Large Peculiar Velocities in the Hydra-Centaurus Supercluster

Distances from the infrared Tully-Fisher relation have been obtained with the Parkes radio telescope for six clusters of galaxies in the Hydra-Centaurus supercluster. One of these clusters is Pavo, a possible extension of the supercluster on the south side of the galactic plane. Three of these clusters show significant positive peculiar velocities of order 500 km s^-1^ in a comoving reference frame in which the observer is at rest with respect to the cosmic microwave background radiation. The net peculiar velocity of the sample suggests that Hydra-Centaurus tends to share the motion of the Local Group in this reference frame. When added to the Arecibo cluster sampLe, the data also fit a model in which two mass concentrations, one at Virgo and one just beyond the centroid of the Parkes sample, perturb the Hubble flow. A full model of the gravitational field in the sample volume based on the distribution of IRAS galaxies provides a better fit to the data without requiring a mass distribution different from the distribution of galaxies. Despite the significance of the peculiar velocities of some clusters, the present sample can only put an upper limit on the rms one-dimensional peculiar velocities in the population of gravitationally perturbed clusters from which it is drawn. This upper limit is v_pec_(rms) < 400 km s^-1^. These measurements of peculiar velocities remain consistent with the expectations for a universe dominated by cold dark matter, although the coherence length of the Virgo-Hydra-Centaurus flow is an uncomfortable constraint.