Magnetohydrodynamic Tests of Galactic Synchrotron Depolarization

Synchrotron emission from cosmic-ray electrons accelerated in the Galactic magnetic field encodes information on the role of turbulence in the magnetized interstellar medium. Although synchrotron radiation is intrinsically highly polarized, the observed fractional polarization f = P / I is below 5% over most of the sky. Despite this marked depolarization, the remaining signal is highly aligned, with median scatter in polarization angle less than 15° within regions of 20° diameter on the sky. We use magnetohydrodynamic (MHD) simulations to determine the extent to which MHD turbulence can reproduce the observed pattern of substantial depolarization with minimal angular scatter. We evaluate both the synchrotron fractional polarization and polarization angle as projected through a suite of simulations with sonic Mach number 0.5 < M S < 6 and magnetic (Alfvén) Mach number 0.5 < M A < 4 . Simulations with a weakly magnetized plasma and strongly supersonic flow ( M A = 2 with 4 < M S < 6 ) match the observed high-latitude sky. The fit improves when the angle between the observer and the seed magnetic field is 15°–30° from perpendicular, in agreement with models of the local inclination angle of the large-scale Galactic magnetic field. Additional improvement occurs if the estimated brightness of the unpolarized Galactic synchrotron emission is reduced, either by subtracting an unpolarized extragalactic component from the observed radio maps or by applying a steeper spectra index when scaling the unpolarized synchrotron intensity from 408 MHz to 30 GHz.