Towards optimal and robust $f_{\rm NL}$ constraints with multi-tracer analyses

Understand

We discuss the potential of the multi-tracer technique to improve observational constraints of the local primordial non-Gaussianity (PNG) parameter $f_{\rm NL}$ from the galaxy power spectrum.

• For two galaxy samples $A$ and $B$, the constraining power is $\propto |b_1^B b_\phi^A - b_1^Ab_\phi^B|$, where $b_1$ and $b_\phi$ are the linear and PNG galaxy bias parameters.
• We show this allows for significantly improved constraints compared to the traditional expectation $\propto |b_1^A - b_1^B|$ based on naive universality-like relations where $b_\phi \propto b_1$.
• Using IllustrisTNG galaxy simulation data, we find that different equal galaxy number splits of the full sample lead to different $|b_1^B b_\phi^A - b_1^Ab_\phi^B|$, and thus have different constraining power.