AMAZE^*

I. The evolution of the mass–metallicity relation at z > 3

¹ INAF - Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy e-mail: maiolino@oa-roma.inaf.it
² National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
³ Dipartimento di Astronomia, Università di Firenze, Largo E. Fermi 2, 50125 Firenze, Italy
⁴ INAF - Istituto di Radioastronomia, Largo E. Fermi 5, 50125 Firenze, Italy
⁵ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁶ Astrophysics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
⁷ Dipartimento di Astronomia, Università di Trieste, via Tiepolo 11, 34131 Trieste, Italy
⁸ INAF - Osservatorio Astronomico di Trieste, via Tiepolo 11, 34131 Trieste, Italy
⁹ Geneva Observatory, Geneva University, 51 chemins des Mailletes, 1290 Sauverny, Switzerland
¹⁰ INAF - Osservatorio Astronomico di Padova, Vicolo Osservatorio 5, 35122 Padova, Italy
¹¹ INAF - Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received: 28 February 2008
Accepted: 24 June 2008

Abstract

We present initial results of an ESO-VLT large programme (AMAZE) aimed at determining the evolution of the mass-metallicity relation at z > 3 by means of deep near-IR spectroscopy. Gas metallicities are measured, for an initial sample of nine star forming galaxies at z ~ 3.5, by means of optical nebular lines redshifted into the near-IR. Stellar masses are accurately determined by using Spitzer-IRAC data, which sample the rest-frame near-IR stellar light in these distant galaxies. When compared with previous surveys, the mass-metallicity relation inferred at z ~ 3.5 shows an evolution much stronger than observed at lower redshifts. The evolution is prominent even in massive galaxies, indicating that z ~ 3 is an epoch of major action in terms of star formation and metal enrichment also for massive systems. There are also indications that the metallicity evolution of low mass galaxies is stronger relative to high mass systems, an effect which can be considered the chemical version of the galaxy downsizing. The mass-metallicity relation observed at z ~ 3.5 is difficult to reconcile with the predictions of some hierarchical evolutionary models. Such discrepancies suggest that at z > 3 galaxies are assembled mostly with relatively un-evolved sub-units, i.e. small galaxies with low star formation efficiency. The bulk of the star formation and metallicity evolution probably occurs once small galaxies are already assembled into bigger systems.