Profiles of Dark Matter Velocity Anisotropy in Simulated Clusters

Details Profiles of Dark Matter Velocity Anisotropy in Simulated Clusters Profiles of Dark Matter Velocity Anisotropy in Simulated Clusters

2011-06-29

arxiv.org/abs/1106.6048v1

Astronomy and Astrophysics

Astrophysics

Cosmology and Extragalactic Astrophysics

7 pages, 11 figures, submitted to ApJ

Scientific paper

Interest in the spatial distribution of dark matter (DM) velocities in galaxy clusters has grown recently in light of the improved capability of determining its degree of anisotropy, $\beta$, from (either separate or joint) analysis of several different sets of measurements. Since cluster evolution is a highly non-linear hierarchical process, detailed theoretical expectations for $\beta$ and its profile can only be obtained from numerical simulations. We report statistical results for $\beta$ from a sample of some 6000 cluster-size halos (at redshift zero) identified in a $\Lambda$CDM hydrodynamical adaptive mesh refinement simulation done with the Enzo code. These include profiles of $\beta$ in clusters with different masses, relaxation states, and at several redshifts, modeled both as spherical and triaxial DM configurations. Specifically, although we find a large scatter in the DM velocity anisotropy profiles of different halos (across elliptical shells extending to at least ~$ 1.5 r_{vir}$), universal patterns are found when these are averaged over halo masses, redshifts, and relaxation stages. These are characterized by a very small velocity anisotropy at the halo center, increasing outward and leveling off at ~ 0.1-0.2 of the virial radius in lower mass and redshift halos. We also find that at radii larger than about $0.2 r_{vir}$, $\beta$ tends to be lower in spherical than in elliptical halos. This finding may help in sharpening the contrast between cluster shape measurements (e.g., by the CLASH project) and results from simulations. Our analysis does not indicate that there is significant correlation (found in some previous studies) between the radial density slope, $\gamma$, and $\beta$ at large radii, $0.3 r_{vir} < r < r_{vir}$.

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