Local primordial non-Gaussian bias at the field level

Abstract Local primordial non-Gaussianity (LPNG) couples long-wavelength cosmological fluctuations to the short-wavelength behavior of galaxies. This coupling is encoded in bias parameters including b ϕ and b δϕ at linear and quadratic order in the large-scale biasing framework. We perform the first field-level measurement of b ϕ and b δϕ using Lagrangian bias and non-linear displacements from N-body simulations. We compare our field level measurements with universality predictions and separate universe results, finding qualitative consistency, but disagreement in detail. We also quantify the information on f NL available in the field given various assumptions on knowledge of b ϕ at fixed initial conditions. We find that it is not possible to precisely constrain f NL when marginalizing over b ϕ f NL even at the field level, observing a 2-3X degradation in constraints between a linear and quadratic biasing model on perturbative field-level mocks, suggesting that a b ϕ prior is necessary to meaningfully constrain f NL at the field level even in this idealized scenario. For simulated dark matter halos, the pure f NL constraints from both linear and quadratic field-level models appear biased when marginalizing over bias parameters including b ϕ and b δϕ due largely to the f NL b ϕ degeneracy. Our results are an important consistency test of the large-scale bias framework for LPNG and highlight the importance of physically motivated priors on LPNG bias parameters for future surveys.