Primordial non-Gaussianity — the effects of relativistic and wide-angle corrections to the power spectrum

Wide-angle and relativistic corrections to the Newtonian and flat-sky approximations are important for accurate modeling of the galaxy power spectrum of next-generation galaxy surveys. In addition to Doppler and Sachs-Wolfe relativistic corrections, we include the effects of lensing convergence, time delay and integrated Sachs-Wolfe. We investigate the impact of these corrections on measurements of the local primordial non-Gaussianity parameter fNL, using two futuristic spectroscopic galaxy surveys, planned for SKAO2 and MegaMapper. In addition to the monopole, we include the quadrupole of the galaxy Fourier power spectrum. The quadrupole is much more sensitive to the corrections than the monopole. The combination with the quadrupole improves the precision on fNL by ∼ 45% and ∼ 63% for SKAO2 and MegaMapper respectively. Neglecting the wide-angle and relativistic corrections produces a shift in fNL which is very sensitive to the magnification bias and the redshift evolution of the comoving number density. In the case of SKAO2, the shift in fNL is negligible — since the contributions to the shift from integrated and non-integrated effects nearly cancel. For MegaMapper, there is only partial cancellation of integrated and non-integrated effects and the shift is ∼ 0.6 σ. We point out that some of the approximations made in the wide-angle and relativistic corrections may artificially suppress the shift in fNL.