CAN STANDARD COSMOLOGICAL MODELS EXPLAIN THE OBSERVED ABELL CLUSTER BULK FLOW

Lauer and Postman (LP) observed that all Abell clusters with redshifts less than 15,000 km s(-1) appear to be participating in a bulk flow o f 689 km s(-1) with respect to the cosmic microwave background. We fin d this result difficult to reconcile with all popular models for large -scale structure formation that assume Gaussian initial conditions. Th is conclusion is based on Monte Carlo realizations of the LP data, dra wn from large particle-mesh N-body simulations for six different model s of the initial power spectrum (standard, tilted, and Omega(0) = 0.3 cold dark matter, hot dark matter, and two variants of the primordial baryon isocurvature model). We have taken special care to treat proper ly the longest-wavelength components of the power spectra. The simulat ions are sampled, ''observed,'' and analyzed as identically as possibl e to the LP cluster sample. Large-scale bulk flows as measured from cl usters in the simulations are in excellent agreement with those measur ed from the grid: the clusters do not exhibit any strong velocity bias on large scales. Bulk flows with amplitude as large as that reported by LP are not uncommon in the Monte Carlo data sets; the distribution of measured bulk flows before error bias subtraction is roughly Maxwel lian, with a peak around 400 km s(-1) However, the chi(2) of the obser ved bulk flow, taking into account the anisotropy of the error ellipso id, is much more difficult to match in the simulations. The models exa mined are ruled out at confidence levels between 94% and 98%.