Dynamics of nearby groups of galaxies: The role of the cosmological constant

Abstract

Context. Different cosmological data are consistent with the presence of an accelerated expansion of the Universe produced by an exotic matter-energy component, dubbed "dark-energy". A cosmological constant is a possible explanation for this expansion since it satisfies most of the observational constraints.Aims. We investigate the consequences of such a component on the dynamics of galaxy groups by attempting to detect possible effects on scales of approximately few Mpc. Methods. The Lemaître-Tolman model was modified by the inclusion of the cosmological constant term and, from numerical solution of the equations of motion, a velocity-distance relation was obtained. This relation depends on two parameters: the central core mass and the Hubble parameter. The non-linear fit of this relation to available data enabled the masses of five nearby groups of galaxies and the Virgo cluster to be obtained as well as estimates of the Hubble constant.Results. The analysis of the present results indicates that the velocity-distance relation derived from the modified Lemaître-Tolman model in addition to that derived from the "canonical" model, provide equally acceptable fits to the existing available data. As a consequence, any robust conclusion about the effects of the cosmological constant on the dynamics of groups could be established. The mean value of the Hubble parameter derived from the present study of local flows is km s-1 Mpc-1. © 2008 ESO.