A NONTHERMAL RADIO HALO SURROUNDING M82

We report radio observations of the starburst galaxy M82 at several wavelengths between 90 and 6 cm. These observations reveal an extensive synchrotron-emitting halo which extends to a diameter of at least 8 kpc. The halo is interpreted in terms of synchrotron-emitting electrons swept outward from the nuclear region by the outflow probably associated with supernova activity. The brightness distribution of the halo is asymmetric, which could be related to either a corresponding asymmetry in the distribution of SNR with respect to the disk, or to the influence of the IGM through which M82 is moving. The spectral index of the emissions is -0.4 +/- 0.1 in the nucleus, steepening to about -1.0 at a radius of 1 kpc along the minor axis. We interpret this result in terms of inverse Compton scattering of the relativistic electrons against IR photons emitted by the nuclear region. There is insufficient power radiated by this mechanism however to contribute significantly to the X-ray halo seen in this galaxy. The wind speed estimate from the inverse Compton argument falls in the range 1000-3000 km s-1. The relativistic particle energy transported outward by the wind is estimated to be approximately 10(40) ergs s-1, or about 2% of the total energy transported in the wind. This result is consistent with the corresponding ratio in SNR, as expected if the wind and relativistic particles originate with the supernova activity in the nucleus.