A Multiwavelength Study on the Fate of Ionizing Radiation in Local Starbursts

TL;DR

This study investigates how ionizing radiation escapes from local starburst and non-starburst galaxies, finding no significant differences in escape fractions or dust absorption, highlighting the complexity of radiation transfer in these environments.

Contribution

It provides a comparative analysis of the spectral energy distributions of starburst and non-starburst galaxies, revealing that variations are larger than systematic differences, and challenges assumptions about ionizing radiation escape.

Findings

No significant difference in ionizing radiation absorption between starbursts and non-starbursts.

Dust in starburst galaxies is hotter than in non-starburst galaxies.

Starburst galaxies do not show higher ultraviolet flux indicating escape of ionizing photons.

Abstract

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 of a sample of local star-forming galaxies, containing thirteen local starburst galaxies and ten 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 is 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 24um, 70um, and 160um 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 GALEX bands that could indicate a high escape fraction of ionizing photons in starburst galaxies. The small H-alpha fractions of the diffuse, warm ionized medium in starburst galaxies are apparently due to temporarily boosted H-alpha 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.