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 bϕ and bδϕ at linear and quadratic order in the large-scale biasing framework. We perform the first field-level measurement of bϕ and bδϕ 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 fNL available in the field given various assumptions on knowledge of bϕ at fixed initial conditions. We find that it is not possible to precisely constrain fNL when marginalizing over bϕfNL 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 bϕ prior is necessary to meaningfully constrain fNL at the field level even in this idealized scenario. For simulated dark matter halos, the pure fNL constraints from both linear and quadratic field-level models appear biased when marginalizing over bias parameters including bϕ and bδϕ due largely to the fNLbϕ 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.