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 5 h-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 Skid) 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.
http://www-hpcc.astro.washington.edu/tools/DENMAX- Publication:
-
New Astronomy
- Pub Date:
- July 1997
- DOI:
- 10.1016/S1384-1076(97)00011-0
- arXiv:
- arXiv:astro-ph/9612007
- Bibcode:
- 1997NewA....2...91G
- Keywords:
-
- METHODS: NUMERICAL;
- GALAXIES: INTERACTIONS;
- LOCAL GROUP;
- DARK MATTER;
- LARGE-SCALE STRUCTURE OF UNIVERSE;
- Astrophysics
- E-Print:
- Latex file (19 pages) + 13 figures. Submitted to New Astronomy. Two MPEG movies were not included. Also available (this time with the movies) at http://www-hpcc.astro.washington.edu/faculty/fabio/index.html