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We investigate the gas-phase metallicity and Lyman Continuum (LyC) escapefraction of a strongly gravitationally lensed, extreme emission-line galaxy atz=3.417, J1000+0221S, recently discovered by the CANDELS team. We deriveionization and metallicity sensitive emission-line ratios from H+K bandLBT/LUCI medium resolution spectroscopy. J1000+0221S shows high ionizationconditions, as evidenced by its enhanced [OIII]/[OII] and [OIII]/Hbeta ratios.Consistently, strong-line methods based on the available line ratios suggestthat J1000+0221S is an extremely metal-poor galaxy, with a metallicity of12+log(O/H) < 7.44 (< 5% solar), placing it among the most metal-poorstar-forming galaxies at z > 3 discovered so far. In combination with its lowstellar mass (2x10^8 Msun) and high star formation rate (5 Msun/yr), themetallicity of J1000+0221S is consistent with the extrapolation to low massesof the mass-metallicity relation traced by Lyman-break galaxies at z > 3, butit is 0.55 dex lower than predicted by the fundamental metallicity relation atz < 2.5. These observations suggest the picture of a rapidly growing galaxy,possibly fed by the massive accretion of pristine gas. Additionally, deepLBT/LBC in the UGR bands are used to derive a limit to the LyC escape fraction,thus allowing us to explore for the first time the regime of sub-L* galaxies atz > 3. We find a 1sigma upper limit to the escape fraction of 23%, which adds anew observational constraint to recent theoretical models predicting thatsub-L* galaxies at high-z have high escape fractions and thus are theresponsible for the reioization of the Universe.

Evidence of very low metallicity and high ionization state in a strongly lensed, star-forming dwarf galaxy at z=3.417