Internal and External Alignment of the Shapes and Angular Momenta of ΛCDM Halos

Abstract

We investigate how the shapes and angular momenta of galaxy and group mass dark matter halos in a ΛCDM N -body simulation are correlated internally, and how they are aligned with respect to the location and properties of surrounding halos. We explore these relationships down to halos of much lower mass (10 11 h −1 M ⊙ ) than previous studies. The halos are triaxial, with c/a ratios of 0.6 ± 0.1 and a mean two-dimensional projected ellipticity of e = 0.24. More massive halos are more flattened. The axis ratios rise out to 0.6 r vir , beyond which they drop. The principal axes, in particular the minor axes, are very well aligned within 0.6 r vir . High mass halos show particularly strong internal alignment. The angular momentum vectors are also reasonably well aligned except between the very outermost and very innermost regions of the halo. The angular momentum vectors tend to align with the minor axes, with a mean misalignment of ∼ 25 • , and lie perpendicular to the major and intermediate axes. The properties of a halo at 0.4 r vir are quite characteristic of the properties at most other radii within the halo. There is a very strong tendency for the minor axes of halos to lie perpendicular to large scale filaments, and a much weaker tendency for the major axes to lie along the filaments. This alignment extends to much larger separations for group and cluster mass halos than for galaxy mass halos. As a consequence, the intrinsic alignments of galaxies are likely weaker than previous predictions, which were based on the shapes of cluster mass halos. The angular momenta of the highest concentration halos tend to point toward other halos. The angular momenta of galaxy mass halos point parallel to filaments, while those of group and cluster mass halos show a very strong tendency to point perpendicular to the filaments. This suggests that group and cluster mass halos acquire most of their angular momentum from major mergers along filaments, while the accretion history of mass and angular momentum onto galaxy mass halos has been smoother.

The alignment and shape of dark matter, stellar, and hot gas distributions in the EAGLE and cosmo-OWLS simulations

“…We note that the computation of shape parameters in a spherical region biases the shape towards higher sphericity. When computing shapes of dark matter haloes in spherical regions it is possible to correct for this effect applying the simple empirical rescaling: S true = S √ 3 as suggested in Bailin & Steinmetz (2005). This correction is not implemented in the results presented here since a similar correction is not available for the other quantities that we present.…”

Baryonic pinching of galactic dark matter halos

“…Probes of the Milky Way halo indicate that it should be rather spherical with f 0.2 and that an oblate structure of f ∼ 0.2 might be preferable (see, e.g., [47] and references therein). Milky Way sized halos formed in dissipationless simulations are usually predicted to be considerably more triaxial and prolate, although a large scatter is expected [48,49,50,51,52,53,54]. Including dissipational baryons into the numerical simulation, and thereby converting the halo prolateness into a slightly oblate halo, might turn out to be essential to produce good agreement with observations.…”

Exploring Halo Substructure with Giant Stars. XI. The Tidal Tails of the Carina Dwarf Spheroidal Galaxy and the Discovery of Magellanic Cloud Stars in the Carina Foreground

“…In contrast, cold dark matter (CDM ) halos tend to have rounder potentials (Stoehr et al 2002;Hayashi et al 2003;Bailin & Steinmetz 2005) so that either the Carina halo is very unusual or an explanation is required for why its embedded luminous component has a rather different spatial distribution than its dark halo.…”