{
  "id": "astro-ph/9606168",
  "version": "v1",
  "published": "1996-06-27T18:46:46.000Z",
  "updated": "1996-06-27T18:46:46.000Z",
  "title": "UV Colors and Extinctions of HII Regions in the Whirlpool Galaxy (M51)",
  "authors": [
    "J. K. Hill",
    "W. H. Waller",
    "R. H. Cornett",
    "R. C. Bohlin",
    "K. -P. Cheng",
    "S. G. Neff",
    "R. W. O'Connell",
    "M. S. Roberts",
    "A. M. Smith",
    "P. M. N. Hintzen",
    "E. P. Smith",
    "T. P. Stecher"
  ],
  "comment": "21 pages, 6 figures, latex, uses AASMS4 and EPSF",
  "doi": "10.1086/303753",
  "categories": [
    "astro-ph"
  ],
  "abstract": "Far-UV (1520 ang.), U, H-alpha, and R images of the interacting Sbc spiral galaxy 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 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 HII 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 and Kuno (1995). The normalized far-UV extinction A(152)/E(B-V) increases with radius 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 ~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 correction for extinction. We attribute the increasing fraction of ``missing'' H-alpha flux with decreasing radius to increasing extinction in the Lyman continuum. Increasing extinction-corrected far-UV flux of the HII regions with decreasing radius 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 ~0.6 times the far-infrared energy flux of M51 observed by IRAS.",
  "revisions": [
    {
      "version": "v1",
      "updated": "1996-06-27T18:46:46.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "hii regions",
      "extinction",
      "whirlpool galaxy",
      "uv colors",
      "dust-absorbed lyman continuum flux"
    ],
    "tags": [
      "journal article"
    ],
    "note": {
      "typesetting": "LaTeX",
      "pages": 21,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}