http://arxiv.org/abs/2107.06887
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). The uncertainties on $b_{\phi}$ and $b_{\phi\delta}$ have a direct, often overlooked impact on the constraints of the local PNG parameter $f_{\rm NL}$, which we study and discuss. For a galaxy survey with $V = 100{\rm Gpc}^3/h^3$ at $z=1$, we find that uncertainties $\Delta b_{\phi} \lesssim 1$ and $\Delta b_{\phi\delta} \lesssim 5$ are needed to yield unbiased constraints on $f_{\rm NL}$ using power spectrum and bispectrum data. We also show why priors on galaxy bias are useful even in analyses that fit for products $f_{\rm NL}b_{\phi}$ and $f_{\rm NL}b_{\phi\delta}$. The strategies we discuss here to deal with galaxy bias uncertainties can be straightforwardly implemented in existing $f_{\rm NL}$ constraint analyses (we provide fits for some of the bias relations). Our results also motivate more works with galaxy formation simulations to refine our understanding of $b_{\phi}$ and $b_{\phi\delta}$ towards improved constraints on $f_{\rm NL}$.
A. Barreira
Thu, 15 Jul 21
11/63
Comments: 17 pages + 2 appendices; 7 figures. Comments welcomed!
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