We present rest-frame optical spectra for a sample of 9 low-mass star-forming galaxies in the redshift range 1.5 < z < 3 which are gravitationally lensed by foreground clusters .
We used Triplespec , an echelle spectrograph at the Palomar 200-inch telescope that is very effective for this purpose as it samples the entire near-infrared spectrum simultaneously .
By measuring the flux of nebular emission lines we derive gas phase metallicities and star formation rates , and by fitting the optical to infrared spectral energy distributions we obtain stellar masses .
Taking advantage of the high magnification due to strong lensing we are able to probe the physical properties of galaxies with stellar masses in the range 7.8 < \log M / M _ { \odot } < 9.4 whose star formation rates are similar to those of typical star-forming galaxies in the local universe .
We compare our results with the locally determined relation between stellar mass , gas metallicity and star formation rate .
Our data are in excellent agreement with this relation , with an average offset \langle \Delta \log ( \mathrm { O / H } ) \rangle = 0.01 \pm 0.08 , suggesting a universal relationship .
Remarkably , the scatter around the fundamental metallicity relation is only 0.24 dex , smaller than that observed locally at the same stellar masses , which may provide an important additional constraint for galaxy evolution models .