Scale Lengths in Dark Matter Halos

Abstract

We investigate a hypothesis regarding the origin of the scalelength in halos formed in cosmological N-body simulations. This hypothesis can be viewed as an extension of an earlier idea put forth by Merritt and Aguilar. Our findings suggest that a phenomenon related to the radial orbit instability is present in such halos and is responsible for density profile shapes. This instability sets a scalelength at which the velocity dispersion distribution changes rapidly from isotropic to radially anisotropic. This scalelength is reflected in the density distribution as the radius at which the density profile changes slope. We have tested the idea that radially dependent velocity dispersion anisotropy leads to a break in density profile shape by manipulating the input of a semi-analytic model to imitate the velocity structure imposed by the radial orbit instability. Without such manipulation, halos formed are approximated by single power-law density profiles and isotropic velocity distributions. Halos formed with altered inputs display density distributions featuring scalelengths and anisotropy profiles similar to those seen in cosmological N-body simulations.

On Universal Halos and the Radial Orbit Instability

“…Our results complement this rule in that they provide an example where a change in dM/dE goes hand in hand with a change in . This connection is absent in the results of Barnes et al (2006); they attempt to account for the effects of the ROI by changing the velocity structure of the infalling shells while leaving their energies unchanged.…”

“…Moreover, Taylor & Navarro (2001) found that / 3 , which they argue is a proxy for the ''phase-space density,'' is well approximated by a pure power law in radius. This observation prompted a number of analytic studies in which the Jeans equation, under the assumption that / 3 is a power law, is solved for the radial density profile ( Hansen 2004;Dehnen & McLaughlin 2005;Austin et al 2005; Barnes et al 2006). Finally, and identified what appears to be a universal relation between the velocity anisotropy parameter profile (r) (see Binney & Tremaine 1987 and eq.…”

“…The connection between the ROI and the structure of dark matter halos was also investigated by Barnes et al (2005), who developed a semianalytic model for halo formation based on the formulation of the secondary infall model of Ryden & Gunn (1987). In their model, the ROI does not arise dynamically, but rather its effects, specifically in the orbit structure assumed for the mass shells, are introduced by hand.…”

The origin of cold dark matter halo density profiles

“…White & Zartisky 1992;Ryden 1993;Sikivie et al 1997;Subramanian et al 2000;Subramanian 2000;Hiotelis 2002;Le Delliou & Henriksen 2003;Shapiro et al 2004;Barnes et al 2005). Applying the collisionless Boltzmann equation to self-similar gravitational collapse, Subramanian (2000) and Subramanian et al (2000) demonstrated that tangential velocities are required to obtain an inner profile that is shallower than that of a singular isothermal sphere.…”

“…Unfortunately, the inner slope predicted by such models is much steeper than that found in 3-dimensional cosmological simulations. Barnes et al 2005) noticed that tangential motions of particles may cause flattening of the inner profiles. N -body simulations by Huss et al (1999) and Hansen & Moore (2004) showed that the inner density profile of a halo is correlated with the degree of velocity anisotropy.…”