The ionization structure of a z=2.37 lensed galaxy that spews ionizing photons

We reveal the ionization conditions within a spectacular gravitationally lensed galaxy, the Sunburst Arc at redshift z=2.37, which is the best example of a distant galaxy that leaks ionizing photons. How ionizing photons escape from galaxies is poorly understood, but it must have happened in order to re-ionize the Universe at high redshift. Narrow-band imagery and grism spectroscopy from the Hubble Space Telescope reveals, thanks to the magnification from gravitational lensing, the ionization state of the gas at spatial scales (100pc) that are normally impossible to access for distant galaxies. Due to spectacular good luck, each of the [O II], [O III], and Hbeta rest-frame optical emission lines fall into narrow-band filters on the WFC3-IR instrument. We analyze the spatially resolved R23 and O32 diagnostics for 20 discrete spatial regions within this single galaxy, as well as the spatially resolved [Ne III]/[O II] ratio. We determine the physical conditions that of the region that leaks ionizing photons, and contrast with the rest of the galaxy, which does not.