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We study the impact that large-scale perturbations of (i) the matter densityand (ii) the primordial gravitational potential with local primordialnon-Gaussianity (PNG) have on galaxy formation using the IllustrisTNG model. Wefocus on the linear galaxy bias $b_1$ and the coefficient $b_\phi$ of thescale-dependent bias induced by PNG, which describe the response of galaxynumber counts to these two types of perturbations, respectively. We perform ourstudy using separate universe simulations, in which the effect of theperturbations is mimicked by changes to the cosmological parameters: modifiedcosmic matter density for $b_1$ and modified amplitude $\mathcal{A}_s$ of theprimordial scalar power spectrum for $b_\phi$. We find that the widely useduniversality relation $b_\phi = 2\delta_c(b_1 - 1)$ is a poor description ofthe bias of haloes and galaxies selected by stellar mass $M_*$, which isinstead described better by $b_\phi(M_*) = 2\delta_c(b_1(M_*) - p)$ with $p \in[0.4, 0.7]$. This is explained by the different impact that matteroverdensities and local PNG have on the median stellar-to-halo-mass relation. Asimple model of this impact allows us to describe the stellar mass dependenceof $b_1$ and $b_\phi$ fairly well. Our results also show a nontrivial relationbetween $b_1$ and $b_\phi$ for galaxies selected by color and black hole massaccretion rate. Our results provide refined priors on $b_\phi$ for local PNGconstraints and forecasts using galaxy clustering. Given that the widely useduniversality relation underpredicts $b_\phi(M_*)$, existing analyses mayunderestimate the true constraining power on local PNG.

Galaxy bias and primordial non-Gaussianity: insights from galaxy formation simulations with IllustrisTNG