Speaker
Description
High-resolution N-body simulations have shown that the large-scale clustering of dark halos is predominantly but not purely dependent on their dark matter mass. At fixed halo mass, the bias factor of halo clustering depends on a variety of secondary properties of halos such as formation time, concentration, spin and shape, an effect known as “assembly bias” which should be related to the variance of the assembly history of halos of a given mass. Both the mass-dependent halo bias and the assembly bias should have an important impact on galaxy formation and evolution. In this work, we use the IllustrisTNG300 dark matter-only simulation to investigate the dependence of the assembly bias on both the definition and the spatial orientation of dark matter halos. We find the long-standing controversial results reported in the literature on spin bias are caused by the different halo definitions adopted in different studies. Furthermore, we find strong anisotropy of the assembly bias when the halo spin or the axis-ratio of halos is considered as the secondary property. The spin bias becomes rather weak when the clustering measurement is limited to the direction along the major axis of halos, and this result depends only weakly on halo mass. A similar but weaker anisotropic effect is found for halo concentration, while no anisotropy is found for either halo formation time or halo shape. We discuss the link of these findings with the spatial alignment of galaxies that has been observed from low-z redshift surveys.
