The Role of the Radial Orbit Instability in Dark Matter Halo Formation and Structure

Abstract

For a decade, N-body simulations have revealed a nearly universal dark matter density profile, which appears to be robust to changes in the overall density of the universe and the underlying power spectrum. Despite its universality, the physical origin of this profile has not yet been well understood. Semi-analytic models by Barnes et al. (2005) have suggested that the density structure of dark matter halos is determined by the onset of the radial orbit instability (ROI). We have tested this hypothesis using N-body simulations of collapsing dark matter halos with a variety of initial conditions. For dynamically cold initial conditions, the resulting halo structures are triaxial in shape, due to the mild aspect of the instability. We examine how variations in initial velocity dispersion affect the onset of the instability, and find that an isotropic velocity dispersion can suppress the ROI entirely, while a purely radial dispersion does not. The quantity σ 2 /v 2 c is a criterion for instability, where regions with σ 2 /v 2 c 1 become triaxial due to the ROI or other perturbations. We also find that the radial orbit instability sets a scale length at which the velocity dispersion changes rapidly from isotropic to radially anisotropic. This scale length is proportional to the radius at which the density profile changes shape, as is the case in the semi-analytic models; however, the coefficient of proportionality is different by a factor of ∼2.5. We conclude that the radial orbit instability is likely to be a key physical mechanism responsible for the nearly universal profiles of simulated dark matter halos.

CLASH-VLT: The mass, velocity-anisotropy, and pseudo-phase-space density profiles of thez= 0.44 galaxy cluster MACS J1206.2-0847

“…For a NFW-like density profile, the γ−β relation would imply isotropic orbits (β ≈ 0) near the center, and more radially anisotropic orbits (β > 0) outside, as observed in DM halos. The radius where β(r) departs from isotropy, r β , is then naturally related to the characteristic scale length r −2 of the DM density profile Bellovary et al 2008). A relation between r −2 and r β has indeed been found in numerically simulated halos (Barnes et al 2007;Mamon et al 2010).…”

“…Galaxies that were accreted by the cluster after the end of violent relaxation, would retain their slightly radial orbital distribution, producing the external β(r). Yet another process capable of isotropizing the initial radial orbits of infalling galaxies is radial orbit instability (ROI, see, e.g., Bellovary et al 2008).…”

The dynamics ofz~ 1 clusters of galaxies from the GCLASS survey

“…There are several processes that could in principle cause orbital isotropization, such as dynamical friction, violent relaxation following major mass accretion by the clusters, modification of the gravitational potential by secular mass accretion (Gill et al 2004), radial orbital instability (Bellovary et al 2008), and interaction with the intra-cluster medium (Dolag et al 2009). Insight in which of these processes is more effective in shaping galaxy orbits can come from an estimate of their relative timescales at different epochs of the cluster evolution.…”

The Effects of Angular Momentum on Halo Profiles

“…The formation of the bar is analogous to the structure formed by an ROI where a slight breaking of a distribution's spherical or cylindrical symmetry evolves exponentially into a significant asymmetry. Bellovary et al (2008) (BDB) showed that for a non-rotating spherical mass, velocity perturbations of the 3D thermal variety disrupted the instability, but purely radial dispersions were ineffective. BDB also made the claim that the ROI is a key physical mechanism contributing to the nearly universal profiles of simulated DM halos.…”

“…A halo's history consists of periods of gentle minor mergers, interrupted occasionally by more violent major mergers. Bellovary et al (2008) performs preliminary simulations of halos subject to controlled minor and major mergers to investigate the realistic role of the ROI, where they speculate that the ROI does not play a significant role in the subsequent relaxation process after a major merger. However, they also state that there are indications of an operational ROI during the minor merger periods, acting on nearly radial tidal streams associated with disrupted subhalos.…”