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Reconstructing Positions \& Peculiar Velocities of Galaxy Clusters within 25000 km/sec: The Bulk Velocity

Abstract

Using a dynamical 3-D reconstruction procedure we estimate the peculiar velocities of R0R\ge0 Abell/ACO galaxy clusters from their measured redshift within 25000 km/sec. The reconstruction algorithm relies on the linear gravitational instability hypothesis, assumes linear biasing and requires an input value of the cluster β\beta-parameter (βcΩ0.6/bc\beta_c \equiv \Omega_{\circ}^{0.6}/b_c), which we estimated in Branchini \& Plionis (1995) to be βc0.21\beta_c\simeq 0.21. The resulting cluster velocity field is dominated by a large scale streaming motion along the Perseus Pisces--Great Attractor base-line directed towards 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 18000\sim 18000 km/sec, we recover extremely well both the local group velocity and direction, in disagreement with the Lauer \& Postman (1994) observation. However, we find a 6%\sim 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. (1995) analyses of observed galaxy motions at 50006000\sim 5000 - 6000 km/sec; it decreases thereafter while at the Lauer \& Postman limiting depth (15000\sim 15000 km/sec) its amplitude is 150\sim 150 km/sec, in comfortable agreement with most cosmological models.Comment: 8 pages, uuencoded compressed tarred postscript file uncluding text and 3 figures. Accepted in ApJ Letter

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