Large-scale motions in the universe: Using clusters of galaxies as tracers

Can clusters of galaxies be used to trace the large-scale peculiar velocity field of the universe? We answer this question by using large-scale cosmological simulations to compare the motions of rich clusters of galaxies with the motion of the underlying matter distribution. Three models are investigated: Ω = 1 and Ω = 0.3 CDM, and Ω = 0.3 PBI models, all normalized to the COBE background fluctuations. We compare the cluster and mass distribution of peculiar velocities, bulk motions, velocity dispersions, and Mach numbers as a function of scale for R ≥ 50 h^-1 Mpc. We also present the large-scale velocity and potential maps of clusters and of the matter. We find that clusters of galaxies trace well the large-scale velocity field and can serve as an efficient tool to constrain cosmological models. The recently reported bulk motion of clusters 689 ± 178 km s^-1 on ∼150 h^-1 Mpc scale (Lauer & Postman 1994) is larger than expected in any of the models studied (≤190 ± 78 km s^-1).