Ultraviolet Colors and Extinctions of H ii Regions in the Whirlpool Galaxy (M51)

Abstract

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. SPACE TELESCOPE SCI INST,BALTIMORE,MD 21218. CALIF STATE UNIV FULLERTON,DEPT PHYS,FULLERTON,CA 92634. UNIV VIRGINIA,CHARLOTTESVILLE,VA 22903. NATL RADIO ASTRON OBSERV,CHARLOTTESVILLE,VA 22903. CALIF STATE UNIV LONG BEACH,DEPT PHYS & ASTRON,LONG BEACH,CA 90840.