ULTRAVIOLET IMAGING TELESCOPE ULTRAVIOLET IMAGES - LARGE-SCALE STRUCTURE, H-II REGIONS, AND EXTINCTION IN M81

Ultraviolet images of M81 obtained by the Ultraviolet Imaging Telescope during the 1990 December Astro-1 spacelab mission allow determination of 2490 and 1520 angstrom fluxes for 46 H II regions and global surface brightness profiles. Comparison photometry in the V band is obtained from a ground-based CCD image. Ultraviolet radial profiles show bulge and exponential disk components, with a local decrease in disk surface brightness inside the inner Lindblad Resonance approximately 4' from the nucleus. The V profile shows typical bulge plus exponential disk structure, with no local maximum in the disk. There is little change of UV color across the disk, although there is a strong gradient in the bulge. The ratio of the near-UV surface brightness to the V band surface brightness in the disk is a factor approximately 4 lower for M81 than for M51. Total near-UV and far-UV UIT fluxes for M81 are consistent with previous measurements by O AO 2. The flux from the bulge is estimated to be 53% of the total in the V band, 31% in the near-UV, and 5% in the far-UV. Observed M152 - V colors of the H II regions are consistent with model spectra for young clusters, after dereddening using A(v) determined from m249 - V and the Galactic extinction curve. The value of A(v), so determined, is 0.4 mag greater on the average than A(v) derived from radio continuum and H-alpha fluxes. This difference is within the error estimates of the ground-based extinction determinations. The observed colors are unlikely to be caused by larger ages than the 3 Myr assumed by the model, since the Lyman continuum flux of a cluster is a strongly decreasing function of age.