Supercluster Infall Models

Abstract

We investigate the structure of the velocity field around large clusters formed in Ω = 1 cosmological N-body simulations, that form large scale structure in reasonable agreement with observations. We can test the applicability of spherical infall models commonly used to measure the mass of the Virgo supercluster and the cosmological density parameter. The spherical model is seen to be reasonably accurate for mean overdensity δ < 3, but only when the infall velocity is averaged over 4π steradians. Large asphericities in the flow field are the norm, even at low overdensi-ties. These asphericities result from significant subclustering seen on small scales and from the non-spherical nature of the exterior mass distribution. The simple spherical model for the Virgocentric flow as a measure for Ω is subject to large systematic and random errors. A much better test is to use a model independent, local criterion, noting that the local peculiar force field determines the local peculiar velocity extremely well both in direction and in amplitude. This relationship which must be valid in linear theory can be extended surprisingly well into the non-linear perturbations of the developing clusters.