First direct metallicity measurement of a lensed star-forming galaxy at z = 1.7

We present the rest-frame optical spectrum of a strongly lensed galaxy at redshift z = 1.7 behind the cluster Abell 1689. We detect the temperature sensitive auroral line [OIII] λ4363, which allows the first direct metallicity measurement for galaxies at z > 1. Our high signal-to-noise spectrum indicates that the target is an extremely low metallicity star-forming galaxy, with 12+log(O/H)=7.5^+0.1 _-0.2 from the direct T_e -based method. We estimate an intrinsic absolute B-band magnitude of M_B = -18.3 ± 0.1, with a stellar mass of 4.4 ± 1.2×10⁸ M _⊙. This galaxy extends the luminosity-metallicity relation of star-forming galaxies at z > 2 by more than an order of magnitude. Given the double nuclei-like morphology and the velocity profile of Hα, we tentatively suggest that it could be a merger or a proto-rotating disk galaxy.