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

Abstract

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.

References

  1. 1

    Ferrière, K. M. The interstellar environment of our galaxy. Rev. Mod. Phys. 73, 1031–1066 (2001)

    ADS  Article  Google Scholar 

  2. 2

    de Avillez, M. A. & Breitschwerdt, D. Global dynamical evolution of the ISM in star forming galaxies. I. High resolution 3D simulations: effect of the magnetic field. Astron. Astrophys. 436, 585–600 (2005)

    ADS  CAS  Article  Google Scholar 

  3. 3

    Chepurnov, A. & Lazarian, A. Extending the big power law in the sky with turbulence spectra from Wisconsin Hα Mapper data. Astrophys. J. 710, 853–858 (2010)

    ADS  CAS  Article  Google Scholar 

  4. 4

    Armstrong, J. W., Rickett, B. J. & Spangler, S. R. Electron density power spectrum in the local interstellar medium. Astrophys. J. 443, 209–221 (1995)

    ADS  Article  Google Scholar 

  5. 5

    Kritsuk, A. G., Norman, M. L., Padoan, P. & Wagner, R. The statistics of supersonic isothermal turbulence. Astrophys. J. 665, 416–431 (2007)

    ADS  Article  Google Scholar 

  6. 6

    Kowal, G., Lazarian, A. & Beresnyak, A. Density fluctuations in MHD turbulence: Spectra, intermittency, and topology. Astrophys. J. 658, 423–445 (2007)

    ADS  Article  Google Scholar 

  7. 7

    Kissmann, R., Kleimann, J., Fichtner, H. & Grauer, R. Local turbulence simulations for the multiphase ISM. Mon. Not. R. Astron. Soc. 391, 1577–1588 (2008)

    ADS  Article  Google Scholar 

  8. 8

    Wieringa, M. H., de Bruyn, A. G., Jansen, D., Brouw, W. N. & Katgert, P. Small scale polarization structure in the diffuse galactic emission at 325 MHz. Astron. Astrophys. 268, 215–229 (1993)

    ADS  Google Scholar 

  9. 9

    Gray, A. D., Landecker, T. L., Dewdney, P. E. & Taylor, A. R. A large-scale, interstellar Faraday-rotation feature of unknown origin. Nature 393, 660–662 (1998)

    ADS  CAS  Article  Google Scholar 

  10. 10

    Haverkorn, M., Katgert, P. & de Bruyn, A. G. Structure in the local Galactic ISM on scales down to 1 pc, from multi-band radio polarization observations. Astron. Astrophys. 356, L13–L16 (2000)

    ADS  Google Scholar 

  11. 11

    Gaensler, B. M. et al. Radio polarization from the inner galaxy at arcminute resolution. Astrophys. J. 549, 959–978 (2001)

    ADS  CAS  Article  Google Scholar 

  12. 12

    Uyanıker, B. & Landecker, T. L. A highly ordered Faraday rotation structure in the interstellar medium. Astrophys. J. 575, 225–233 (2002)

    ADS  Article  Google Scholar 

  13. 13

    Shukurov, A. & Berkhuijsen, E. M. Faraday ghosts: depolarization canals in the galactic radio emission. Mon. Not. R. Astron. Soc. 342, 496–500 (2003)

    ADS  Article  Google Scholar 

  14. 14

    Haverkorn, M. & Heitsch, F. Canals beyond Mars: beam depolarization in radio continuum maps of the warm ISM. Astron. Astrophys. 421, 1011–1019 (2004)

    ADS  Article  Google Scholar 

  15. 15

    Fletcher, A. & Shukurov, A. Depolarization canals and interstellar turbulence. EAS Publ. Ser. 23, 109–128 (2007)

    CAS  Article  Google Scholar 

  16. 16

    McClure-Griffiths, N. M. et al. The Southern Galactic Plane Survey: the test region. Astrophys. J. 551, 394–412 (2001)

    ADS  CAS  Article  Google Scholar 

  17. 17

    Newton-McGee, K. J. Radio Polarimetry as a Probe of Interstellar Magnetism 83–87. Ph.D. thesis, Univ. Sydney. (2009)

    Google Scholar 

  18. 18

    Ransom, R. R., Uyanıker, B., Kothes, R. & Landecker, T. L. Probing the magnetized interstellar medium surrounding the planetary nebula Sh 2–216. Astrophys. J. 684, 1009–1017 (2008)

    ADS  Article  Google Scholar 

  19. 19

    Gaustad, J. E., McCullough, P. R., Rosing, W. & Van Buren, D. A robotic wide-angle Hα survey of the southern sky. Publ. Astron. Soc. Pacif. 113, 1326–1348 (2001)

    ADS  Article  Google Scholar 

  20. 20

    Parker, Q. A. et al. The AAO/UKST SuperCOSMOS Hα survey. Mon. Not. R. Astron. Soc. 362, 689–710 (2005)

    ADS  CAS  Article  Google Scholar 

  21. 21

    Beresnyak, A., Lazarian, A. & Cho, J. Density scaling and anisotropy in supersonic magnetohydrodynamic turbulence. Astrophys. J. 624, L93–L96 (2005)

    ADS  Article  Google Scholar 

  22. 22

    Lemaster, M. N. & Stone, J. M. Dissipation and heating in supersonic hydrodynamic and MHD turbulence. Astrophys. J. 691, 1092–1108 (2009)

    ADS  Article  Google Scholar 

  23. 23

    Eyink, G. L. Besov spaces and the multifractal hypothesis. J. Stat. Phys. 78, 353–375 (1995)

    ADS  MathSciNet  Article  Google Scholar 

  24. 24

    Burkhart, B., Falceta-Gonçalves, D., Kowal, G. & Lazarian, A. Density studies of MHD interstellar turbulence: statistical moments, correlations and bispectrum. Astrophys. J. 693, 250–266 (2009)

    ADS  Article  Google Scholar 

  25. 25

    Whitley, D. A genetic algorithm tutorial. Stat. Comput. 4, 65–85 (1994)

    Article  Google Scholar 

  26. 26

    Hill, A. S. et al. The turbulent warm ionized medium: emission measure distribution and MHD simulations. Astrophys. J. 686, 363–378 (2008)

    ADS  CAS  Article  Google Scholar 

  27. 27

    Burkhart, B., Stanimirovic´, S., Lazarian, A. & Kowal, G. Characterizing magnetohydrodynamic turbulence in the Small Magellanic Cloud. Astrophys. J. 708, 1204–1220 (2010)

    ADS  Article  Google Scholar 

  28. 28

    Gaensler, B. M., Madsen, G. J., Chatterjee, S. & Mao, S. A. The vertical structure of warm ionised gas in the Milky Way. Publ. Astron. Soc. Aust. 25, 184–200 (2008)

    ADS  CAS  Article  Google Scholar 

  29. 29

    Haverkorn, M., Gaensler, B. M., McClure-Griffiths, N. M., Dickey, J. M. & Green, A. J. The Southern Galactic Plane Survey: polarized radio continuum observations and analysis. Astrophys. J. 167 (suppl.). 230–238 (2006)

    ADS  CAS  Article  Google Scholar 

  30. 30

    Cho, J. & Lazarian, A. Compressible magnetohydrodynamic turbulence: mode coupling, scaling relations, anisotropy, viscosity-damped regime and astrophysical implications. Mon. Not. R. Astron. Soc. 345, 325–339 (2003)

    ADS  Article  Google Scholar 

Download references

Acknowledgements

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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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). https://doi.org/10.1038/nature10446

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