SEARCHING FOR LARGE-SCALE STRUCTURE IN DEEP RADIO SURVEYS

We calculate the expected amplitude of the dipole and higher spherical harmonics in the angular distribution of radio galaxies, The median r edshift of radio sources in existing catalogues is z similar to 1, whi ch allows us to study large-scale structure on scales between those ac cessible to present optical and infrared surveys, and that of the cosm ic microwave background (CMB). The dipole is a result of two effects w hich turn out to be of comparable magnitude: (i) our motion with respe ct to the CMB, and (ii) large-scale structure, parametrized here by a family of cold dark matter power-spectra, We make specific predictions for the Green Bank 1987 (87GB) and Parkes-MIT-NRAO (PMN) catalogues, which in our combined catalogue include similar to 40000 sources brigh ter than 50 mJy at 4.85 GHz, over about 70 per cent of the sky. For th ese relatively sparse catalogues both the motion and large-scale struc ture dipole effects are expected to be smaller than the Poisson shot n oise. However, we detect dipole and higher harmonics in the combined 8 7GB-PMNraw catalogue which are far larger than expected. We attribute this to a 2 per cent flux mismatch between the two catalogues. Ad hoc corrections made in an effort to match the catalogues may suggest a ma rginal detection of a dipole, To detect a dipole and higher harmonics unambiguously, a catalogue with full sky coverage and similar to 10(6) sources is required, We also investigate the existence and extent of the supergalactic plane in the above catalogues. In a strip of +/- 10 degrees of the standard supergalactic equator, we find a 3 sigma detec tion in PMNraw, but only l sigma in 87GB(raw). We briefly discuss the implications of ongoing surveys such as FIRST and NVSS and follow-up r edshift surveys.