Magnetic field surrounding the starburst nucleus of the galaxy M82 from polarized dust emission

Magnetic fields may play an important role in the star-formation process, e specially in the central regions of 'starburst' galaxies where star formati on is vigorous. But the field directions are very difficult to determine in the dense molecular gas out of which the stars form, so it has hitherto be en impossible to test this hypothesis. Dust grains in interstellar clouds t end to be magnetically aligned, and it is possible to determine the alignme nt direction based on the polarization of optical light due to preferential extinction along the long axes of the aligned grains(1). This technique wo rks, however, only for diffuse gas, not for the dense molecular gas. Here w e report observations of polarized thermal emission from the aligned dust g rains in the central region of M82, which directly traces(2) the magnetic f ield structure (as projected onto the plane of the sky). Organized field li nes are seen around the brightest star-forming regions, while in the dusty halo the field lines form a giant magnetic bubble possibly blown out by the galaxy's 'superwind'.