Context : Galaxy metallicities have been measured to redshift z \sim 2 by gas-phase oxygen abundances of the interstellar medium using the R _ { 23 } and N2 methods .
Galaxy stellar metallicities provide crucial data for chemical evolution models but have not been assessed reliably much outside the local Universe .

Aims : We determine the iron-abundance , stellar metallicity of star-forming galaxies at redshift z \sim 2 , homogeneously-selected and observed as part of the Galaxy Mass Assembly ultra-deep Spectroscopic Survey ( GMASS ) .

Methods : We compute the equivalent width ( EW ) of a rest-frame mid-ultraviolet ( mid-UV ) , photospheric absorption-line index , the 1978 Å index , found to vary monotonically with stellar metallicity by Rix , Pettini and collaborators ( R04 ) , in model star-forming galaxy ( SFG ) spectra created using the theoretical massive star models of Pauldrach and coworkers , and the evolutionary population synthesis code Starburst99 .
The 1978 Å index is sensitive to Fe III transitions and measures the iron-abundance , stellar metallicity .
To accurately determine the 1978 Å index EW , we normalise and combine 75 SFG spectra from the GMASS survey to produce a spectrum corresponding to a total integration time 1652.5 hours ( and a signal-to-noise ratio \sim 100 for our 1.5 Å binning ) of FORS2 spectroscopic observations at the Very Large Telescope .

Results : We measure a iron-abundance , stellar metallicity of log ( Z / Z _ { \odot } ) = -0.574 \pm 0.159 for our spectrum representative of a galaxy of stellar mass 9.4 \times 10 ^ { 9 } M _ { \odot } assuming a Chabrier initial mass function ( IMF ) .
We find that the R04 model SFG spectrum for log ( Z / Z _ { \odot } ) = -0.699 solar metallicity provides the best description of our GMASS coadded spectrum .
For similar galaxy stellar mass , our stellar metallicity is \sim 0.25 dex lower than the oxygen-abundance , gas-phase metallicity quantified by Erb and collaborators ( E06 ) for UV-selected star-forming galaxies at z = 2 .

Conclusions : We measure the iron-abundance , stellar metallicity of star-forming galaxies at redshift z \sim 2 by analysing the 1978 Å index in a spectrum created by combining 75 galaxy spectra from the GMASS survey .
We find that our measurement is \sim 0.25 dex lower than the oxygen-abundance gas-phase metallicity at similar values of galaxy stellar mass .
We conclude that we are witnessing the establishment of a light-element overabundance in galaxies as they are being formed at redshift z \sim 2 .
Our measurements are indeed reminiscent of the \alpha -element enhancement seen in the likely progenitors of these starburst galaxies at low-redshift , i.e .
galactic bulges and early-type galaxies .