The parameter f_NL measures the local non-Gaussianity in the primordial energy fluctuations of the Universe, with any deviation from f_NL=0 providing key constraints on inflationary models. Galaxy clustering is sensitive to f_NL at large scale modes and the next generation of galaxy surveys will approach a statistical error of 1σ(f_NL)=1. However, the systematic errors on these constraints are dominated by the degeneracy of f_NL with the galaxy bias parameters b_1 (galaxy overdensities caused by mass perturbations) and b_ϕ (galaxy overdensities caused by primordial potential perturbations). It has been shown that the assumed scaling of b_ϕ(z)=2 delta_c (b_1(z)-1) is not accurate for realistically simulated galaxies, and depends both on the galaxy selection and the way that galaxies are modeled.
To address this, I leveraged my CAMELS-SAM pipeline to explore how varying parameters of galaxy formation affects b_ϕ and b_1 for various galaxy selections. I ran separate-universe N-body simulations of L=205 h^-1 cMpc and N=1280^3 to measure b_ϕ, and run 55 unique instances of the Santa Cruz semi-analytic model with varying parameters of stellar and AGN feedback.
In my paper (https://arxiv.org/pdf/2602.04987), I looked at how the behavior and evolution of a SC-SAM model's stellar-, SFR- and sSFR- to halo mass relationships track well with how b_1 and b_ϕ(b_1) change across redshift and selection for the SC-SAM. I found our variations of the SC-SAM encapsulate the b_ϕ behavior previously measured in IllustrisTNG, the Munich SAM, and the Galacticus SAM. Finally, I identified sSFR selections as particularly robust to varied galaxy modeling. tldr: galaxies do NOT follow universality at all, and the details of galaxy formation can really affect what your f_NL measurement can be!
In my student Annie's paper (https://arxiv.org/pdf/2604.21790 ; thanks to Elisabeth Krause for sharing her PhD student!), we present a framework to construct physically motivated, observation-conditioned priors on b_ϕ by marginalizing over galaxy formation uncertainties. Using the data from CAMELS-SAM and the paper described just above, we constructed a distribution of b_ϕ conditioned on observations for a the DESI Emission Line Galaxy-like sample. Conditioning on the observed z=0 SMF or SHMR decreases σ{bϕ} from 0.69 to 0.08 and 0.02 respectively -- reductions of 88% and 97% -- with consistent results when conditioning on the observed data directly. Despite substantial shifts in the galaxy formation posteriors, the resulting b_ϕ distributions remain mutually consistent across all observables. The SMF and SHMR are found to carry sufficient constraining power to reduce the galaxy formation uncertainty in b_ϕ relevant for f_NL inference with next-generation spectroscopic surveys.
PDFs below are arXiV versions -- look at the links for the most recent updates after peer review!