Ellipsoidal halo finders and implications for models of triaxial halo formation

Abstract

We describe an algorithm for identifying ellipsoidal haloes in numerical simulations, and quantify how the resulting estimates of halo mass and shape differ with respect to spherical halo finders. Haloes become more prolate when fit with ellipsoids, the difference being most pronounced for the more aspherical objects. Although the ellipsoidal mass is systematically larger, this is less than 10% for most of the haloes. However, even this small difference in mass corresponds to a significant difference in shape. We quantify these effects also on the initial mass and deformation tensors, on which most models of triaxial collapse are based. By studying the properties of protohaloes in the initial conditions, we find that models in which protohaloes are identified in Lagrangian space by three positive eigenvalues of the deformation tensor are tenable only at the masses well-above $M_{\ast }$. The overdensity $\delta$ within almost any protohalo is larger than the critical value associated with spherical collapse (increasing as mass decreases); this is in good qualitative agreement with models which identify haloes requiring that collapse have occured along all three principal axes, each axis having turned around from the universal expansion at a different time. The distributions of initial values are in agreement with the simplest predictions associated with ellipsoidal collapse, assuming initially spherical protohaloes, collapsed around random positions which were sufficiently overdense. However, most protohaloes are not spherical and departures from sphericity increase as protohalo mass decreases. [Abridged]

18 pages, 17 figures, accepted for publication in MNRAS

References in corpus (5)

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• The Population of Dark Matter Subhaloes: Mass Functions and Average Mass Loss Rates
• An improved model for the formation times of dark matter haloes
• Environmental dependence in the ellipsoidal collapse model
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Cited by in corpus (11)

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• Universality of dark matter haloes shape over six decades in mass: Insights from the Millennium XXL and SBARBINE simulations
• The mass dependence of dark matter halo alignments with large-scale structure
• Evolution and Statistics of Non-Sphericity of Dark Matter Halos from Cosmological N-Body Simulation
• The formation of CDM haloes II: collapse time and tides
• Statistics of Dark Matter Halos in the Excursion Set Peak Framework
• Shape Profiles and Orientation Bias for Weak and Strong Lensing Cluster Halos
• Getting in shape with minimal energy. A variational principle for protohaloes
• Constrained simulations and excursion sets: understanding the risks and benefits of `genetically modified' haloes