Research on the Formation and Activity of Galactic Nuclear Gas Disks Including Magnetohydrodynamical Processes

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Three-dimensional global magnetohydrodynamic (MHD) simulations have been carried out to study the mechanism of the formation and activity of galactic nuclear gas disks. The initial state is a gas torus threaded by weak toroidal magnetic fields. We assumed that the gas torus is rotating in the gravitational potential of stars and the dark matter. We found (1) magnetorotational instability growing in the torus drives magnetic turbulence, (2) interstellar gas in falls by transporting angular momentum through Maxwell stress and forms a nuclear gas disk, (3) galactic mean magnetic fields are amplified up to 1 micro G, (4) the direction of mean azimuthal magnetic fields reverses with period of about 1Gyr, and (5) the reversal of mean magnetic fields is driven by the buoyant escape of magnetic flux from the gas disk to the galactic halo. We also carried out global 3D MHD simulations of the nuclear gas disk and found that magnetic loops similar to those observed inside 1kpc of our galaxy are formed by the buoyant rise of the magnetic flux from the gas disk.Numerical simulations indicated that poloidal magnetic flux is carried toward the galactic center. We performed 2D and 3D global resistive MHD simulations of jet formation from disks threaded by vertical magnetic fields and showed that mass accretion is partly driven by the angular momentum extraction by jets. The stability of magnetically driven jets has been studied by using 3D MHD code in Cartesian coordinates. We found that although the kink instability grows in regions where magnetic field lines are strongly twisted, it does not disrupt the jet.We implemented 3D MHD simulation engine based on the Roe scheme to the MHD simulator CANS (Coordinated Astronomical Numerical Software). The manuals and web-pages of CANS have been translated into English and used in the Asian Winter School on Numerical Astrophysics, held in March 13-17, 2006, at Chiba University.