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Context: Galaxy metallicities have been measured to redshift z~2 by gas-phaseoxygen abundances of the interstellar medium using the R23 and N2 methods.Galaxy stellar metallicities provide crucial data for chemical evolution modelsbut have not been assessed reliably much outside the local Universe. Aims: Wedetermine the iron-abundance, stellar metallicity of star-forming galaxies(SFGs) at redshift z~2, observed as part of the Galaxy Mass Assembly ultra-deepSpectroscopic Survey (GMASS). Methods: We compute the equivalent width of arest-frame mid-ultraviolet, photospheric absorption-line index, the 1978 indexfound to vary monotonically with stellar metallicity by Rix and collaborators.We normalise and combine 75 SFG spectra from the GMASS survey to produce aspectrum corresponding to a total integration time 1652.5 hours (and asignal-to-noise ratio ~100 for our 1.5 angstrom binning) of FORS2 spectroscopicobservations at the Very Large Telescope. Results: We measure aniron-abundance, stellar metallicity of log (Z/Zsolar) = -0.574+/-0.159 for ourspectrum representative of a galaxy of stellar mass 9.4 x 10^9 Msolar assuminga Chabrier IMF. We find that the R04 model SFG spectrum for log (Z/Zsolar) =-0.699 solar metallicity provides the best description of our GMASS coaddedspectrum. For similar galaxy stellar mass, our stellar metallicity is ~0.25 dexlower than the oxygen-abundance, gas-phase metallicity quantified by Erb andcollaborators for UV-selected star-forming galaxies at z=2. Conclusions: Weconclude that we are witnessing the establishment of a light-elementoverabundance in galaxies as they are being formed at redshift z~2. Ourmeasurements are reminiscent of the alpha-element enhancement seen inlow-redshift, galactic bulges and early-type galaxies. (Abridged)

GMASS Ultradeep Spectroscopy of Galaxies at redshift z~2. I. The stellar metallicity