Unclustered tracers remain unclustered

The lack of primordial non-Gaussianity response of bias-zero tracers

¹ Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain
² Departamento de Física Teórica, Universidad Autónoma de Madrid, 28049 Madrid, Spain
³ Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
⁴ Centro de Investigación Avanzada en Física Fundamental (CIAFF), Universidad Autónoma de Madrid, ES-28049 Madrid, Spain
⁵ Instituto de Física, Universidad Nacional Autónoma de México, Apdo. Postal 20-364 Ciudad de México, Mexico

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 30 December 2025
Accepted: 14 February 2026

Abstract

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 f_NL = 100 and f_NL = −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 (δ_t, R) and measure the linear bias (b₁) and PNG response (b_ϕ). Most δ_t, R bins show a PNG response compatible with b_ϕ = 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 (b_ϕ = 2δ_crit(b₁ − 1)) for b₁ > 1, but the b_ϕ(b₁) curve flattens to 0 below |b₁|< 1. Hence, we find b_ϕ ≈ 0 for all bias-zero tracers considered. The complex δ_t, 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.