We study the structure and substructure of Lambda-CDM halos using a suite of high-resolution cosmological N-body simulations. Our analysis of the substructure population of dark matter halos focuses on their mass and peak circular velocity functions, as well as their spatial distribution and dynamics. In our analysis, we consider the whole population of subhalos physically associated with the main halo, defined as those that have, at some time, crossed within the virial radius of the main progenitor. We find that this population extends beyond 3 times the virial radius and includes objects on unorthodox orbits, several of which travel at velocities approaching the nominal escape speed from the system. We trace the origin of these unorthodox orbits to the tidal dissociation of bound groups, which results in the ejection of some systems along tidal streams. This process primarily influences low-mass systems leading to clear mass-dependent biases in their spatial distribution and kinematics: the lower the subhalo mass at accretion time the more concentrated and kinematically hotter their descendant population. When quantified in terms of present day subhalo mass these trends disappear, presumably due to the increased effect of dynamical friction and tidal stripping on massive systems. We confirm several of these results using the ultra-high resolution Aquarius simulations, which extend the dynamic range of the subhalo mass function by nearly 3 orders of magnitude. Using these simulations we confirm that the substructure mass function follows a power-law, $dN/dM\propto M^{-1.9}$, and exhibits very little halo-to-halo scatter. This implies that the total mass in substructure within a given halo is bounded to a small fraction of the total halo mass, with the smooth component dominating the halo inner regions. Using the Aquarius simulations we study the structure of galaxy-sized Lambda-CDM halos. We find that the spherically averaged density profiles become increasingly shallow toward the halo center, with no sign of converging to an asymptotic power-law; a radial dependence accurately described by the Einasto profile. In our highest resolution run we resolve scales approaching 100 pc, at which point the maximum asymptotic slope is $\approx -0.89$, confidently ruling out recent claims for cusps as steep as $r^{-1.2}$. We find that the spherically averaged density and velocity dispersion profiles are not universal, but rather show subtle but significant deviations from self-similarity. Intriguingly, departures from self-similarity are minimized when cast in terms of the phase-space density profile, $\rho/\sigma^3$, suggesting an intimate scaling between densities and velocity dispersions across the system. The phase-space density profiles follow a power-law with radius, $r^{-1.875}$, identical to that of Bertschinger's similarity solution for self-similar infall onto a point mass in an otherwise unperturbed Einstein-de Sitter universe.

We study the structure and substructure of Lambda-CDM halos using a suite of high-resolution cosmological N-body simulations. Our analysis of the substructure population of dark matter halos focuses on their mass and peak circular velocity functions, as well as their spatial distribution and dynamics. In our analysis, we consider the whole population of subhalos physically associated with the main halo, defined as those that have, at some time, crossed within the virial radius of the main progenitor. We find that this population extends beyond 3 times the virial radius and includes objects on unorthodox orbits, several of which travel at velocities approaching the nominal escape speed from the system. We trace the origin of these unorthodox orbits to the tidal dissociation of bound groups, which results in the ejection of some systems along tidal streams. This process primarily influences low-mass systems leading to clear mass-dependent biases in their spatial distribution and kinematics: the lower the subhalo mass at accretion time the more concentrated and kinematically hotter their descendant population. When quantified in terms of present day subhalo mass these trends disappear, presumably due to the increased effect of dynamical friction and tidal stripping on massive systems. We confirm several of these results using the ultra-high resolution Aquarius simulations, which extend the dynamic range of the subhalo mass function by nearly 3 orders of magnitude. Using these simulations we confirm that the substructure mass function follows a power-law, $dN/dM\propto M^{-1.9}$, and exhibits very little halo-to-halo scatter. This implies that the total mass in substructure within a given halo is bounded to a small fraction of the total halo mass, with the smooth component dominating the halo inner regions. Using the Aquarius simulations we study the structure of galaxy-sized Lambda-CDM halos. We find that the spherically averaged density profiles become increasingly shallow toward the halo center, with no sign of converging to an asymptotic power-law; a radial dependence accurately described by the Einasto profile. In our highest resolution run we resolve scales approaching 100 pc, at which point the maximum asymptotic slope is $\approx -0.89$, confidently ruling out recent claims for cusps as steep as $r^{-1.2}$. We find that the spherically averaged density and velocity dispersion profiles are not universal, but rather show subtle but significant deviations from self-similarity. Intriguingly, departures from self-similarity are minimized when cast in terms of the phase-space density profile, $\rho/\sigma^3$, suggesting an intimate scaling between densities and velocity dispersions across the system. The phase-space density profiles follow a power-law with radius, $r^{-1.875}$, identical to that of Bertschinger's similarity solution for self-similar infall onto a point mass in an otherwise unperturbed Einstein-de Sitter universe.