Low-Mach-number turbulence in interstellar gas revealed by radio polarization gradients

Abstract

The interstellar medium of the Milky Way is multiphase, magnetized and turbulent. Turbulence in the interstellar medium produces a global cascade of random gas motions, spanning scales ranging from 100 parsecs to 1,000 kilometres (ref. 4). Fundamental parameters of interstellar turbulence such as the sonic Mach number (the speed of sound) have been difficult to determine, because observations have lacked the sensitivity and resolution to image the small-scale structure associated with turbulent motion. Observations of linear polarization and Faraday rotation in radio emission from the Milky Way have identified unusual polarized structures that often have no counterparts in the total radiation intensity or at other wavelengths, and whose physical significance has been unclear. Here we report that the gradient of the Stokes vector (Q, U), where Q and U are parameters describing the polarization state of radiation, provides an image of magnetized turbulence in diffuse, ionized gas, manifested as a complex filamentary web of discontinuities in gas density and magnetic field. Through comparison with simulations, we demonstrate that turbulence in the warm, ionized medium has a relatively low sonic Mach number, M(s) ≲ 2. The development of statistical tools for the analysis of polarization gradients will allow accurate determinations of the Mach number, Reynolds number and magnetic field strength in interstellar turbulence over a wide range of conditions.

The Link Between Turbulence, Magnetic Fields, Filaments, and Star Formation in the Central Molecular Zone Cloud G0.253+0.016

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“…Observational studies of turbulence in the warm ionised medium indicate a transonic (Hill et al 2008;Gaensler et al 2011;Burkhart et al 2012) regime. Rewriting Eq.…”

Radio Synchrotron Fluctuation Statistics as a Probe of Magnetized Interstellar Turbulence

“…Recently, Gaensler et al (2011) and Burkhart et al (2012) have shown that the skewness and kurtosis of polarization gradients of synchrotron emission can constrain the sonic Mach number in the warm ionized medium (WIM). This technique has been applied by these authors and Iacobelli et al (2014) to observations, all of which demonstrate that the WIM is approximately transonic.…”