THE VELOCITY-FIELD IN THE LOCAL SUPERCLUSTER

The velocity field in the Local Supercluster (LSC) is studied using a spiral galaxy sample out to 3000 km s-1. We describe the field with a model that includes spherically symmetric Virgocentric infall, "great attractor" (GA) infall, and random motion of the "Local Anomaly" with respect to its surroundings. The model is optimized using a maximum-likelihood method, which is able to reduce various sample selection effects. Our best estimates for the amplitudes of the Virgocentric and the GA flow at the position of the Local Group (LG) are, respectively, 197 ± 37 and 400 ± 91 km s-1. The Local Anomaly shows a motion of 230 km s-1 toward (l, b) = (205°, 11°) with an effective radius of 540 km s-1. The total peculiar velocity of the LG with respect to the GA agrees very well with its motion in the cosmic microwave background (CMB) frame. Our fitting method is able to compare the mass and galaxy number distributions, and implies that galaxies are more clustered than the underlying mass in the LSC. The Hubble velocity of the Virgo Cluster, as a self-adjusted scale parameter in our model fit, is found to be 1422 ± 43 km s-1, which corresponds to a Hubble constant of (90 ± 3) × 10-0.2(μ31) km s-1, if μ is the distance modulus of Virgo. Finally, Monte Carlo experiments are performed to test the reliability of our fitting technique. An interesting result from the experiments is that the GA's tidal effect on the LSC cannot be misinterpreted as part of the Virgo flow, when the GA infall is turned off in the model fit. Instead, the LG's total velocity toward Virgo would be underestimated, which in turn would cause us to underestimate the Hubble velocity of the Virgo Cluster. The Monte Carlo experiments are also used to estimate the observed scatter in the Tully-Fisher (TF) relation due to random motions of galaxies in the LSC, which then allows us to estimate the intrinsic dispersion of the TF relation. If the peculiar velocity dispersion of galaxies in the LSC is 200 km s-1, we find an upper limit to the intrinsic dispersion of the TF relation of 0.36 mag.