Some assembly required: assembly bias in massive dark matter halos

TL;DR

This paper investigates the presence and nature of assembly bias in massive dark matter halos, revealing that the bias depends on how halo age is defined and highlighting issues with standard halo mass definitions.

Contribution

The study demonstrates that assembly bias exists in massive halos and clarifies how different definitions of halo age affect the observed bias, emphasizing the importance of physically motivated halo definitions.

Findings

Assembly bias is significant in halos with M~10^{15} M_sun.

No correlation between bias and half-mass time at M~10^{14} M_sun.

Bias depends on the halo age definition and mass measurement method.

Abstract

We study halo assembly bias for cluster-sized halos. Previous work has found little evidence for correlations between large-scale bias and halo mass assembly history for simulated cluster-sized halos, in contrast to the significant correlation found between bias and concentration for halos of this mass. This difference in behavior is surprising, given that both concentration and assembly history are closely related to the same properties of the linear-density peaks that collapse to form halos. Using publicly available simulations, we show that significant assembly bias is indeed found in the most massive halos with $M\sim 10^{15}M_\odot$, using essentially any definition of halo age. For lower halo masses $M\sim 10^{14}M_\odot$, no correlation is found between bias and the commonly used age indicator $a_{0.5}$, the half-mass time. We show that this is a mere accident, and that significant assembly bias exists for other definitions of halo age, including those based on the time when the halo progenitor acquires some fraction $f$ of the ultimate mass at $z=0$. For halos with $M_{\rm vir}\sim 10^{14}M_\odot$, the sense of assembly bias changes sign at $f=0.5$. We explore the origin of this behavior, and argue that it arises because standard definitions of halo mass in halo finders do not correspond to the collapsed, virialized mass that appears in the spherical collapse model used to predict large-scale clustering. Because bias depends strongly on halo mass, these errors in mass definition can masquerade as or even obscure the assembly bias that is physically present. More physically motivated halo definitions using splashback should be free of this particular defect of standard halo finders.