The dynamics of disk galaxy mergers

Abstract

The long and short term evolution of mergers involving disk and disk-halo galaxies has been studied by using self-gravitating N-body models with N~2000. The stability of isolated, pure disk models has been examined and it has been found that such models can be made stable to global non-axisymmetric modes by the use of sufficient gravity softening. Disks stablised by softened gravity have been used to investigate the spin dependence of zero impact parameter collisions and the effect of halos on the severity of the merger. The dynamical importance of a halo to the behaviour of a disk inside its optical scale length has been assessed and used in constructing self-consistent, disk-halo models. The long term evolution of the two systems NGC4038/9 and NGC4676 has been modelled. It is found that the outcome of these mergers is a tumbling, bar-like system whose morphology is similar to that of elliptical galaxies. However the bars have large streaming motions which cause them to show larger values of V/σ in their roundest projection than do elliptical galaxies. It is proposed that apparently round, low mass ellipticals may show large values of V/σ and that mass may be an important parameter in the V/a-e plane. Experiments on the collision of massive ellipticals and disks have been conducted and it has been found that the shell structures reported by Malin and Carter (1981) are consistent with them being formed by such collisions. A theory for the formation and evolution of shells, based on a phase wrapping process , has been developed and applied to observations of shell ellipticals If the observed shell structures can be attributed to a single infall event then the shells are not consistent with the central optical galaxy being the sole component of the potential in which the shell stars move. Spectroscopic observations of the shell ellipticals NGC3923 and NGC1344 have been conducted. Both galaxies are kinematically similar to other ellipticals. NGC3923 has a number of features in its rotation curve that are suggestive of the structures expected from the phase wrapping theory of shell formation.