Streaming motions of abell clusters: new evidence for a high-amplitude bulk flow on very large scales

Abstract

Streaming motions of galaxies and clusters provide the only method for probing the distribution of mass, as opposed to light, on scales of 20 - 100 h(^-1)Mpc. This thesis presents a new survey of the local peculiar velocity field, based upon Fundamental Plane (FP) distances for an all-sky sample of 56 clusters to cz = 12000 km s(^-1). Central velocity dispersions have been determined from new spectroscopic data for 429 galaxies. From new R-band imaging data the FP photometric parameters (effective diameter and effective surface brightness) have been measured for 324 galaxies. The new spectroscopic and photometric data have been carefully combined with an extensive body of measurements compiled from the literature, to yield a closely homogeneous catalogue of FP data for 725 early type galaxies. Fitting the inverse FP relation to the merged catalogue yields distance estimates with a scatter of 22% per galaxy, resulting in cluster distance errors of 2-13%. The distances are consistent, on a cluster-by-cluster basis, with those determined from Tully-Fisher-Fisher studies and from earlier FP determinations. The distances are marginally inconsistent with distance estimates based on brightest cluster galaxies, but this disagreement can be traced to a few highly discrepant clusters. The resulting peculiar velocity field is dominated by a bulk streaming component, with amplitude of 810 ± 180km s(^-1) (directed towards l = 260 ,b = -5 ), a result which is robust against a range of potential systematic effects. The flow direction is ~35 from the CMB dipole and ~15 from the X-ray cluster dipole direction. Two prominent superclusters (the Shapley Concentration and the Horologium-Reticulum Supercluster) may contribute significantly to the generation of this flow. More locally, there is no far- side infall into the 'Great Attractor' (GA), apparently due to the opposing pull of the Shapley Concentration. A simple model of the flow in this direction suggests that the GA region generates no more than ~60% of the Local Group's motion in this direction. Contrary to some previous studies, the Perseus-Pisces supercluster is found to exhibit no net streaming motion. On small scales the velocity field is extremely quiet, with an rms cluster peculiar velocity of < 270 km s(^-1) in the frame defined by the bulk-flow. The results of this survey suggest that very distant mass concentrations contribute significantly to the local peculiar velocity field. This result is difficult to accommodate within currently popular cosmological models, which have too little large-scale power to generate the observed flow. The results may instead favour models with excess fluctuation power on 60-150h(^-1)Mpc scales.