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

Turbulence in the Milky Way Turbulence is a well-studied phenomenon in disciplines ranging from meteorology to cardiology, but astrophysical turbulence — acting over physical scales spanning 10 15 orders of magnitude — stretches current theories to their limits. Until now, insight into interstellar turbulence has been limited by our inability to image the small-scale structure associated with turbulent motions. This paper presents the first direct images of turbulence in interstellar gas. Using radio-continuum images of a patch of the Galactic plane, observed with the Australia Telescope Compact Array, Gaensler et al . report that the gradient of the Stokes vector ( Q,U ) provides an image of magnetized turbulence in diffuse ionized gas, seen as a complex filamentary web of discontinuities in gas density and magnetic field. The turbulence in the warm ionized medium has a relatively low sonic Mach number of approximately Mach 2. The interstellar medium of the Milky Way is multiphase 1 , magnetized 2 and turbulent 3 . 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 5 , 6 , 7 . 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 8 , 9 , 10 , 11 , 12 , and whose physical significance has been unclear 13 , 14 , 15 . 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.