http://arxiv.org/abs/2304.06823
Recent high-resolution 53-$\mu$m polarimetric observations from SOFIA/HAWC+ 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 amodified” 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 themodified” DCF results are improved significantly ($\sim$3-22\%). Applying our “modified” DCF method to the CND revealed B-field strengths of $\sim1 – 16$ mG in the northern arm, $\sim1 – 13$ mG in the eastern arm, and $\sim3 – 27$ mG in the western arm at spatial scales $\lesssim1$ pc. The balance between turbulent gas energy (kinetic + 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.
J. Guerra, E. Lopez-Rodriguez, D. Chuss, et. al.
Mon, 17 Apr 23
24/51
Comments: Submitted to ApJ; Under review
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