| Abstract
| - Aims. Cosmic reionization is an important process occurring in the early epochs of the Universe. However, because of observational limitations due to the opacity of the intergalactic medium to Lyman continuum photons, the nature of ionizing sources is still not well constrained. While high-redshift star-forming galaxies are thought to be the main contributors to the ionizing background at z> 6 , it is impossible to directly detect their ionizing emission. Therefore, looking at intermediate redshift analogues ( z ~ 2−4 ) can provide useful hints about cosmic reionization. Methods. We investigate the physical properties of one of the best Lyman continuum emitter candidate at z = 3.212 found in the GOODS-S/CANDELS field with photometric coverage from the U to the MIPS 24 μm band and VIMOS/VLT and MOSFIRE/Keck spectroscopy. These observations allow us to derive physical properties such as stellar mass, star formation rate, age of the stellar population, dust attenuation, metallicity, and ionization parameter, and to determine how these parameters are related to the Lyman continuum emission. Results. Investigation of the UV spectrum confirms a direct spectroscopic detection of the Lyman continuum emission with S/ N> 5 . Non-zero Ly α flux at the systemic redshift and high Lyman- α escape fraction ( fesc(Ly α) ≥ 0.78 ) suggest a low H i column density. The weak C and Si low-ionization absorption lines are also consistent with a low covering fraction along the line of sight. The subsolar abundances are consistent with a young and extreme starburst. The [O iii] λλ4959,5007 +H β equivalent width (EW) is one of the largest reported for a galaxy at z> 3 ( EW( [ O iii ] λλ4959,5007 + H β) ≃ 1600 Å , rest-frame; 6700 Å observed-frame) and the near-infrared spectrum shows that this is mainly due to an extremely strong [O iii] emission. The large observed [O iii]/[O ii] ratio ( >10) and high ionization parameter are consistent with prediction from photoionization models in the case of a density-bounded nebula scenario. Furthermore, the EW([O iii] λλ4959,5007 +H β) is comparable to recent measurements reported at z ~ 7−9 , in the reionization epoch. We also investigate the possibility of an AGN contribution to explain the ionizing emission but most of the AGN identification diagnostics suggest that stellar emission dominates instead. Conclusions. This source is currently the first high- z example of a Lyman continuum emitter exhibiting indirect and direct evidences of a Lyman continuum leakage and having physical properties consistent with theoretical expectation from Lyman continuum emission from a density-bounded nebula. A low H i column density, low covering fraction, compact star formation activity, and a possible interaction/merging of two systems may contribute to the Lyman continuum photon leakage.
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