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

TL;DR

This study uses magnetohydrodynamic simulations to explore how rotating, weakly magnetized accretion flows around supermassive black holes generate asymmetric, unsteady bipolar outflows through magnetic bubble growth and complex disk motions.

Contribution

It presents a new model linking magnetic bubble growth and accretion dynamics to outflow formation, explaining asymmetry and unsteadiness without strong radiation or jets.

Findings

Magnetic bubbles are essential for outflow growth.

Outflows are asymmetric due to complex accretion motions.

Inward drifting magnetic patches cause unsteadiness.

Abstract

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 $\sim$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 magnetic bubbles for our model that corresponds to a necessary condition for outflow growth. We found that the north-south asymmetric 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 non-steady 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, NGC 1377.