Primordial non-Gaussianity and the NRAO VLA Sky Survey

The NRAO VLA Sky Survey (NVSS) is the only data set that allows an accurate determination of the auto-correlation function (ACF) on angular scales of several degrees for active galactic nuclei at z ~= 1. Surprisingly, the ACF is found to be positive on large scales while, in the framework of the standard hierarchical clustering scenario with Gaussian primordial perturbations, it should be negative for a redshift-independent effective halo mass of order of that found for optically selected quasars. We show that a small primordial non-Gaussianity (NG) can add sufficient power on very large scales to account for the observed NVSS ACF. The best-fit value of the parameter f NL, quantifying the amplitude of primordial NG of local type, is f NL = 62 ± 27 (1σ error bar) and 25 < f NL < 117 (2σ confidence level), corresponding to a detection of NG significant at the ~3σ confidence level. The minimal halo mass of NVSS sources is found to be M min = 1012.47±0.26 h -1 M sun (1σ) strikingly close to that of optically selected quasars. We discuss caveats and possible physical and systematic effects that can have an impact on the results.