http://arxiv.org/abs/1610.04231
The clustering of dark-matter haloes with fixed mass depends on their formation history, an effect known as assembly bias. We investigate the origin of this phenomenon using zoom N-body simulations. We follow the formation of seven galaxy-sized haloes selected using a definition of collapse time that generates strong assembly bias. Haloes at redshift zero are classified according to the time in which the physical volume containing their final mass becomes stable. For `stalled’ haloes this happens at z~1.5 while for `accreting’ haloes this has not happened yet. The zoom simulations confirm that stalled haloes do not grow in mass while accreting haloes show a net inflow. The reason is that accreting haloes are located at the nodes of a network of thin filaments which feed them. Conversely, each stalled halo lies within a prominent filament that is thicker than the halo size. Infalling material from the surroundings becomes part of the filament while matter within it recedes from the halo. We conclude that assembly bias originates from quenching halo growth due to tidal forces following the formation of non-linear structures in the cosmic web. Also the internal dynamics of the haloes change: the velocity anisotropy profile is biased towards radial (tangential) orbits in accreting (stalled) haloes. Finally, we extend the excursion-set theory to account for these effects. A simple criterion based on the ellipticity of the linear tidal field combined with the spherical collapse model provides excellent predictions for both classes of haloes.
M. Borzyszkowski, C. Porciani, E. Romano-Diaz, et. al.
Mon, 17 Oct 16
43/53
Comments: 17 pages, 16 figures, submitted to MNRAS, comments welcome
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