The scale-dependent signature of primordial non-Gaussianity in the large-scale structure of cosmic reionization

Abstract

The rise of large-scale structure in the universe depends upon the statistical distribution of initial density fluctuations generated by inflation. While the simplest models of inflation predict an almost perfectly Gaussian distribution, more-general models predict primordial deviations from Gaussianity that observations might yet be sensitive enough to detect. Recent measurements of the cosmic microwave background (CMB) temperature anisotropy bispectrum by the Planck collaboration have significantly tightened observational limits on the level of primordial non-Gaussianity (PNG) in the Universe, but they are still far from the level predicted by the simplest models of inflation. Probing levels of PNG below CMB sensitivities will require other methods, such as searching for the statistical imprint of PNG on the clustering of galactic haloes. During the cosmic epoch of reionization (EoR), the first stars and galaxies released radiation into the intergalactic medium (IGM) that created ionized patches whose large-scale geometry and evolution reflected the underlying abundance and large-scale clustering of the star-forming galaxies. This statistical connection between ionized patches in the IGM and galactic haloes suggests that observations of reionization may provide another means of constraining PNG. We employ the linear perturbation theory of reionization and semi-analytic models based on the excursion-set formalism to model the effects of PNG on the EoR. We quantify the effects of PNG on the large-scale structure of reionization by deriving the ionized density bias, i.e. the ratio of the ionized atomic to total matter overdensities in Fourier space, at small wavenumber. Just as previous studies found that PNG creates a scale-dependent signature in the halo bias, so, too, we find a scale-dependent signature in the ionized density bias. Our results, which differ significantly from previous attempts in the literature to characterize this PNG signature, will be applied elsewhere to predict its observable consequences, e.g. in the cosmic 21 cm background. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.