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We study the matter distribution in and around dark halos in the standard Λ cold dark matter (ΛCDM) universe. We use the outputs of high resolution N-body simulations to compute the mean surface density profile around galactic size dark matter halos. The result is compared with the observed surface density profile from the lensing magnification measurement of quasars in Sloan Digital Sky Survey (SDSS) recently reported by Menard et al. (2010). We show that the ΛCDM simulations reproduce the observed surface density profile remarkably well over a wide range of radial distance of 10−104h−1kpc. The mean surface density profile at small separation lengths is consistent with the theoretical prediction that dark halos have a universal NavarroFrenk-White density profile. The observed single power-law of Σ ∝ R−0.8 over the distance range is explained by a simple analytic model based on a halo approach. We also study the distribution of dark matter in the outer part of the halos by measuring the mass enclosed within αRvirial, as a function of α . Our simulations suggest that more than a half of total matter is contained in about 10% volume associated with galactic halos. Assuming the SDSS galaxies are hosted by dark halos with mass 2×1012 −10M⊙, the total mass around the halos amounts to 30 percent of the total mass in the local universe.

The distribution of dark matter around galaxies : Mean surface density profile and outer structure