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 [O III] 4363, 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.We estimate
an intrinsic absolute B band magnitude of M_{B}=-18.3 \pm 0.1$, with a stellar
mass of 4.4$\pm1.2\times10^{8}$ M$_{\odot}$. 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 velocity
profile of \ha, we tentatively suggest that it could be a merger or a
proto-rotating disk galaxy.
