THE QDOT AND CLUSTER DIPOLES - EVIDENCE FOR A LOW-OMEGA-0 UNIVERSE

We have reanalysed the QDOT survey in order to investigate the convergence properties of the estimated dipole and the consequent reliability of the derived value of OMEGA0(0.6)/b. We find that there is no compelling evidence that the QDOT dipole has converged within the limits of reliable determination and completeness. The value of OMEGA0 derived by Rowan-Robinson et al. (1990) should therefore be considered only as an upper limit. Furthermore, we find strong evidence that the shell between 140 and 160 h-1 Mpc does contribute significantly to the total dipole anisotropy and therefore to the motion of the Local Group with respect to the cosmic microwave background. This shell contains the Shapley concentration, but we argue that this concentration itself cannot explain all the gravitational acceleration produced by it; there must exist a coherent anisotropy which includes this structure, but extends greatly beyond it. With the QDOT data alone, we cannot determine precisely the magnitude of any such anisotropy, but any contribution to the Local Group motion from large scales would favour a value of OMEGA0(0.6)/b(IRAS) less-than-or-equaL-to 0.6, smaller than previous estimates based on IRAS galaxies; such a result would be consistent with the dipole measured from samples of rich clusters, which are much more complete at large depths.