Structure of Magnetic Tower Jets in Stratified Atmospheres

Based on a new approach on modeling the magnetically dominated outflows fromAGNs (Li et al. 2006), we study the propagation of magnetic tower jets ingravitationally stratified atmospheres (such as a galaxy cluster environment)in large scales ($>$ tens of kpc) by performing three-dimensionalmagnetohydrodynamic (MHD) simulations. We present the detailed analysis of theMHD waves, the cylindrical radial force balance, and the collimation ofmagnetic tower jets. As magnetic energy is injected into a small central volumeover a finite amount of time, the magnetic fields expand down the backgrounddensity gradient, forming a collimated jet and an expanded ``lobe'' due to thegradually decreasing background density and pressure. Both the jet and lobesare magnetically dominated. In addition, the injection and expansion produce ahydrodynamic shock wave that is moving ahead of and enclosing the magnetictower jet. This shock can eventually break the hydrostatic equilibrium in theambient medium and cause a global gravitational contraction. This contractionproduces a strong compression at the head of the magnetic tower front and helpsto collimate radially to produce a slender-shaped jet. At the outer edge of thejet, the magnetic pressure is balanced by the background (modified) gaspressure, without any significant contribution from the hoop stress. On theother hand, along the central axis of the jet, hoop stress is the dominantforce in shaping the central collimation of the poloidal current. The system,which possesses a highly wound helical magnetic configuration, never quitereaches a force-free equilibrium state though the evolution becomes much slowerat late stages. The simulations were performed without any initialperturbations so the overall structures of the jet remain mostly axisymmetric.Comment: 9 pages, 11 figures, 1 table, accepted for publication in Ap