HUBBLE IMAGING OF THE IONIZING RADIATION FROM A STAR-FORMING GALAXY AT Z = 3.2 WITH f esc > 50%

Star-forming galaxies are considered to be the leading candidate sources dominating cosmic reionization at z > 7: the search for analogs at moderate redshift showing Lyman continuum (LyC) leakage is currently an active line of research. We have observed a star-forming galaxy at z = 3.2 with Hubble/WFC3 in the F336W filter, corresponding to the 730-890 Å rest-frame, and detected LyC emission. This galaxy is very compact and also has a large Oxygen ratio [O III]λ5007/[O II]λ3727 (≳10). No nuclear activity is revealed from optical/near-infrared spectroscopy and deep multi-band photometry (including the 6Ms X-ray Chandra observations). The measured escape fraction of ionizing radiation spans the range 50%-100%, depending on the intergalactic medium (IGM) attenuation. The LyC emission is measured at mF336W = 27.57 ± 0.11 (with signal-to-noise ratio (S/N) = 10) and is spatially unresolved, with an effective radius of Re < 200 pc. Predictions from photoionization and radiative transfer models are in line with the properties reported here, indicating that stellar winds and supernova explosions in a nucleated star-forming region can blow cavities generating density-bounded conditions compatible to optically thin media. Irrespective of the nature of the ionizing radiation, spectral signatures of these sources over the entire electromagnetic spectrum are of central importance for their identification during the epoch of reionization when the LyC is unobservable. Intriguingly, the Spitzer/IRAC photometric signature of intense rest-frame optical emissions ([O III]λλ4959,5007 + Hβ) recently observed at z ≃ 7.5-8.5 is similar to what is observed in this galaxy. Only the James Webb Space Telescope will measure optical line ratios at z > 7, allowing a direct comparison with the lower-redshift LyC emitters, such as that reported here.