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


The interstellar medium of the Milky Way is multiphase1, magnetized2 and turbulent3. 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 motion5,6,7. 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 wavelengths8,9,10,11,12, and whose physical significance has been unclear13,14,15. 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, Ms 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.

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Figure 1: Total intensity ( I ) and linearly polarized intensity ( Q, U, P ) for an 18-deg2 region of the Southern Galactic Plane Survey29.
Figure 2: |P | for an 18-deg2 region of the Southern Galactic Plane Survey.
Figure 3: |P| derived from propagation of linear radio polarization through three different isothermal simulations of magnetized turbulence.

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We thank S. Brown, A. Hill, R. Kissmann, A. MacFadyen, M.-M. Mac Low, E. Petroff, P. Slane and X. Sun for discussions. The Australia Telescope Compact Array is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO. B.M.G. and T.R. acknowledge the support of the Australian Research Council through grants FF0561298, FL100100114 and FS100100033. B.B. acknowledges support from the National Science Foundation Graduate Research Fellowship and the NASA Wisconsin Space Grant Institution. A.L. acknowledges the support of the National Science Foundation through grant AST0808118 and of the Center for Magnetic Self-Organization in Astrophysical and Laboratory Plasmas. We thank the staff of the Australia Telescope National Facility, especially M. Calabretta, R. Haynes, D. McConnell, J. Reynolds, R. Sault, R. Wark and M. Wieringa, for their support of the Southern Galactic Plane Survey.

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Authors and Affiliations



J.M.D., N.M.Mc.–G., B.M.G. and A.J.G. carried out the original observations. B.M.G., N.M.Mc.–G. and T.R. produced the polarization images from the raw data. B.M.G., M.H., K.J.N.–Mc., R.D.E and N.M.Mc.–G. worked together to develop the gradient technique, and B.M.G. then applied the gradient technique to the images. B.B. and A.L. performed the simulations and the statistical analysis. B.M.G. led the writing of the paper and the interpretation of results. All authors discussed the results and commented on the manuscript.

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Correspondence to B. M. Gaensler.

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Gaensler, B., Haverkorn, M., Burkhart, B. et al. Low-Mach-number turbulence in interstellar gas revealed by radio polarization gradients. Nature 478, 214–217 (2011).

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