Probing the Large-scale Velocity Field with Clusters of Galaxies

What is the role of clusters of galaxies in probing the large-scale velocity field of the universe? We investigate the distribution of peculiar velocities of clusters of galaxies in the popular low-density (Ω = 0.3) flat Cold-Dark-Matter (CDM) cosmological model, which best fits many large-scale structure observations. An Ω = 1 CDM model is also studied for comparison. We find that clusters of galaxies are efficient tracers of the large-scale velocity field. The clusters exhibit a Maxwellian distribution of peculiar velocities, as expected from Gaussian initial density fluctuations. The cluster 3-D velocity distribution for the Ω = 0.3 model peaks at v ∼ 400 km s, and extends to high velocities of v ∼ 1200 km s. The rms peculiar velocity of the clusters is 440 km s. Approximately 10% of all model clusters move with high peculiar velocities of v ≥ 700 km s. The observed velocity distribution of clusters of galaxies is compared with the predictions from cosmological models. The observed data exhibit a larger velocity tail than seen in the model simulations; however, due to the large observational uncertainties, the data are consistent at a ∼ 3σ level with the model predictions, and with a Gaussian initial density field. The large peculiar velocities reported for some clusters of galaxies (v ≥ 3000 km s) are likely to be overestimated, if the current model is viable. Subject headings: cosmology: theory – galaxies: clustering