The Local Group as a test of cosmological models

Abstract

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 Ω = 1 (SCDM) nor Ω = 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 Ω. 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 Skid77http://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.