Where does non-Universality in Assembly Bias come from?

TL;DR

This paper investigates the origins of non-universality in assembly bias of dark matter halos, using the Separate Universe framework and simulations, revealing how selection criteria influence bias relations across cosmologies.

Contribution

It provides a theoretical explanation for non-universality in assembly bias based on halo selection methods and validates it with N-body simulations across different cosmologies.

Findings

Universal bias for mass-selected halos explained by the Separate Universe framework.

Non-universality in concentration-selected halos varies with halo mass and cosmology.

High-redshift, high-bias objects' non-universality is due to selection effects.

Abstract

Constraints on local primordial non-Gaussianity (LPnG) obtained from galaxy power spectra are limited by the perfect degeneracy between the LPnG parameter $f_{\rm NL}$ and the bias parameter $b_{\phi}$ which encodes the response of galaxy clustering to a change in the amplitude of primordial curvature fluctuations. For galaxies observed by galaxy surveys, the relation between $b_{\phi}$ and the galaxy bias $b_{g}$ is poorly understood and differs significantly from the universal mass function ansatz. In this paper, we investigate this non-universality in the context of dark-matter halos using the Separate Universe framework, focussing on dark-matter halos selected by mass and/or concentration. We show that the Separate Universe framework provides a natural explanation of the observed universality in the bias of dark-matter halos selected purely by their mass, independent of the spherical collapse picture of halo formation. We further propose an explanation for the observed non-universality in halos selected by concentration and corroborate it with $N$-body simulations in scale-free (EdS) and $\Lambda\text{CDM}$ cosmologies. In particular, we show that the relation between $b_{\phi}$ and halo bias $b_{h}$ for halos selected by concentration in matter-dominated cosmologies tends towards universality at the highest halo masses due to such halos gravitationally dominating their environment throughout their evolution. We also argue that concentration-selected halos of lower masses exhibit non-universality due to their mass accretion being significantly affected by the gravitational influence of neighbouring, more massive halos. Our results suggest that any non-universality in high redshift ($z\gtrsim 3$), high-bias objects observed by realistic galaxy surveys is entirely an artifact of the associated selection function.