Dependence of the dynamical properties of light-cone simulation dark matter halos on their environment

¹ Physics Dept., Aristotle Univ. of Thessaloniki, Thessaloniki 54124, Greece
e-mail: mchira@physics.auth.gr
² National Observatory of Athens, P. Pendeli, Athens, Greece
³ African Institute for Mathematical Sciences, 6 Melrose Road, Muizenberg, 7945 Cape Town, South Africa

Received: 1 September 2020
Accepted: 2 January 2021

Abstract

Aims. We study the dependence of the dynamical properties of massive (M ≥ 1.5 × 10¹³ M_⊙ h⁻¹) dark matter halos on their environment in a whole-sky ΛCDM light-cone simulation extending to z ∼ 0.65. The properties of interest for this study are the halo shape (parametrized via its principal axes), spin and virialization status, the alignment of halo spin and shape, as well as the shape-shape and spin-spin alignments among halo neighbors.

Methods. We define the halo environment using the notion of halo isolation status determined by the distance to its nearest neighbor. This defines a maximum spherical region around each halo devoid of other halos, above the catalog threshold mass. We consider “close halo pairs” to be pairs separated by a distance that is lower than a specific threshold. In order to decontaminate our results from the known dependence of halo dynamical properties on mass, we used a random sampling procedure to compare properties of similar halo abundance distributions.

Results. We find that: (a) there is a strong dependence on the part of the halo properties with regard to their environment, confirming that isolated halos are more aspherical and more prolate with lower spin values; (b) correlations between halo properties exist and are mostly independent of the halo environment; (c) halo spins are aligned with the minor axis, regardless of halo shape; and (d) close halo neighbors have their major axes statistically aligned, while they show a slight but statistically significant preference for anti-parallel spin directions. The latter result is enhanced for the case of close halo pairs in low-density environments. Furthermore, we find a tendency for the spin vectors to be oriented perpendicular to the line that connects such close halo pairs.