GMASS ultradeep spectroscopy of galaxies at z∼2 -: I.: The stellar metallicity

Context. Galaxy metallicities have been measured to redshift z similar to 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 similar to 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 angstrom 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 angstrom index is sensitive to Fe III transitions and measures the iron-abundance, stellar metallicity. To accurately determine the 1978 angstrom 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 h (and a signal-to- noise ratio similar to 100 for our 1.5 angstrom binning) of FORS2 spectroscopic observations at the Very Large Telescope. Results. We measure a iron-abundance, stellar metallicity of log(Z/Z(circle dot)) = -0.574 +/- 0.159 for our spectrum representative of a galaxy of stellar mass 9.4 x 10(9) M-circle dot assuming a Chabrier initial mass function (IMF). We find that the R04 model SFG spectrum for log (Z/Z(circle dot)) = -0.699 solar metallicity provides the best description of our GMASS coadded spectrum. For similar galaxy stellar mass, our stellar metallicity is similar to 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 similar to 2 by analysing the 1978 angstrom index in a spectrum created by combining 75 galaxy spectra from the GMASS survey. We find that our measurement is similar to 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 similar to 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.