A large-scale, interstellar Faraday-rotation feature of unknown origin

Abstract

The disk of the Milky Way contains free electrons and magnetic fields which contribute significantly to the energetics of the interstellar medium1. The concentrations of electrons and magnetic fields are too low to be detected by direct methods, but may be investigated using Faraday rotation, a wavelength-dependent shift in linear polarization angle induced by a magneto-ionic medium2. Structures in polarization angle arising from Faraday rotation have been detected recently at long radio wavelengths3. These structures are disorganized and filamentary, probably arising from interstellar gas in the vicinity of the Sun. Here we report a more distant, highly ordered Faraday-rotation structure of elliptical shape, with its long axis parallel to the plane of the Galaxy. The feature appears to be located in an inter-arm region of the Milky Way, between the spiral arm containing the Sun and the next outer (Perseus) spiral arm. Within the elliptical region, small-scale structure which characterizes the turbulence seen in adjacent regions of the interstellar medium is absent. The origin of this magneto-ionic feature is uncertain, but it must arise from an organization of the magnetic-field and electron-density distributions on a scale of the order of 50 parsecs (165 light years).

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Figure 1: Grey-scale image of polarization angle made from DRAO data smoothed to 2′.
Figure 2: Grey-scale images of Stokes Q (top) and U (bottom), smoothed to 2′, corresponding to the area shown in Fig. 1.

References

  1. 1

    Jura, M. in Interstellar Processes (eds Hollenbach, D. J. & Thronson, H. A. Jr) 3–17 (Reidel, Dordrecht, (1987)).

    Google Scholar 

  2. 2

    Lang, K. R. Astrophysical Formulae (Springer, Berlin, (1974)).

    Google Scholar 

  3. 3

    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 

  4. 4

    Normandeau, M., Taylor, A. R. & Dewdney, P. E. The Dominion Radio Astrophysical Observatory Galactic Plane Survey pilot project: the W3/W4/W5/HB3 region. Astrophys. J. Suppl. 108, 279–299 (1997).

    ADS  Article  Google Scholar 

  5. 5

    Smegal, R. J., Landecker, T. L., Vaneldik, J. F., Routledge, D. & Dewdney, P. E. Aperture synthesis polarimetry: application to the Dominion Radio Astrophysical Observatory synthesis telescope. Radio Sci. 32, 643–656 (1997).

    ADS  Article  Google Scholar 

  6. 6

    Salter, C. J. & Brown, R. L. in Galactic and Extragalactic Radio Astronomy 2nd edn (eds Verschuur, G. L. & Kellermann, K. I.) 1–36 (Springer, New York, (1988)).

    Google Scholar 

  7. 7

    Westerhout, G. Asurvey of the continuous radiation from the Galactic system at a frequency of 1390 Mc/s. Bull. Astron. Inst. Neth. 14, 215–260 (1958).

    ADS  Google Scholar 

  8. 8

    Gardner, F. F. & Whiteoak, J. B. Polarization of cosmic radio waves. Annu. Rev. Astron. Astrophys. 4, 245–292 (1966).

    ADS  Article  Google Scholar 

  9. 9

    Braunsfurth, E. Neutral hydrogen in the Cas OB6 association. Astron. Astrophys. 117, 297–303 (1983).

    ADS  CAS  Google Scholar 

  10. 10

    Broten, N. W., McLeod, J. M. & Vallée, J. P. Catalogue of unambiguous (Faraday-thin, one-component, spectrum-selected) rotation measures for galaxies and quasars. Astrophys. Space Sci. 141, 303–331 (1988).

    ADS  Article  Google Scholar 

  11. 11

    Vallée, J. P. in Fundamentals of Cosmic Physics Vol. 19, 1–89 (Gordon & Breach, Amsterdam, (1988)).

    Google Scholar 

  12. 12

    Taylor, J. H. & Cordes, J. M. Pulsar distances and the Galactic distribution of free electrons. Astrophys. J. 411, 674–684 (1993).

    ADS  Article  Google Scholar 

  13. 13

    Rand, R. J. & Kulkarni, S. R. The local Galactic magnetic field. Astrophys. J. 343, 760–772 (1989).

    ADS  Article  Google Scholar 

  14. 14

    Beck, R. & Hoernes, P. Magnetic spiral arms in the galaxy NGC6946. Nature 379, 47–49 (1996).

    ADS  CAS  Article  Google Scholar 

  15. 15

    Fan, Z. & Lou, Y.-Q. Origin of the magnetic spiral arms in the galaxy NGC4946. Nature 383, 800–802 (1996).

    ADS  CAS  Article  Google Scholar 

  16. 16

    Wilkinson, A. & Smith, F. G. Characteristics of the local Galactic magnetic field determined from background polarization surveys. Mon. Not. R. Astron. Soc. 167, 593–611 (1974).

    ADS  Article  Google Scholar 

  17. 17

    Beuermann, K., Kanbach, G. & Berkhuijsen, E. M. Radio structure of the Galaxy: thick disk and thin disk at 408 MHz. Astron. Astrophys. 153, 17–34 (1985).

    ADS  Google Scholar 

  18. 18

    Digel, S. W., Lyder, D. A., Philbrick, A. J., Puche, D. & Thaddeus, P. Alarge-scale CO survey towards W3, W4, and W5. Astrophys. J. 458, 561–575 (1996).

    ADS  Article  Google Scholar 

  19. 19

    Leahy, J. P. Small-scale variations in Galactic Faraday rotation. Mon. Not. R. Astron. Soc. 226, 433–446 (1987).

    ADS  Article  Google Scholar 

  20. 20

    Brouw, W. N. & Spoelstra, T. A. Th. Linear polarization of the Galactic radio emission at frequencies between 408 and 1411 MHz. Reductions. Astron. Astrophys. Suppl. Ser. 26, 129–146 (1976).

    ADS  Google Scholar 

  21. 21

    Kallas, E. & Reich, W. A21 cm radio continuum survey of the Galactic Plane between l = 93° and l = 162°. Astron. Astrophys. Suppl. Ser. 42, 227–243 (1980).

    ADS  Google Scholar 

  22. 22

    Roger, R. S. Low frequency absorption due to IC1805 and IC1848. Astrophys. J. 155, 831–840 (1969).

    ADS  Article  Google Scholar 

  23. 23

    Dubois, P. in Library and Information Services in Astronomy (eds Wilkins, G. A. & Stevens-Rayburn, S.) 146 (US Naval Observatory, Washington DC, (1988)).

    Google Scholar 

  24. 24

    Dyson, J. E. & Williams, D. A. Physics of the Interstellar Medium 173 (Wiley, New York, (1980)).

    Google Scholar 

Download references

Acknowledgements

We thank M. Normandeau (Univ. Calgary; now Univ. California at Berkeley) for the acquisition and initial processing of the data, and A. G. Willis (DRAO) for some later processing. The Dominion Radio Astrophysical Observatory is operated as a national facility by the National Research Council of Canada. The Canadian Galactic Plane Survey is a Canadian project with international partners, and is supported by the Natural Sciences and Engineering Research Council of Canada.

Author information

Affiliations

Authors

Corresponding author

Correspondence to A. D. Gray.

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Gray, A., Landecker, T., Dewdney, P. et al. A large-scale, interstellar Faraday-rotation feature of unknown origin. Nature 393, 660–662 (1998). https://doi.org/10.1038/31413

Download citation

Further reading

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.