ryle et al. in 1950. The following sequence of Cambridge Catalogues, reaching sucessively fainter objects, is well known as a powerful tool to locate very distant extragalactic sources. The potential of deep probing by radio surveys seems to have been first realized by Mills (1952). On the basis of his own catalogue, which suggested isotropic distribution of all sources outside the Milky Way and an intensity distribution conforming to the -1.5 power law, he considered the origin of radio radiation from extragalactic objects an equally probable hypothesis to that of the then generally favoured ``radio stars'' in the solar vicinity. Fig. 7 is taken from the Second Cambridge Catalogue (Shakeshaft et al. 1955) where the weaker sources, however, were not confirmed by the southern survey with the `Mills cross' (Pawsey 1957) and by the Third Cambridge catalogue. They are now considered to be artifacts. The first quasars identified (3C48 and 3C273, and others) obtained their names from the Third Cambridge Catalogue (Edge et al. 1959).

Figure 7. 2C-Radio map of the sky (Shakeshaft et al. 1955). ``Map showing the distribution of radio sources in galactic coordinates. The open circles represent the sources of large angular diameter, and in both cases the sizes indicates the flux density of the sources.''

With the advent of satellite astronomy short wavelength surveys became possible. The first results from the UHURU satellite were published in 1971. The final catalogue of this successful mission includes 339 sources (Forman et al. 1978) as shown in Fig. 8. Hard x-rays and -rays were observed with Cos B. While most of the individual sources are located in the Milky way, the diffuse high energy background has cosmological significance (Pinkau 1979), with successively lower density fluctuation expected at z 10 (soft x-rays) and z 100 (hard x-rays and -rays) (Silk 1970).

Figure 8. 4U-X-ray map of the sky (Forman et al. 1978). ``The sources ... are displayed in galactic coordinates. The size of the symbols representing the sources is proportional to the logarithm of the peak source intensity.'' The 'extragalactic distribution' in the log N - log S diagram follows the S-1.5 power law.

A far reaching infrared survey was carried out with the IRAS satellite in 1985/86. It has influenced observational cosmology significantly. The brightest and most massive galaxies seem to be strong infrared emitters. Quasars show infrared brightness correlated with their x-ray fluxes. The infrared background radiation has become important as the short wavelength end of the microwave background. Another infrared background with density fluctuations of the order 1 appears at z 1 (Partridge 1988).