2010ApJ...725.2029H

The fate of ionizing radiation is vital for understanding cosmic ionization, energy budgets in the interstellar and intergalactic medium, and star formation rate indicators. The low observed escape fractions of ionizing radiation have not been adequately explained, and there is evidence that some starbursts have high escape fractions. We examine the spectral energy distributions (SEDs) of a sample of local star-forming galaxies, containing 13 local starburst galaxies and 10 of their ordinary star-forming counterparts, to determine if there exist significant differences in the fate of ionizing radiation in these galaxies. We find that the galaxy-to-galaxy variations in the SEDs are much larger than any systematic differences between starbursts and non-starbursts. For example, we find no significant differences in the total absorption of ionizing radiation by dust, traced by the 24 µm, 70 µm, and 160 µm MIPS bands of the Spitzer Space Telescope, although the dust in starburst galaxies appears to be hotter than that of non-starburst galaxies. We also observe no excess ultraviolet flux in the Galaxy Evolution Explorer bands that could indicate a high escape fraction of ionizing photons in starburst galaxies. The small Hα fractions of the diffuse, warm ionized medium (WIM) in starburst galaxies are apparently due to temporarily boosted Hα luminosity within the star-forming regions themselves, with an independent, constant WIM luminosity. This independence of the WIM and starburst luminosities contrasts with WIM behavior in non-starburst galaxies and underscores our poor understanding of radiation transfer in both ordinary and starburst galaxies.