Stellar metallicity of star-forming galaxies at z  ~  3^⋆

¹ INAF – Osservatorio Astrofisico di Arcetri, Firenze, Italy
e-mail: veronica@arcetri.astro.it
² Harvard-Smithsonian Center for Astrophysics, 60 Garden street, Cambridge, MA 02138, USA
³ INAF – Osservatorio Astronomico di Roma, Monte Porzio Catone, Italy
⁴ Dipartimento di Astronomia, Università di Firenze, Firenze, Italy
⁵ Kyoto University, Japan

Received: 22 September 2011
Accepted: 9 December 2011

Abstract

The stellar metallicity is a direct measure of the amount of metals present in a galaxy, since a large part of the metals lie in its stars. In this paper, we investigate new stellar metallicity indicators suitable for high-z galaxies by studying the stellar photospheric absorption lines in the rest-frame ultraviolet, hence sampling predominantly young hot stars. We defined these new indicators based on the equivalent widths (EW) of selected features using theoretical spectra created with the evolutionary population synthesis code Starburst99. We used them to compute the stellar metallicity for a sample of ultraviolet-selected galaxies at z > 3 from the AMAZE (Assessing the Mass-Abundance redshift Evolution) survey using very deep (37 h per object) VLT/FORS spectra. Moreover, we applied these new metallicity indicators to eight additional high redshift galaxies studied in literature. We then compared stellar and gas-phase metallicities measured from the emission lines for all these galaxies, finding that within the errors the two estimates are in good agreement, with possible tendency for stellar metallicities to be lower than the gas phase ones. For the first time, we study the stellar mass-stellar metallicity relation at z > 3. We find that the metallicity of young, hot stars in galaxies at z ~ 3 have similar values of the aged stars in local SDSS galaxies, in contrast to findings for the gas phase metallicity.