The physical origins of low-mass spin bias

TL;DR

This paper investigates the origin of low-mass halo spin bias, revealing that splashback haloes near massive structures cause the observed inversion of bias trends, linking spin bias to assembly bias.

Contribution

It demonstrates that splashback haloes are responsible for the inversion of low-mass spin bias and clarifies the connection between spin bias and assembly bias.

Findings

Splashback haloes cause the inversion of spin bias at low masses.

Removing splashback haloes restores the intrinsic mass dependence of spin bias.

The crossover mass scale is determined by splashback halo abundance.

Abstract

At $z=0$, higher-spin haloes with masses above $\log(\text{M}_{\text{c}}/h^{-1}\text{M}_\odot)\simeq 11.5$ have a higher bias than lower-spin haloes of the same mass. However, this trend is known to invert below this characteristic crossover mass, $\text{M}_{\text{c}}$. In this paper, we measure the redshift evolution and scale dependence of halo spin bias at the low-mass end and demonstrate that the inversion of the signal is entirely produced by the effect of splashback haloes. These low-mass haloes tend to live in the vicinity of significantly more massive haloes, thus sharing their large-scale bias properties. We further show that the location of the redshift-dependent crossover mass scale $\text{M}_{\text{c}}(z)$ is completely determined by the relative abundance of splashbacks in the low- and high-spin subpopulations. Once splashback haloes are removed from the sample, the intrinsic mass dependence of spin bias is recovered. Since splashbacks have been shown to account for some of the assembly bias signal at the low-mass end, our results unveil a specific link between two different secondary bias trends: spin bias and assembly bias.