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

Abstract

We use hydrodynamical separate universe simulations with the IllustrisTNG model to predict the local primordial non-Gaussianity (PNG) bias parameters $b_{\varphi }$ and $b_{\varphi \delta }$, which enter at leading order in the galaxy power spectrum and bispectrum. This is the first time that $b_{\varphi \delta }$ is measured from either gravity-only or galaxy formation simulations. For dark matter halos, the popular assumption of universality overpredicts the $b_{\varphi \delta }(b_1)$ relation in the range $1\lesssim b_1\lesssim 3$ by up to $\Delta b_{\varphi \delta }\sim 3$ ($b_1$ is the linear density bias). The adequacy of the universality relation is worse for the simulated galaxies, with the relations $b_{\varphi }(b_1)$ and $b_{\varphi \delta }(b_1)$ 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 $b_{\varphi }$ and $b_{\varphi \delta }$ have a direct, often overlooked impact on the constraints of the local PNG parameter $f_{\mathrm{NL}}$, which we study and discuss. For a survey with $V=100{\mathrm{Gpc}}^3/h^3$ at $z=1$, uncertainties $\Delta b_{\varphi }\lesssim 1$ and $\Delta b_{\varphi \delta }\lesssim 5$ around values close to the fiducial can yield relatively unbiased constraints on $f_{\mathrm{NL}}$ using power spectrum and bispectrum data. We also show why priors on galaxy bias are useful even in analyses that fit for products $f_{\mathrm{NL}}{b}_{\varphi }$ and $f_{\mathrm{NL}}{b}_{\varphi \delta }$. The strategies we discuss to deal with galaxy bias uncertainties can be straightforwardly implemented in existing $f_{\mathrm{NL}}$ constraint analyses (we provide fits for some of the bias relations). Our results motivate more works with galaxy formation simulations to refine our understanding of $b_{\varphi }$ and $b_{\varphi \delta }$ towards improved constraints on $f_{\mathrm{NL}}$.

18 pages + 2 appendices; 8 figures. Comments welcomed! v2: added references and the new Fig. 6 with an additional forecast (same conclusions). v3: minor comments added following the referee report

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• Constraining primordial non-Gaussianity from DESI quasar targets and Planck CMB lensing
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• Learning to Concentrate: Multi-tracer Forecasts on Local Primordial Non-Gaussianity with Machine-Learned Bias
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• Large Scale Limit of the Observed Galaxy Power Spectrum
• Primordial non-Gaussianity and non-Gaussian Covariance
• PNG-UNITsims: Halo clustering response to primordial non-Gaussianities as a function of mass
• Multi-tracer power spectra and bispectra: Formalism
• Modal compression of the redshift-space galaxy bispectrum
• The local PNG bias of neutral Hydrogen, ${\mathrm{H}}_{\mathrm{I}}$
• Halo assembly bias from a deep learning model of halo formation
• Wide-angle effects in the galaxy bispectrum
• Quijote-PNG: Quasi-maximum likelihood estimation of Primordial Non-Gaussianity in the non-linear halo density field
• Radio-optical synergies at high redshift to constrain primordial non-Gaussianity
• Validating galaxy clustering models with Fixed & Paired and Matched-ICs simulations: application to Primordial Non-Gaussianities
• What does a cosmological experiment really measure? Covariant posterior decomposition with normalizing flows
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• Non-Gaussian assembly bias from a semi-analytic galaxy formation model
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• Constraining $f_{NL}$ using the Large-Scale Modulation of Small-Scale Statistics
• Constraining primordial non-Gaussianity from the large scale structure two-point and three-point correlation functions
• Squeezing information from radio surveys to probe the primordial Universe
• A revisited Correction to the Halo Mass Function for local-type Primordial non-Gaussianity
• Quijote PNG: The information content of the halo power spectrum and bispectrum