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

Far-UV (1520 ang.), U, H-alpha, and R images of the interacting Sbc spiralgalaxy M51 were obtained by the Ultraviolet Imaging Telescope (UIT) and at Mt.Laguna Observatory. The mu(152)-mu(U) radial gradient of >1 mag, becoming bluerwith increasing radius, is attributed primarily to a corresponding radialextinction gradient. Magnitudes in both UV bands and H-alpha fluxes arereported for 28 HII regions. Optical extinctions for the 28 corresponding UVsources are computed from the measured m(152)-U colors by fitting to theoptical extinctions of Nakai and Kuno (1995). The normalized far-UV extinctionA(152)/E(B-V) increases with radius or decreasing metallicity, from 5.99 to6.54, compared with the Galactic value 8.33. The best-fit m(152)-U color for noextinction, -3.07, is the color of a model solar metallicity starburst of age~2.5 Myr with IMF slope -1.0. HII regions show decreasing observed H-alphafluxes with decreasing radius, relative to the H-alpha fluxes predicted fromthe observed f(152) for age 2.5 Myr, after correction for extinction. Weattribute the increasing fraction of ``missing'' H-alpha flux with decreasingradius to increasing extinction in the Lyman continuum. Increasingextinction-corrected far-UV flux of the HII regions with decreasing radius isprobably a result of the corresponding increasing column density of theinterstellar gas resulting in larger mass OB associations. The estimateddust-absorbed Lyman continuum flux is ~0.6 times the far-infrared energy fluxof M51 observed by IRAS.Comment: 21 pages, 6 figures, latex, uses AASMS4 and EPS