Magnetically supported tori in active galactic nuclei

We explore the concept that the gas tori inferred to exist around the central black holes in many active galactic nuclei (AGN) have significant magnetic pressure support. We derive the basic equations for magnetically supported tori with a general axisymmetric B field involving toroidal and poloidal components. We treat strong and weak field limits assuming the aspect ratio R/a larger than unity, where R is the major radius of the torus, and a is the minor radius. For strong fields, the major radius is influenced by the B field, whereas for weak fields only the minor radius is affected by the field. In the strong field limit, stability to kink perturbations is possible only for R/a approaching unity, just where our theory breaks down. The weak field tori tend to be kink stable for R/a < ∼ 13. This suggests that magnetically supported tori in nature need to be “fat” in order to be stable. This appears to be consistent with observations of tori in AGN. We note that some fat disks around young stars and around more evolved stars represent magnetically supported tori.