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 3-D velocity distribution for the $\Omega=0.3$ model peaks at $v \sim 400$ km s$^{-1}$, and extends to high velocities of $v \sim 1200$ 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 of $v \ge 700$ 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 $\sim 3\sigma$ level with the model predictions, and with a Gaussian initial density field. The large peculiar velocities reported for some clusters of galaxies ($v \geq 3000$ km s$^{-1}$) are likely to be overestimated, if the current model is viable. 
Neta A. Bahcall, Renyue Cen and Mirt Gramann (Princeton University   Observatory, Princeton, NJ)