Spontaneous Formation of Outflows Powered by Rotating Magnetized Accretion Flows in a Galactic Center

We investigate how magnetically driven outflows are powered by a rotating, weakly magnetized accretion flow onto a supermassive black hole using axisymmetric magnetohydrodynamic simulations. Our proposed model focuses on the accretion dynamics on an intermediate scale between the Schwarzschild radius and the galactic scale, which is similar to 1-100 pc. We demonstrate that a rotating disk formed on a parsec-scale acquires poloidal magnetic fields via accretion, and this produces an asymmetric bipolar outflow at some point. The formation of the outflow was found to follow the growth of strongly magnetized regions around disk surfaces (magnetic bubbles). The bipolar outflow grew continuously inside the expanding bubbles. We theoretically derived the growth condition of the magnetic bubbles for our model that corresponds to a necessary condition for outflow growth. We found that the north-south asymmetrical structure of the bipolar outflow originates from the complex motions excited by accreting flows around the outer edge of the disk. The bipolar outflow comprises multiple mini-outflows and downflows (failed outflows). The mini-outflows emanate from the magnetic concentrations (magnetic patches). The magnetic patches exhibit inward drifting motions, thereby making the outflows unsteady. We demonstrate that the inward drift can be modeled using a simple magnetic patch model that considers magnetic angular momentum extraction. This study could be helpful for understanding how asymmetric and nonsteady outflows with complex substructures are produced around supermassive black holes without the help of strong radiation from accretion disks or entrainment by radio jets such as molecular outflows in radio-quiet active galactic nuclei, e.g., NGC 1377.

Automated summary

This study investigates the magnetically driven outflows powered by a rotating, weakly magnetized accretion flow onto a supermassive black hole through axisymmetric magnetohydrodynamic simulations. The results show that a rotating disk formed on a parsec-scale acquires poloidal magnetic fields via accretion, leading to the formation of an asymmetric bipolar outflow. The growth of strongly magnetized regions around disk surfaces is found to be responsible for the formation of the outflow, which continuously grows inside expanding magnetic bubbles. The north-south asymmetrical structure of the bipolar outflow originates from the complex motions excited by accreting flows around the outer edge of the disk.