THE ARC AND OTHER STRUCTURES IN THE CENTER OF M81

Multi-configuration VLA lambda 6 cm and lambda 20 cm radio continuum o bservations of the extended emission surrounding the mini-Seyfert nucl eus of M81 are presented and compared with optical emission-line image s and with near-IR images. The extended radio emission is dominated by a large, highly polarized, nonthermal are with a projected length of 0.9 kpc in the plane of the sky and centered 45'' (0.8 kpc) northeast of the nucleus. The radio are, which has no apparent optical counterpa rt, is connected by three bridges to the extended emission of the cent ral source. The two outer bridges give the are the appearance of a gia nt loop coming off the central source; the middle bridge has a nonther mal spectral index. In the are, the magnetic field perpendicular to th e line-of-sight is parallel to the curve of the are. Most likely, the are is highly inclined to the plane of the thermal gas and its shape i s governed by magnetic and cosmic-ray pressures. It may be a large-sca le version of the Galactic Center Lobe in the center of the Milky Way or a small-scale version of the nuclear radio lobes extending perpendi cular to the disk in certain edge-on Seyfert galaxies. The middle brid ge and a nonthermal ''little jet'' (2.5''=44 pc long) at the nucleus l ine up at a position angle of similar to 30 degrees and could be suppl ying cosmic ray-electrons to the are. In addition, two oval structures are detected in the radio continuum. The larger one coincides with th e near-infrared mini-bar, The smaller one (possibly an inclined gas di sk) has a different position angle and no near-infrared counterpart. A prominent radio continuum filament extends west from the northern maj or axis of the smaller oval but does not correspond to either an Her f ilament or an obvious dust lane. (C) 1996 American Astronomical Societ y.