Towards optimal and robust f_nl constraints with multi-tracer analyses
Alexandre Barreira, E. Krause
Abstract
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 ∝ | b B 1 b A ϕ - b A 1 b B ϕ |, where b 1 and b ϕ are the linear and PNG galaxy bias parameters. We show this allows for significantly improved constraints compared to the traditional expectation ∝ | b A - b B | based on naive universality-like relations where b ϕ ∝ b 1 . Using IllustrisTNG galaxy simulation data, we find that different equal galaxy number splits of the full sample lead to different | b B 1 b A ϕ - b A 1 b B ϕ |, 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 b ϕ ≠ 0. Importantly, since these are constraints on f nl b ϕ and not f nl , they do not require priors on the b ϕ ( b 1 ) relation. For direct constraints on f nl , we show that multi-tracer constraints can be significantly more robust than single-tracer to b ϕ misspecifications and uncertainties; this relaxes the precision and accuracy requirements for b ϕ 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 b ϕ ( b 1 ) relation.