The nature of assembly bias - II. Halo spin

TL;DR

This paper investigates how the spin parameter influences the clustering of massive dark matter haloes, revealing a distinct assembly bias related to spin that differs from age-related bias and suggesting different underlying physical mechanisms.

Contribution

It demonstrates that halo spin introduces a unique assembly bias not mitigated by existing peak height redefinitions, highlighting different origins for spin and age assembly biases.

Findings

High-spin haloes are more strongly clustered than low-spin haloes of the same mass.

Redefining peak height using spin removes the bias related to spin but introduces a bias related to relative age.

Assembly bias with respect to spin may be driven by filamentary accretion in dense environments.

Abstract

We study an assembly-type bias parametrized by the dimensionless spin parameter that affects massive structures. In numerical simulations higher spin haloes are more strongly clustered than lower spin haloes of equal mass. We detect a difference of over a 30 per cent in the clustering strength for dark matter haloes of 10^13-10^14 Msun, which is similar to the result of Bett et al. We explore whether the dependence of clustering strength on halo spin is removed if we apply the redefinition of overdensity peak height proposed by Lacerna & Padilla (Paper I) obtained using assembly ages. We find that this is not the case due to two reasons. Firstly, only a few objects of low-virial mass are moved into the mass range where the spin introduces an assembly bias after using this redefinition. Secondly, this formalism does not alter the mass of massive objects. We then repeat the process of finding the redefined peak height of Paper I but using the spin. In this case, the new masses show no spin-related assembly bias but they introduce a previously absent assembly bias with respect to relative age. From this result, we conclude that the assembly-type bias with respect to the halo spin has a different origin than with respect to assembly age. The former may be due to the material from filaments, which is accreted by massive haloes, that is enhanced in high-density environments, thus causing more extreme spin values without significantly changing the formation age of the halo. In addition, high-mass objects may correspond, in some cases, to a different peak height than that suggested by their mass in numerical simulations, providing a possible explanation for the assembly bias with respect to spin. (abridged)