Assembly bias in the local PNG halo bias and its implication for f_NL constraints

We use N-body simulations to study halo assembly bias (i.e., the dependence of halo clustering on properties beyond total mass) in the density and primordial non-Gaussianity (PNG) linear bias parameters b₁ and b_ϕ, respectively. We consider concentration, spin and sphericity as secondary halo properties, for which we find a clear detection of assembly bias for b₁ and b_ϕ. At fixed total mass, halo spin and sphericity impact b₁ and b_ϕ in a similar manner, roughly preserving the shape of the linear b_ϕ(b₁) relation satisfied by the global halo population. Halo concentration, however, drives b₁ and b_ϕ in opposite directions. This induces significant changes to the b_ϕ(b₁) relation, with higher concentration halos having higher amplitude of b_ϕ(b₁). For z = 0.5 and b₁ ≈ 2 in particular, the population comprising either all halos, those with the 33% lowest or those with the 33% highest concentrations have a PNG bias of b_ϕ ≈ 3, b_ϕ ≈ -1 and b_ϕ ≈ 9, respectively. Varying the halo concentration can make b_ϕ very small and even change its sign. These results have important ramifications for galaxy clustering constraints of the local PNG parameter f_NL that assume fixed forms for the b_ϕ(b₁) relation. We illustrate the significant impact of halo assembly bias in actual data using the BOSS DR12 galaxy power spectrum: assuming that BOSS galaxies are representative of all halos, the 33% lowest or the 33% highest concentration halos yields σ_fNL = 44, 165, 19, respectively. Our results suggest taking host halo concentration into account in galaxy selection strategies to maximize the signal-to-noise on f_NL. They also motivate more simulation-based efforts to study the b_ϕ(b₁) relation of halos and galaxies.