Ultraviolet colors and extinctions of H II regions in the whirlpool galaxy (M51)

Far-UV (wavelength 1520 Angstrom), U, H alpha, 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 mu(152) - mu(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 H alpha 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 similar to 2.5 Myr with IMF slope -1.0. H rr regions show decreasing observed H alpha fluxes with decreasing radius, relative to the H alpha fluxes predicted from the observed f(152) for age 2.5 Myr, after the H alpha and f(152) are corrected for extinction. We attribute the increasing fraction of ''missing'' Ha 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 similar to 0.6 times the far-infrared energy flux of M51 observed by IRAS.