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 ({OMEGA} = 0.3) flat cold dark matter (CDM) cosmological model, which best fits many large-scale structure observations. An {OMEGA} = 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 three-dimensional velocity distribution for the {OMEGA} = 0.3 model peaks at v ~ 400 km s^- 1^ and extends to high velocities of v ~ 1200 km s^-1^. The rms peculiar velocity of the clusters is 440 km s^-1^. Approximately 10% of all model clusters move with high peculiar velocities v => 700 km s^-1^. 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 ~3 σ, 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^-1^)are likely to be overestimated, if the current model is viable.