Unclustered tracers remain unclustered

Constraining primordial non-Gaussianities (PNGs) is one of the main goals of new-generation large-scale galaxy surveys. It had been proposed that unclustered tracers (with bias b1 = 0) could be optimal for PNG studies, and that these could be found by selecting galaxies in bins of their local density. We tested this hypothesis in state-of-the-art simulations from the PNG-UNITsim suite with local fNL = 100 and fNL = −20. We considered different parent tracer catalogues: all haloes together, haloes in broad mass bins, and halo occupation distribution (HOD) models for luminous red galaxies (LRGs) and quasars (QSOs). We then classify these tracers by their local density (δt, R) and measure the linear bias (b1) and PNG response (bϕ). Most δt, R bins show a PNG response compatible with bϕ = 0 for all haloes or the low-mass bin (log M < 11). For high-mass haloes (log M> 12), QSO or LRG, we recover a trend closer to the universality relation (bϕ = 2δcrit(b1 − 1)) for b1 > 1, but the bϕ(b1) curve flattens to 0 below |b1|< 1. Hence, we find bϕ ≈ 0 for all bias-zero tracers considered. The complex δt, R-based selection causes their clustering to strongly deviate from simple assumptions, namely the universality relation and Poisson shot noise, hindering their capability to constrain PNGs.