Ultraviolet Colors and Extinctions of HiiRegions in the Whirlpool Galaxy (M51)

Far-UV (wavelength 1520 Å), U , Hα, and R images of the interacting Sbc spiral galaxy M51 were obtained by the Ultraviolet Imaging Telescope (UIT) during the Astro-2 Spacelab mission of 1995 March and at Mount Laguna Observatory. The μ 152 - μ U radial gradient of over 1 mag, becoming bluer with increasing radius, is attributed primarily to a corresponding radial extinction gradient. Magnitudes in both UV bands and Hα fluxes are reported for 28 H II regions. Optical extinctions for the 28 corresponding UV sources are computed from the measured m 152 - U colors by fitting to the optical extinctions of Nakai & Kuno. The estimated normalized far-UV extinction A 152 / E ( B - V ) increases with increasing Galactocentric distance (decreasing metallicity), from 5.99 to 6.54, compared with the Galactic value 8.33. The best-fit m 152 - U color for no extinction, -3.07, is the color of a model solar metallicity starburst of age ~2.5 Myr with IMF slope -1.0. H II regions show decreasing observed Hα fluxes with decreasing radius, relative to the Hα fluxes predicted from the observed f 152 for age 2.5 Myr, after the Hα and f 152 are corrected for extinction. We attribute the increasing fraction of "missing" Hα flux with decreasing radius to increasing extinction in the Lyman continuum. The increasing extinction-corrected far-UV flux of the H II regions with decreasing distance to the nucleus is probably a result of the corresponding increasing column density of the interstellar gas resulting in larger mass OB associations. The estimated dust-absorbed Lyman continuum energy flux is ~0.6 times the far-infrared energy flux of M51 observed by IRAS .