The dynamical state of groups of galaxies

Groups are likely sites for interaction and merging between galaxies, and may evolve significantly in a fraction of a Hubble time. This paper presents a series of N-body experiments modelling the evolution of groups of 5 to 10 galaxies with massive dark haloes, starting from a wide range of initial conditions. Galaxies with massive haloes ‘stick together’ on encounter and subsequently merge. Galaxy–galaxy and galaxy–background interactions extract orbital energy from the galaxies, which collect at the centre of the system. As a result, the dynamical mass and time-scales inferred using the galaxies significantly underestimate the true values for the system as a whole. Present-epoch groups, even those with apparently short ($$\sim0.1 \enspace H^{-1}_0$$) dynamical times, are probably relatively young objects which have only just collapsed; nevertheless, these groups have already undergone significant dynamical evolution, segregating the galaxies from the mass. These results imply that (i) many group members are merger remnants, (ii) groups of galaxies evolve significantly on a dynamical time-scale, and (iii) the value of Ω estimated using the virial M/L ratios of groups is too low by a factor of ∼3 or more.