CMB constraint on non-Gaussianity in isocurvature perturbations

We study the CMB constraints on non-Gaussianity in CDM isocurvature perturbations. Non-Gaussian isocurvature perturbations can be produced in various models at the very early stage of the Universe. Since the isocurvature perturbations little affect the structure formation at late times, CMB is the best probe of isocurvature non-Gaussianity at least in the near future. In this paper, we focus on non-Gaussian curvature and isocurvature perturbations of the local-type, which are uncorrelated and in the form ζ = ζG+(3/5)fNL(ζG2−(ζG2)) and S = SG+fNL(ISO)(SG−(SG2)), and constrain the non-linearity parameter of isocurvature perturbations, fNL(ISO), as well as the curvature one fNL. For this purpose, we employ several state-of-art techniques for the analysis of CMB data and simulation. Assuming that isocurvature perturbations are subdominant, we apply our method to the WMAP 7-year data of temperature anisotropy and obtain constraints on a combination α2fNL(ISO), where α is the ratio of the power spectrum of isocurvature perturbations to that of the adiabatic ones. When the adiabatic perturbations are assumed to be Gaussian, we obtained a constraint α2fNL(ISO) = 40±66 assuming the power spectrum of isocurvature perturbations is scale-invariant. When we assume that the adiabatic perturbations can also be non-Gaussian, we obtain fNL = 38±24 and α2fNL(ISO) = −8±72. We also discuss implications of our results for the axion CDM isocurvature model