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Star-forming galaxies are considered to be the leading candidate sources thatdominate the cosmic reionization at z>7, and the search for analogs at moderateredshift showing Lyman continuum (LyC) leakage is currently a active line ofresearch. We have observed a star-forming galaxy at z=3.2 with Hubble/WFC3 inthe F336W filter, corresponding to the 730-890A rest-frame, and detect LyCemission. This galaxy is very compact and also has large Oxygen ratio[OIII]5007/[OII]3727 (>=10). No nuclear activity is revealed fromoptical/near-infrared spectroscopy and deep multi-band photometry (includingthe 6Ms X-ray, Chandra). The measured escape fraction of ionizing radiationspans the range 50-100\%, depending on the IGM attenuation. The LyC emission isdetected at S/N=10 with m(F336W)=27.57+/-0.11 and it is spatially unresolved,with effective radius R_e<200pc. Predictions from photoionization and radiativetransfer models are in line with the properties reported here, indicating thatstellar winds and supernova explosions in a nucleated star-forming region canblow cavities generating density-bounded conditions compatible with opticallythin media. Irrespective to the nature of the ionizing radiation, spectralsignatures of these sources over the entire electromagnetic spectrum are ofcentral importance for their identification during the epoch of reionization,when the LyC is unobservable. Intriguingly, the Spitzer/IRAC photometricsignature of intense rest-frame optical emissions ([OIII]+Hbeta) observedrecently at z~7.5-8.5 is similar to what is observed in this galaxy. Only theJames Webb Space Telescope will measure optical line ratios at z>7 allowing adirect comparison with lower redshift LyC emitters, as reported here.

Hubble imaging of the ionizing radiation from a star-forming galaxy at z=3.2 with fesc>50%