2003/01/14 by A. V. Maccio', Andrea V. Maccio, Giuseppe Murante +3
Chemistry · Physics and Astronomy · #Advanced Physical and Chemical Molecular Interactions #High-Energy Particle Collisions Research #Theoretical and Computational Physics #astro-ph
paper · pdf · doi:10.1086/373944
published as Astrophys.J. 588 (2003) 35-49 · 19 Pages, 15 figures, ApJ in press
arxiv created 2003/01/14 · openalex publication_date 2003/05/01 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/07/30
We show that dark matter halos in N -body simulations have a boundary layer (BL), which neatly separates dynamically bound mass from unbound materials. We define T ( r ) and W ( r ) as the differential kinetic and potential energy of halos and evaluate them in spherical shells. We notice that in simulated halos such differential quantities fulfill the following properties: (1) the differential virial ratio = -2 T / W has at least one persistent (resolution-independent) minimum , such that, close to , (2) the function w = - d log W / d log r has a maximum, while (3) the relation ( ) ≃ w ( ) holds. BLs are set where these three properties are fulfilled, in halos found in simulations of "tilted" Einstein-de Sitter and ΛCDM models, run ad hoc, using the ART and GADGET codes; their presence is confirmed in larger simulations of the same models with a lower level of resolution. Here we find that ~97% of the ~300 clusters (per model) we have with M > 4.2 × 10 14 h -1 M ☉ own a BL. Those clusters that appear not to have a BL are seen to be undergoing major merging processes and to grossly violate spherical symmetry. The radius ≡ r c has significant properties. First of all, the mass M c it encloses almost coincides with the mass M dyn , evaluated from the velocities of all particles within r c , according to the virial theorem. Also, materials at r > r c are shown not be in virial equilibrium. Using r c we can then determine an individual density contrast Δ c for each virialized halo, which we compare with the "virial" density contrast Δ v ≃ 178Ω (where Ω m is the matter density parameter) obtained assuming a spherically symmetric and unperturbed fluctuation growth. As expected, for each mass scale, Δ v is within the range of values Δ c . However, the spread in Δ c is wide, while the average Δ c is ~25% smaller than the corresponding Δ v . We argue that the matching of properties derived under the assumption of spherical symmetry must be a consequence of an approximate sphericity, after violent relaxation destroyed features related to ellipsoidal nonlinear growth. On the contrary, the spread of the final Δ c is an imprint of the different initial three-dimensional geometries of fluctuations and of the variable environment during their collapse, as suggested by a comparison of our results with the Sheth & Tormen analysis.