Towards optimal and robust f_nl constraints with multi-tracer analyses

Abstract

We discuss the potential of the multi-tracer technique to improve observational constraints of the local primordial non-Gaussianity (PNG) parameter f nl from the galaxy power spectrum. For two galaxy samples A and B, the constraining power is ∝ |bB1 bAϕ - bA1 bBϕ |, where b 1 and bϕ are the linear and PNG galaxy bias parameters. We show this allows for significantly improved constraints compared to the traditional expectation ∝ |bA - bB | based on naive universality-like relations where bϕ ∝ b 1. Using IllustrisTNG galaxy simulation data, we find that different equal galaxy number splits of the full sample lead to different |bB1 bAϕ - bA1 bBϕ |, and thus have different constraining power. Of all of the strategies explored, splitting by g-r color is the most promising, more than doubling the significance of detecting f nl bϕ ≠ 0. Importantly, since these are constraints on f nl bϕ and not f nl, they do not require priors on the bϕ (b 1) relation. For direct constraints on f nl, we show that multi-tracer constraints can be significantly more robust than single-tracer to bϕ misspecifications and uncertainties; this relaxes the precision and accuracy requirements for bϕ priors. Our results present new opportunities to improve our chances to detect and robustly constrain f nl, and strongly motivate galaxy formation simulation campaigns to calibrate the bϕ (b 1) relation.