The Strength of the Sheared Magnetic Field in the Galactic's Circum-Nuclear Disk

Abstract

Recent high-resolution 53-$μ$m polarimetric observations from SOFIA/HAWC+ have revealed the inferred plane-of-the-sky magnetic field (B-field) orientation in the Galactic center's Circum-Nuclear Disk (CND). The B-field is mostly aligned with the steamers of ionized material falling onto Sgr A* at large, differential velocities (shear). In such conditions, estimating the B-field strength with the ``classical" Davis-Chandrasekhar-Fermi (DCF) method does not provide accurate results. We derive a ``modified'' DCF method by solving the ideal MHD equations from first principles considering the effects of a large-scale, shear flow on the propagation of a fast magnetosonic wave. In the context of the DCF approximation, both the value of the shear and its Laplacian affect the inferred B-field strength. Using synthetic polarization data from MHD simulations for a medium dominated by shear flows, we find that the ``classical'' DCF determines B-field strengths only within $>50$\% of the true value where the ``modified" DCF results are improved significantly ($\sim$3-22\%). Applying our ``modified'' DCF method to the CND revealed B-field strengths of 1 - 16 mG in the northern arm, 1 - 13 mG in the eastern arm, and 3 - 27 mG in the western arm at spatial scales $\lesssim1$ pc, with median values of $5.1\pm0.8$, $4.0\pm1.2$, and $8.5\pm2.3$ mG, respectively. The balance between turbulent gas energy (kinetic plus hydrostatic) and turbulent magnetic energy densities suggest that, along the magnetic-field-flow direction, magnetic effects become less dominant as the shear flow increases and weakens the B-field via magnetic convection. Our results indicate that the transition from magnetically to gravitationally dominated accretion of material onto Sgr A* starts at distances $\sim$ 1 pc.