Reconstructing Positions and Peculiar Velocities of Galaxy Clusters within 25,000 Kilometers per Second: The Bulk Velocity

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Abstract
Using a dynamical three-dimensional reconstruction procedure, we estimate the peculiar velocities of R ≥ 0 Abell/ACO galaxy clusters from their measured redshift within 25,000 km s-1. The reconstruction algorithm relies on the linear gravitational instability hypothesis, assumes linear biasing, and requires an input value of the cluster β-parameter (βc ≡ Ω0.60/bc), which we estimated in Branchini & Plionis to be βc 0.21. The resulting cluster velocity field is dominated by a large-scale streaming motion along the Perseus-Pisces/Great Attractor baseline directed toward the Shapley concentration, in qualitative agreement with the galaxy velocity field on smaller scales. Fitting the predicted cluster peculiar velocities to a dipole term, in the Local Group frame and within a distance of ~18,000 km s-1, we recover extremely well both the Local Group velocity and direction, in disagreement with the Lauer & Postman observation. However, we find a ~6% probability that their observed velocity field could be a realization of our corresponding one, if the latter is convolved with their large distance-dependent errors. Our predicted cluster bulk velocity amplitude agrees well with that deduced by the POTENT and the da Costa et al. analyses of observed galaxy motions at ~5000-6000 km s-1; it decreases thereafter, while, at the Lauer & Postman limiting depth (~15,000 km s-1), its amplitude is ~150 km s-1, in comfortable agreement with most cosmological models.
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