GMASS ultradeep spectroscopy of galaxies at z ~ 2 ^*

I. The stellar metallicity

¹ Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy e-mail: halliday@arcetri.astro.it
² Laboratoire AIM, CEA/DSM – CNRS – Université Paris Diderot, DAPNIA/SAp, Orme des Merisiers, 91191 Gif-sur-Yvette, France
³ Dipartimento di Astronomia, Alma Mater Studiorum, Universitá di Bologna, Via Ranzani 1, 40127 Bologna, Italy
⁴ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁵ Osservatorio Astronomico di Padova, vicolo dell'Osservatorio 5, 35122 Padova, Italy
⁶ National Optical Astronomy Observatory, 950 North Cherry Avenue, Tuscon, AZ 85719, USA
⁷ Dipartimento di Astronomia, Universitá di Padova, vicolo dell'Osservatorio 3, 35122 Padova, Italy
⁸ Laboratoire d'Astrophysique de Marseille (UMR 6110), CNRS, Université de Provence, BP 8, 13376 Marseille Cedex 12, France
⁹ European Southern Observatory, Karl-Schwarschild-Str. 2, 85748 Garching, Germany

Received: 14 September 2007
Accepted: 18 December 2007

Abstract

Context.Galaxy metallicities have been measured to redshift by gas-phase oxygen abundances of the interstellar medium using the R₂₃ 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 , 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 for our spectrum representative of a galaxy of stellar mass   assuming a Chabrier initial mass function (IMF). We find that the R04 model SFG spectrum for log  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 .

Conclusions.We measure the iron-abundance, stellar metallicity of star-forming galaxies at redshift 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 . 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.