Showing papers by "Julio F. Navarro published in 2001"

The Phase - space density profiles of cold dark matter halos

Abstract: We examine the coarse-grained phase-space density pro—les of a set of recent, high-resolution simula- tions of galaxy-sized cold dark matter (CDM) halos. Over two and a half decades in radius the phase- space density closely follows a power law, o/p3 P r~a, with aB 1.875. This behavior closely matches the self-similar solution obtained by Bertschinger for secondary infall of gas onto a point-mass perturber in a uniformly expanding universe. On the other hand, the density pro—le corresponding to Bertschingers solution (a power law of slope r2a~6) diUers signi—cantly from the density pro—les of CDM halos. CDM halo density pro—les are clearly not power laws, and they have logarithmic slopes that steepen gradually with radius, roughly as described by Navarro, Frenk, & White (NFW). We show that isotropic, spher- ically symmetric equilibrium mass distributions with power-law phase-space density pro—les form a one- parameter family of structures controlled by the ratio of the local velocity dispersion to the ii natural ˇˇ velocity dispersion at some —ducial radius For i \ a \ 1.875, one recovers the r 0 ; i \ 4nGo(r 0 )r2 0 /p2(r 0 ). power-law solution o P r2a~6 .A si increases, the density pro—les become quite complex but still diverge as r2a~6 near the center. For i larger than some critical value solutions become nonphysical, i crit (a), leading to negative densities near the center. The critical solution, corresponds to the case i \ i crit , where the phase-space density distribution is the narrowest compatible with the power-law phase-space density strati—cation constraint. Over three decades in radius, the critical solution follows closely an NFW pro—le, although its logarithmic slope asymptotically approaches (2a/5 \( 0.75 (rather than (1) at very small radii. Our results thus suggest that the NFW pro—le is the result of a hierarchical assembly process that preserves the phase-space strati—cation of Bertschingers spherical infall model but ii mixes ˇˇ the system maximally, perhaps as a result of repeated merging, leading to a relatively uniform phase- space density distribution across the system. This —nding oUers intriguing clues as to the origin of the similarity in the structure of dark matter halos formed in hierarchically clustering universes. Subject headings: cosmology: theorydark mattergalaxies: formationgalaxies: structure ¨ methods: analyticalmethods: numerical On-line material: color —gures

Halo Substructure and Disk Heating in a Lambda CDM Universe

Abstract: We examine recent suggestions that substructure in cold dark matter (CDM) halos may be in conflict with the presence of thin, dynamically fragile stellar disks. N-body simulations of an isolated disk/bulge/halo model of the Milky Way that includes several hundred dark matter satellites with masses, densities and orbits derived from high-resolution cosmological CDM simulations indicate that substructure at $z=0$ plays only a minor dynamical role in the heating of the disk over several Gyrs. This is because the orbits of satellites in present-day CDM halos seldom take them near the disk, where their tidal effects are greatest. Unless the effects of substructure are very different at earlier times, our models suggest that substructure might not preclude virialized CDM halos from being acceptable hosts of thin stellar disks like that of the Milky Way.

Halo Substructure and Disk Heating in a Λ Cold Dark Matter Universe

Abstract: We examine recent suggestions that substructure in cold dark matter (CDM) halos may be in conflict with the presence of thin, dynamically fragile stellar disks. N-body simulations of an isolated disk/bulge/halo model of the Milky Way that includes several hundred dark matter satellites with masses, densities, and orbits derived from high-resolution cosmological CDM simulations indicate that substructure at z = 0 plays only a minor dynamical role in the heating of the disk over several gigayears. This is because the orbits of satellites in present-day CDM halos seldom take them near the disk, where their tidal effects are greatest. Unless the effects of substructure are very different at earlier times, our models suggest that substructure might not preclude virialized CDM halos from being acceptable hosts of thin stellar disks like that of the Milky Way.

The Phase-Space Density Profiles of Cold Dark Matter Halos

Abstract: We examine the coarse-grained phase-space density profiles of a set of recent, high-resolution simulations of galaxy-sized Cold Dark Matter (CDM) halos. Over two and a half decades in radius the phase-space density closely follows a power-law, $\rho/\sigma^3 \propto r^{-\alpha}$, with $\alpha = 1.875$. This behaviour matches the self-similar solution obtained by Bertschinger for secondary infall in a uniformly expanding universe. On the other hand, the density profile corresponding to Bertschinger's solution (a power-law of slope $r^{2\alpha-6}$) differs significantly from the density profiles of CDM halos. We show that isotropic mass distributions with power-law phase-space density profiles form a one-parameter family of structures controlled by $\kappa$, the ratio of the velocity dispersion to the peak circular velocity. For $\kappa=\alpha=1.875$ one recovers the power-law solution $\rho \propto r^{2\alpha-6}$. For $\kappa$ larger than some critical value, $\kappa_{cr}$, solutions become non-physical, leading to negative densities near the center. The critical solution, $\kappa =\kappa_{cr}$, has the narrowest phase-space density distribution compatible with the power-law phase-space density stratification constraint. Over three decades in radius the critical solution is indistinguishable from an NFW profile. Our results thus suggest that the NFW profile is the result of a hierarchical assembly process that preserves the phase-space stratification of Bertschinger's infall model but which ``mixes'' the system maximally, perhaps as a result of repeated merging.

The Inner Structure of Cold Dark Matter Halos

Abstract: I report on recent progress in our understanding of the structure of CDM halos, and in particular of the inner mass profile of galaxy-sized systems. Numerical simulations have consistently shown that the density profiles of CDM halos steepen monotonically from the center outwards, with slopes shallower than isothermal near the center and steeper than isothermal near the virial radius. Ongoing debate centers on the precise radial dependence of the logarithmic slope, as well as on whether it approaches a well defined asymptotic central value. The latest high-resolution simulations suggest that the circular velocity profile is well approximated by the model proposed by Navarro, Frenk & White (NFW). On the other hand, the radial dependence of the slope of the density profile differs modestly, but significantly, from the model proposed by NFW. As a result, NFW fits tend to underestimate the density at radii just inside the scale radius. Rather than implying a very steep (-1.5) inner divergent slope, I argue that the data is actually best represented by a model where the density profile becomes increasingly shallow with radius, with little sign of approach to a well-defined asymptotic value. A model where the {\it phase-space density profile} is a power law accounts well for these results and suggests that the innermost slope may be as shallow as -0.75. These conclusions are supported by a thorough numerical convergence study that elucidates the effect of numerical parameters such as the timestep, gravitational softening, and particle number, on the mass profile of simulated dark matter halos.

Substructure in CDM Halos and the Heating of Stellar Disks

Abstract: Numerical simulations have revealed the presence of long-lived substructure in Cold Dark Matter (CDM) halos These surviving cores of past merger and accretion events vastly outnumber the known satellites of the Milky Way This finding has prompted suggestions that substructure in cold dark matter (CDM) halos may be incompatible with observation and in conflict with the presence of thin, dynamically fragile stellar disks N-body simulations of a disk/bulge/halo model of the Milky Way that includes several hundred dark matter satellites with masses, densities and orbits derived from high-resolution cosmological CDM simulations indicate that substructure plays only a minor dynamical role in the heating of the disk This is because the orbits of satellites seldom take them near the disk, where their tidal effects are greatest We conclude that substructure might not preclude virialized CDM halos from being acceptable hosts of thin stellar disks like that of the Milky Way