Amaze

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.

On the universality of luminosity–metallicity and mass–metallicity relations for compact star-forming galaxies at redshifts 0 < z < 3

“…The mass-metallicity relation by Tremonti et al (2004) is often used as a benchmark for comparisons with similar relations obtained for low-z and high-z galaxies (e.g. Lee et al 2006;Maiolino et al 2008;Amorín et al 2010;Manucci et al 2010;Maier et al 2014;Steidel et al 2014;Zahid et al 2011Zahid et al , 2012Zahid et al , 2013Zahid et al , 2014a. However, we note that Tremonti et al (2004) selected all star-forming galaxies, including galaxies with low-excitation H ii regions, while only galaxies with high-excitation H ii regions are present in high-z samples.…”

“…The detailed comparison of strong-line methods (e.g. Kewley & Dopita 2002;Kewley & Ellison 2008;Maiolino et al 2008) shows that they are not consistent, non-monotonic (e.g. the R23-method, Pagel et al 1979;Kewley & Dopita 2002), depend on parameters different from metallicity and result in oxygen abundances, which can deviate by ∼ 0.5 dex or more.…”

“…The location of stacked data for star-forming galaxies at z > 2 by Cullen et al (2014) and of data for ninety six star-forming galaxies at 2.0 < z < 2.6 by Steidel et al (2014) are shown by dark-grey filled stars and open circles, respectively. Star-forming galaxies at z ∼ 3.5 by Maiolino et al (2008) and at z ∼ 2 by Maier et al (2014) are shown by dark-grey filled squares and crosses, respectively. Thirty star-forming galaxies with redshifts z ∼ 3.4 by Troncoso et al (2014) are indicated by asterisks.…”

“…They derived oxygen abundances by using strong-line O3N 2 calibration, similar to that applied for our sample. To derive metallicities Cullen et al (2014) and Troncoso et al (2014) used empirical calibrations by Maiolino et al (2008), which also were calibrated with the use of galaxy samples, where oxygen abundances were derived by the Temethod. However, for consistency reasons we selected only high-z galaxies with [O iii] λ4959/Hβ 1 and recalculated their oxygen abundances using Eqs.…”

“…This implies that differences between low-z and high-z galaxies can be introduced, at least in part, by different selection criteria and different methods used for the determination of global galaxy parameters such as metallicities and stellar masses. Maiolino et al (2008) found a strong evolution of the stellar mass-metallicity relation with redshift in the sense that high-z (z ∼ 3.5) galaxies are more metal-poor com-pared to low-z galaxies for the same stellar mass. This effect is more pronounced for low-mass galaxies.…”