Two 100-Mpc-scale structures in the three-dimensional distribution of radio galaxies and their implications

We present unequivocal evidence for a huge (approximate to80 x 100 x 100 Mpc(3)) superstructure at redshift z = 0.27 in the three-dimensional distribution of radio galaxies from the Texas-Oxford NVSS Structure 08(h) region (TONS08) sample, confirming tentative evidence for such a structure from the 7C redshift survey (7CRS). A second, newly discovered superstructure is also found less securely at redshift 0.35 (of dimensions approximate to100 x 100 x 100 Mpc(3)). We present full observational details on the TONS08 sample which was constructed to probe structures in the redshift range 0 less than or similar to z less than or similar to 0.5 by matching NVSS sources with objects in APM catalogues to obtain a sample of optically bright (E approximate to R less than or equal to 19.83), radio-faint (1.4-GHz flux density S-1.4 greater than or equal to 3 mJy) radio galaxies in the same 25 deg(2) area as part II of the 7CRS. Out of the total sample size of 84 radio galaxies, at least 25 are associated with the two approximate to100 Mpc-scale superstructures. We use quasi-linear structure formation theory to estimate the number of such structures expected in the TONS08 volume if the canonical value for radio galaxy bias is assumed. Under this assumption, the structures represent approximate to4-5sigma peaks in the primordial density field and their expected number is low (similar to10(-2) -10(-4)). Because the TONS08 survey was designed to follow up a previously, if tentatively, identified superstructure in the 7CRS, the probability of finding two superstructures in TONS08 is uncertain but, assuming that the tentative detection of the z = 0.35 superstructure is real, must lie between similar to10(-4) and similar to10(-5), depending on how representative the TONS08 region proves to be. We show that similar structures (with similarly low probabilities) are also found in previous radio galaxy redshift surveys, but their significance has not been fully appreciated because they have been traced by very small numbers of radio galaxies. Fortunately, there are several plausible explanations (many of which are testable) for these low probabilities in the form of potential mechanisms for boosting the bias on large scales. These include the association of radio galaxies with highly biased rich clusters in superstructures, enhanced triggering by group-group mergers and enhanced triggering and/or redshift space distortion in collapsing systems as the growth of superstructures moves into the non-linear regime.