Constraining primordial non-Gaussianity via a multitracer technique with surveys by Euclid and the Square Kilometre Array

We forecast future constraints on local-type primordial non-Gaussianity parameter fNL with a photometric galaxy survey by Euclid, a continuum galaxy survey by Square Kilometre Array (SKA), and their combination. We derive a general expression for the covariance matrix of the power spectrum estimates of multiple tracers to show how the so-called multitracer technique improves constraints on fNL. In particular we clarify the role of the overlap fraction of multiple tracers and the division method of the tracers. Our Fisher matrix analysis indicates that stringent constraints of σ(fNL)1 can be obtained even with a single survey, assuming five mass bins. When Euclid and SKA phase 1 (2) are combined, constraints on fNL are improved to σ(fNL)=0.61(0.50).