Dynamics of galaxies with triaxial haloes

Abstract

We study the stability of trajectories near the disk plane of galaxy models with a triaxial dark matter halo component. We also examine the effect of weak discreteness noise, rapidly rotating bar perturbations and weak dissipation on these trajectories. The latter effect is studied both by adding a dissipative component to the force law and by using particle simulations. If the matter distribution is triaxial and has a constant density core, dissipation leads to inflow of material inside the core radius to the centre (since no non-self-intersecting closed orbits exist in the central areas). This leads to the formation of central masses which in turn destabilise the trajectories of any stars formed in these regions. In particular, even if the gas settles by dissipation into a flat disk, stars formed in that disk will later form a bulge like distribution the extent of which would be related to the core radius of the halo and the original asymmetry in the plane. The process of gas inflow is regulated by the fact that too large and condensed a central mass leads to the creation of stable closed loop orbits in the central area around which the gas can move. This would appear to stop the accumulation of central mass before it becomes large enough for rapid loss of halo triaxiality. It was found that weak discreteness noise can increase the fraction of such trajectories significantly and can therefore have important consequences for the modelling of galaxies. The addition of rapidly rotating bar perturbations also increases the degree of instability dramatically. So if bars can form in triaxial haloes they are likely to be quickly destroyed leaving a bulge like structure behind (which may explain the absence of bars in surveys of high redshift galaxies). If they do survive, their main effect on the gas dynamics is to create attractors other than the centre, around which the gas can move. We discuss some possible consequences of the aforementioned effects. In particular, it is suggested that the halo core radius and initial asymmetry may be important in determining the relative disk-halo contribution to the rotation curve of a galaxy - and hence its Hubble type.