The arc and other structures in the center of M81

Multi-configuration VLA lambda 6 cm and lambda 20 cm radio continuum observations of the extended emission surrounding the mini-Seyfert nucleus of M81 are presented and compared with optical emission-line images 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 counterpart, is connected by three bridges to the extended emission of the central source. The two outer bridges give the are the appearance of a giant loop coming off the central source; the middle bridge has a nonthermal spectral index. In the are, the magnetic field perpendicular to the 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 is governed by magnetic and cosmic-ray pressures. It may be a large-scale version of the Galactic Center Lobe in the center of the Milky Way or a small-scale version of the nuclear radio lobes extending perpendicular to the disk in certain edge-on Seyfert galaxies. The middle bridge and a nonthermal ''little jet'' (2.5''=44 pc long) at the nucleus line up at a position angle of similar to 30 degrees and could be supplying cosmic ray-electrons to the are. In addition, two oval structures are detected in the radio continuum. The larger one coincides with the near-infrared mini-bar, The smaller one (possibly an inclined gas disk) has a different position angle and no near-infrared counterpart. A prominent radio continuum filament extends west from the northern major axis of the smaller oval but does not correspond to either an Her filament or an obvious dust lane. (C) 1996 American Astronomical Society.