The Origin and Optical Depth of Ionizing Photons in the Green Pea
  Galaxies
  Our understanding of radiative feedback and star formation in galaxies at
high redshift is hindered by the rarity of similar systems at low redshift.
However, the recently identified Green Pea (GP) galaxies are similar to
high-redshift galaxies in their morphologies and star formation rates and are
vital tools for probing the generation and transmission of ionizing photons.
The GPs contain massive star clusters that emit copious amounts of high-energy
radiation, as indicated by intense [OIII] 5007 emission and HeII 4686 emission.
We focus on six GP galaxies with high ratios of [O III] 5007,4959/[O II] 3727
~10 or more. Such high ratios indicate gas with a high ionization parameter or
a low optical depth. The GP line ratios and ages point to chemically
homogeneous massive stars, Wolf-Rayet stars, or shock ionization as the most
likely sources of the He II emission. Models including shock ionization suggest
that the GPs may have low optical depths, consistent with a scenario in which
ionizing photons escape along passageways created by recent supernovae. The GPs
and similar galaxies can shed new light on cosmic reionization by revealing how
ionizing photons propagate from massive star clusters to the intergalactic
medium.
