GMASS ultradeep spectroscopy of galaxies at z ~ 2 . I. The stellar metallicity

Context: Galaxy metallicities have been measured to redshift z∼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∼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 h (and a signal-to-noise ratio 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_⊙) = -0.574±0.159 for our spectrum representative of a galaxy of stellar mass 9.4×10⁹ M_⊙ assuming a Chabrier initial mass function (IMF). We find that the R04 model SFG spectrum for log (Z/Z_⊙) = -0.699 solar metallicity provides the best description of our GMASS coadded spectrum. For similar galaxy stellar mass, our stellar metallicity is 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∼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 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∼2. Our measurements are indeed reminiscent of the α-element enhancement seen in the likely progenitors of these starburst galaxies at low-redshift, i.e. galactic bulges and early-type galaxies.

Based on observations obtained at the ESO Very Large Telescope

(VLT) as part of the Large Programme 173.A-0687 (the Galaxy Mass

Assembly ultradeep Spectroscopic Survey).