http://arxiv.org/abs/1405.7367

We use two cosmological simulations of structure formation in the standard LCDM scenario to study the evolutionary histories of dark-matter haloes and to characterize the Lagrangian (protohalo) regions from which they form. We focus our analysis on haloes identified at redshift z_id=0 and show that the classic ellipsoidal collapse model developed by Bond & Myers systematically overestimates their collapse time. If one imposes that halo collapse takes place at z_id, this model requires starting from a significantly lower linear density contrast than what is measured in the simulations at the Lagrangian locations of halo formation. Attempting to explain this discrepancy, we test two key assumptions of the model. First, we show that the tides felt by collapsing haloes due to the surrounding large-scale structure evolve non-linearly. Although this effect becomes increasingly important for low-mass haloes that reside in high-density regions, accounting for it in the ellipsoidal collapse model only marginally improves the agreement with N-body simulations. Second, we track the time evolution of the physical volume occupied by forming haloes and show that, after turnaround, it generally stabilizes at a well-defined time, z_c>z_id. Contrary to the basic assumption of extended Press-Schechter theory based on the excursion-set approach, our results suggest that most haloes have already assembled their final mass into a coherent structure prior to z_id, and that the ‘collapse redshift’, z_c, increases with decreasing halo mass. We discuss the implications of this result for understanding the origin of the mass-dependence and scatter in the linear threshold for halo formation. Finally, we show that, when tuned for collapse at z_c, a modified version of the ellipsoidal collapse model that also accounts for the triaxial nature of protohaloes predicts their linear density contrast in an unbiased way.

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M. Borzyszkowski, A. Ludlow and C. Porciani
Fri, 30 May 14
12/74

Comments: 15 pages, 11 figures