Scale-Free Thin Discs with an Isopedic Magnetic Field

Abstract

Global stationary configurations of both aligned and logarithmic spiral MHD perturbations are constructed analytically within an axisymmetric background ofrazor-thin scale-free gas disc, which is embedded in an axisymmetric gravitational potential of a dark matter halo and involves an isopedic magnetic field almost vertically threaded through the disc plane. The scale-free index $\beta$ of the disc rotation speed $v_{\theta}\propto R^{-\beta}$ falls in the rangeof $(-1/2 , 1/2)$ where $R$ is the cylindrical radius. With the holding-back of a deep background dark matter halo potential, the isopedic magnetic field may be strong enough to allow for the magnetic tension force overtaking the disc self-gravity, which can significantly influence global stationary MHD perturbation configurations and stability properties of the scale-free disc system. Only for stationary logarithmic spiral MHD perturbations with a perturbation scale-free index $\beta_1=1/4$ or for aligned stationary MHD perturbations, can the MHD disc maintain a constant radial flux of angularmomentum. The variable radial flux of angular momentum in the radial direction corresponds to a complex dispersion relation. The marginal instabilities foraxisymmetric MHD disturbances are also examined for a special case as an example. When the magnetic tension force overtakes the disc self-gravity, the scale-free disc can be completely stable against axisymmetric MHD disturbances of all wavelengths. We predict the possible existence of an isopedicallymagnetized gas disc system in rotation primarily confined by a massive dark matter halo potential.