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

Abstract 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 b 1 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 b 1 and b ϕ . At fixed total mass, halo spin and sphericity impact b 1 and b ϕ in a similar manner, roughly preserving the shape of the linear b ϕ ( b 1 ) relation satisfied by the global halo population. Halo concentration, however, drives b 1 and b ϕ in opposite directions. This induces significant changes to the b ϕ ( b 1 ) relation, with higher concentration halos having higher amplitude of b ϕ ( b 1 ). For z = 0.5 and b 1 ≈ 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 ( b 1 ) 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 f NL = 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 ( b 1 ) relation of halos and galaxies.