Ultraviolet Colors and Extinctions of H II Regions 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 μ152 - μU 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 m152 - U colors by fitting to the optical extinctions of Nakai & Kuno. The estimated normalized far-UV extinction A152/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 m152 - 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 f152 for age 2.5 Myr, after the Hα and f152 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.