First magnetic field mapping around a starburst galaxy nucleus: 
a new view of M82

Magnetic fields may play an important role in starburst galaxies, channeling the flow of gas towards the nucleus - but this hypothesis has never been directly tested. Optical polarimetry can trace preferential extinction along the long axes of magnetically aligned grains but is confused by scattering, while radio polarimetry detects synchrotron emission from electrons spiralling around field lines, but is affected by Faraday dispersion.

Recent developments in submillimetre polarimetry allow us to use thermal emission from dust grains to not only detect the aligning field, but concentrate on the dense nuclear regions of active star formation (rather than foreground dust and diffuse gas). The first target with the SCUBA Polarimeter was the M82 starburst galaxy, which lies nearby at about 3.25 Mpc and has a dusty 1 kpc-wide torus symmetrical about the nucleus. There are at least 100 stellar superclusters inside the torus, plus a powerful wind that blows grains up into the halo (Alton et al. 1999).

Figure 1: vectors (derived from 850 micron polarimetry) show the magnetic field directions, superimposed on a smoothed image of the dust emission (at 450 microns). Vectors above the 50% flux level are shown in blue, and those below in red, for clarity. The polarization percentage ranges from 0.7-9.1% and the median significance level is 6-sigma so the directions are defined to within about 5 degrees.

This image shows the polarimetry results (see also Greaves et al. 2000). The magnetic field appears quite organised, with two main components visible. In the bright torus, the vectors point towards the nucleus, and since partially ionized material flows more easily along field lines than across them, this field structure assist in channelling clouds towards the starburst nucleus. Outside the torus, the vectors mainly lie parallel to the flux contours, suggesting a bubble-shaped field with a diameter of at least 1 kpc. This has never been seen in another galaxy before, although there may be a counterpart in changes of magnetic field direction over large scales in the solar vicinity (Vallee 1997).

Many normal galaxies have poloidal fields driven by winds, or flattened fields in the plane of spiral arms. M82 shows these features in near-IR polarimetry (Jones 1998), where cold dust is traced in absorption, but the submiilimetre polarimetry traces an entirely new configuration in the dust of star-forming regions. In the future, we hope to model the magnetised wind, and also to observe more starbursts to test the universality of these phenomena.

Alton P.B., Davies J.I. & Bianchi S., 1999, A&A 343, 51
Greaves, J.S., Holland, W.S., Jenness, T. & Hawarden, T.G., 2000, Nature 404, 732
Jones, T.J., 1998, BAAS 193, abstr. 76.03
Vall'ee J.P., 1997, Fund. Cosmic Phys. 19, 1