Predictions for local PNG bias in the galaxy power spectrum and bispectrum and the consequences for $f_{\rm NL}$ constraints

Understand

We use hydrodynamical separate universe simulations with the IllustrisTNG model to predict the local primordial non-Gaussianity (PNG) bias parameters $b_{\phi}$ and $b_{\phi\delta}$, which enter at leading order in the galaxy power spectrum and bispectrum.

• This is the first time that $b_{\phi\delta}$ is measured from either gravity-only or galaxy formation simulations.
• For dark matter halos, the popular assumption of universality overpredicts the $b_{\phi\delta}(b_1)$ relation in the range $1 \lesssim b_1 \lesssim 3$ by up to $\Delta b_{\phi\delta} \sim 3$ ($b_1$ is the linear density bias).
• The adequacy of the universality relation is worse for the simulated galaxies, with the relations $b_{\phi}(b_1)$ and $b_{\phi\delta}(b_1)$ being generically redshift-dependent and very sensitive to how galaxies are selected (we test total, stellar and black hole mass, black hole mass accretion rate and color).