Lyman Continuum Extinction by Dust in H II Regions of Galaxies

We examine Lyman continuum extinction (LCE) in H II regions by comparing infrared fluxes of 49 H II regions in the Galaxy, M31, M33, and the Large Megellanic Cloud with estimated production rates of Lyman continuum photons. A typical fraction of Lyman continuum photons that contribute to hydrogen ionization in the H II regions of three spiral galaxies is ≲50%. The fraction may become smaller as the metallicity (or dust-to-gas ratio) increases. We examine the LCE effect on estimated star formation rates of galaxies. The correction factor for the Galactic dust-to-gas ratio is 2–5.