FAR UV IMAGING OF M-81 AND COMPARISON TO OTHER SPIRAL TRACERS

We analyze a digitized UV image of M 81 at 2000 angstrom and compare it with observations in the B and I bands, in Halpha, HI, and at radio wavelengths of 6 cm and 20 cm. The UV morphology shows several important features: 1) an unresolved (R < 10") nuclear source surrounded by a small, bright oval distortion of radius approximately almost-equal-to 50" (800 pc) powered by bulge stars, 2) a prominent, slightly elliptical ring at R almost-equal-to 250" (4 kpc) as previously detected on HI images, and 3) two bright spiral arms which emerge tangentially from the ring but appear significantly distorted near R = 400" (6 kpc). SN 1993J apparently exploded in a star cluster marked by a minor UV peak on the 4 kpc ring. Comparison of the axisymmetric disk in the UV, B and I bands shows a continuous increase of the disk scale length factor alpha-1 when going toward the shorter wavelengths. There is a significant colour gradient in (UV-I) and in (UV-I)0 between R = 300" - 600". We use the extinction-corrected UV flux along the spiral arms as an index of the SFR and compare it with free-free radio emission and Halpha emission. All three tracers of star formation have a primary maximum at R = 325", a minimum at R = 400" and a secondary maximum at R = 475". The massive star formation rate per unit mass of gas is significantly higher at the primary maximum than elsewhere in the galaxy. Beyond the primary maximum the SFR/M(gas) ratio is fairly constant along the arms until it drops in the outer part of M 81. The relation between massive star formation and the amplitude of the linear density wave is studied by comparing arm/interarm constrasts as a function of galactic radius for the UV, B and I bands and for the nonthermal 20 cm radio continuum. In arm contrasts, all the tracers show two maxima separated by a minimum at r almost-equal-to 6.3 kpc. The primary star formation maximum coincides with a stellar density-wave maximum but additional star-forming mechanisms (orbit crowding, cloud collisions) are needed to explain the increase of the amplitude of the primary maximum relative to the secondary maximum when going towards tracers of younger stars. In the CO-poor galaxy M 81, the average massive star- formation rate per unit mass of gas in only a factor 2-3 smaller than in the CO-rich galaxy M 51. The nonthermal radio emission from the spiral arms does not simply follow the density wave amplitude but requires an input of cosmic ray electrons from Population I objects.