Large-scale assembly bias of dark matter halos

TL;DR

This paper precisely measures how dark matter halo bias depends on properties beyond mass using innovative simulations, revealing consistent results with previous studies and providing new insights into the origins of assembly bias.

Contribution

It introduces a novel 'separate universe' simulation method to measure assembly bias and compares results with previous work, also exploring analytical explanations.

Findings

Bias depends on halo concentration, spin, accretion rate, and ellipticity.

Results agree with previous small-scale measurements.

Analytical insights into assembly bias origins from the excursion set peaks model.

Abstract

We present precise measurements of the assembly bias of dark matter halos, i.e. the dependence of halo bias on other properties than the mass, using curved "separate universe" N-body simulations which effectively incorporate an infinite-wavelength matter overdensity into the background density. This method measures the LIMD (local-in-matter-density) bias parameters $b_n$ in the large-scale limit. We focus on the dependence of the first two Eulerian biases $b^E_1$ and $b^E_2$ on four halo properties: the concentration, spin, mass accretion rate, and ellipticity. We quantitatively compare our results with previous works in which assembly bias was measured on fairly small scales. Despite this difference, our findings are in good agreement with previous results. We also look at the joint dependence of bias on two halo properties in addition to the mass. Finally, using the excursion set peaks model, we attempt to shed new insights on how assembly bias arises in this analytical model.