Far-UV ( wavelength 1520 Å ) , 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 Mt .
Laguna Observatory .
The \mu _ { 152 } - \mu _ { U } radial gradient of over a magnitude , 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 \markcite nakaia ( 1995 ) .
The normalized far-UV extinction A _ { 152 } / E ( B - V ) increases with increasing galactocentric distance or 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 \sim 2.5 Myr with IMF slope -1.0 .
HII 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 ” H \alpha 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 flux is \sim 0.6 times the far-infrared energy flux of M51 observed by IRAS .

\keywords galaxies : individual : M51 , HII Regions , ultraviolet : general