Ellipsoidal halo finders and implications for models of triaxial halo formation

TL;DR

This paper introduces an ellipsoidal halo finder algorithm for simulations, showing it affects mass and shape estimates, and discusses implications for models of triaxial halo formation based on initial conditions.

Contribution

The paper presents a new ellipsoidal halo finder algorithm and analyzes its impact on halo shape, mass, and initial conditions, advancing understanding of triaxial halo formation models.

Findings

Ellipsoidal mass estimates are systematically larger but less than 10% for most haloes.

Protohaloes are generally not spherical, with deviations increasing at lower masses.

Models with three positive eigenvalues of the deformation tensor are valid only above a certain mass threshold.

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 $M_*$. 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]