1997NewA....2...91G

The dynamics of the Local Group and its environment provide a unique challenge to cosmological models. The velocity field within 5h^-1 Mpc of the Local Group (LG) is extremely "cold". The deviation from a pure Hubble flow, characterized by the observed radial peculiar velocity dispersion, is measured to be ∼60 km s^-1. We compare the local velocity field with similarly defined regions extracted from N-body simulations of Universes dominated by cold dark matter (CDM). This test is able to strongly discriminate between models that have different mean mass densities. We find that neither the Omega = 1 (SCDM) nor Omega = 0.3 (OCDM) cold dark matter models can produce a single candidate Local Group that is embedded in a region with such small peculiar velocities. For these models, we measure velocities dispersion between 300-700 km s^-1 and 150-300 km s^-1, respectively, more than twice the observed value. Although both CDM models fail to produce environments similar to those of our Local Group on a scale of a few Mpc, they can give rise to many binary systems that have similar orbital properties as the Milky Way-Andromeda system. The local, gravitationally induced bias of halos in the CDM "Local Group" environment, if defined within a sphere of 10 Mpc around each Local Group is ∼1.5, independent of Omega. No biasing scheme could reconcile the measured velocity dispersions around Local Groups with the observed one. Identification of binary systems using a halo finder (named Skid (http://www-hpcc.astro.washington.edu/tools/DENMAX) based on a local density maxima search instead of a simple linking algorithm, gives a much more complete sample. We show that a standard "friend-of-friends" algorithm would miss about 40% of the LG candidates present in the simulations.