Unclustered tracers remain unclustered: the lack of primordial non-Gaussianity response of bias-zero tracers

Abstract

Constraining primordial non-Gaussianities (PNG) is one of the main goals of new-generation large-scale galaxy surveys. It had been proposed that unclustered tracers (with bias $b_1=0$) could be optimal for PNG studies, and that these could be found by selecting galaxies in bins of their local density. Here, we test this hypothesis in state-of-the-art simulations from the PNG-UNITsim suite with local $f_{\mathrm{NL}}=100$ and $f_{\mathrm{NL}}=-20$. We consider different parent tracer catalogues: all halos together, halos in large mass bins, and HOD models for LRGs and QSO. We then classify these tracers by their local density ($\hat{I}{´}_{t,R}$) and measure the linear bias ($b_1$) and PNG-response ($b_{\ddot{I}}•$). Most $\hat{I}{´}_{t,R}$ bins show a PNG-response compatible with $b_{\ddot{I}}•=0$ for all halos or the low-mass bin (log$M<11$). For high-mass halos (log$M>$12), QSO or LRG, we recover a trend closer to the universality relation ($b_{\ddot{I}}•=2\hat{I}{´}_{\mathrm{crit}}(b_1-1)$) for $b_1>1$, but the $b_{\ddot{I}}•(b_1)$ curve flattens to 0 below $|b_1|<1$. Hence, we find $b_{\ddot{I}}•\approx 0$ for all bias-zero tracers considered. The complex $\hat{I}{´}_{t,R}$-based selection causes their clustering to strongly deviate from simple assumptions, namely the universality relation and Poisson shot noise, hindering their capability to constrain PNG.

in review in A&A letters