Indicator functions: distilling the information from Gaussian random fields

Abstract

A random Gaussian density field contains a fixed amount of Fisher information on the amplitude of its power spectrum. For a given smoothing scale, however, that information is not evenly distributed throughout the smoothed field. We investigate which parts of the field contain the most information by smoothing and splitting the field into different levels of density (using the formalism of indicator functions), deriving analytical expressions for the information content of each density bin in the joint-probability distribution (given a distance separation). When we choose one particular distance regime (i.e. cells separated by 60–80 h−1 Mpc), we find that the information in that range peaks at moderately rare densities (where the number of smoothed survey cells is roughly of order of magnitude of 100). Counter-intuitively, we find that, for a finite survey volume (again at a particular distance range), indicator function analysis can outperform conventional two-point statistics while using only a fraction of the total survey cells, and we explain why. In light of recent developments in marked statistics (such as the indicator power spectrum and density-split clustering), this result elucidates how to optimize sampling for effective extraction of cosmological information.