The assembly bias of dark matter haloes to higher orders

TL;DR

This study uses a large simulation to measure higher order clustering of dark matter haloes, revealing strong mass and internal structure dependence of bias parameters up to fourth order, with implications for galaxy clustering models.

Contribution

It provides the first measurement of halo bias parameters up to fourth order across a wide mass range using a novel counts-in-cells technique.

Findings

Rarest haloes are 100 times more clustered than dark matter.

Bias parameters strongly depend on halo mass for M > M_*.

Internal structure influences bias, with more concentrated haloes less clustered for M > M_*.

Abstract

We use an extremely large volume ($2.4h^{-3}{\rm Gpc}^{3}$), high resolution N-body simulation to measure the higher order clustering of dark matter haloes as a function of mass and internal structure. As a result of the large simulation volume and the use of a novel ``cross-moment'' counts-in-cells technique which suppresses discreteness noise, we are able to measure the clustering of haloes corresponding to rarer peaks than was possible in previous studies; the rarest haloes for which we measure the variance are 100 times more clustered than the dark matter. We are able to extract, for the first time, halo bias parameters from linear up to fourth order. For all orders measured, we find that the bias parameters are a strong function of mass for haloes more massive than the characteristic mass $M_{*}$. Currently, no theoretical model is able to reproduce this mass dependence closely. We find that the bias parameters also depend on the internal structure of the halo up to fourth order. For haloes more massive than $M_{*}$, we find that the more concentrated haloes are more weakly clustered than the less concentrated ones. We see no dependence of clustering on concentration for haloes with masses $M<M_{*}$; this is contrary to the trend reported in the literature when segregating haloes by their formation time. Our results are insensitive to whether haloes are labelled by the total mass returned by the friends-of-friends group finder or by the mass of the most massive substructure. This implies that our conclusions are not an artefact of the particular choice of group finding algorithm. Our results will provide important input to theoretical models of galaxy clustering.