Powerful radio sources as cosmological probes: Where are they ?

Large-area radio surveys find that radio sources are relatively sparsely distributed on the sky. At

3 mJy the surface density is of the order of 40 sources per square degree. However, it has been shown that mJy-level radio surveys trace large-scale structure on scales inaccessible to other wavelengths (Cress et al. 1996, Magliocchetti et al. 1998), falling between the scales probed by the CMB and optical/IR surveys such as 2dF and IRAS. In particular radio surveys have two significant advantages over other wavelengths: (i) radio emission is unattenuated by the intervening medium: observations have established radio sources can be reliably traced to high redshift (Shaver et al. 1996) and (ii) the average redshift of radio sources in complete samples is high, typically 1 for

50 mJy.

The potential of the latest generation of large-area mJy-level radio surveys (e.g. NVSS, WENSS, SUMSS & FIRST) to reveal large-scale structure is now well-recognised (Wall 1998). However the primary constraint in using these surveys is that the redshift distribution N(z) is undetermined. Analyses of spatial clustering require this distribution to convert 2D radio survey data to 3D using the `cosmic Limber' equation (Loan, Wall & Lahav 1997) This is where evolution and beaming models of the total radio source sky can play a useful role. Unified-scheme based evolution and beaming models predict N(z) - reducing the effort required from spectroscopic follow-up of large samples of radio survey data which is a long and difficult process.

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