Predictions for local PNG bias in the galaxy power spectrum and bispectrum and the consequences for f NL constraints

Abstract

We use hydrodynamical separate universe simulations with the IllustrisTNG model to predict the local primordial non-Gaussianity (PNG) bias parameters b ϕ and b ϕδ, which enter at leading order in the galaxy power spectrum and bispectrum. This is the first time that b ϕδ is measured from either gravity-only or galaxy formation simulations. For dark matter halos, the popular assumption of universality overpredicts the b ϕδ(b 1) relation in the range 1 ≲ b 1 ≲ 3 by up to Δ b ϕδ ∼ 3 (b 1 is the linear density bias). The adequacy of the universality relation is worse for the simulated galaxies, with the relations b ϕ(b 1) and b ϕδ(b 1) being generically redshift-dependent and very sensitive to how galaxies are selected (we test total, stellar and black hole mass, black hole mass accretion rate and color). The uncertainties on b ϕ and b ϕδ have a direct, often overlooked impact on the constraints of the local PNG parameter f NL, which we study and discuss. For a survey with V = 100 Gpc3/h3 at z=1, uncertainties Δ b ϕ ≲ 1 and Δ b ϕδ ≲ 5 around values close to the fiducial can yield relatively unbiased constraints on f NL using power spectrum and bispectrum data. We also show why priors on galaxy bias are useful even in analyses that fit for products f NL b ϕ and f NL b ϕδ. The strategies we discuss to deal with galaxy bias uncertainties can be straightforwardly implemented in existing f NL constraint analyses (we provide fits for some of the bias relations). Our results motivate more works with galaxy formation simulations to refine our understanding of b ϕ and b ϕδ towards improved constraints on f NL.