ISM excitation and metallicity of star-forming galaxies at z~3.3 from near-IR spectroscopy

TL;DR

This study investigates the gas-phase metallicity, ionization, and stellar properties of star-forming galaxies at z~3.3 using near-infrared spectroscopy, revealing a lower metallicity and weak SFR-metallicity correlation compared to local galaxies.

Contribution

It provides new measurements of metallicity and ionization parameters at high redshift, and examines the evolution of the mass-metallicity relation with a model accounting for star formation efficiency.

Findings

High-redshift galaxies have lower metallicity than local counterparts.

No strong correlation between SFR and metallicity at z~3.3.

Evolution of the mass-metallicity relation can be explained by a weak change in star formation efficiency.

Abstract

We study the relationship between stellar mass, star formation rate (SFR),ionization state, and gas-phase metallicity for a sample of 41 normal star-forming galaxies at $3 \lesssim z \lesssim 3.7$. The gas-phase oxygen abundance, ionization parameter, and electron density of ionized gas are derived from rest-frame optical strong emission lines measured on near-infrared spectra obtained with Keck/MOSFIRE. We remove the effect of these strong emission lines in the broad-band fluxes to compute stellar masses via spectral energy distribution fitting, while the SFR is derived from the dust-corrected ultraviolet luminosity. The ionization parameter is weakly correlated with the specific SFR, but otherwise the ionization parameter and electron density do not correlate with other global galaxy properties such as stellar mass, SFR, and metallicity. The mass-metallicity relation (MZR) at $z\simeq3.3$ shows lower metallicity by $\simeq 0.7$ dex than that at $z=0$ at the same stellar mass. Our sample shows an offset by $\simeq 0.3$ dex from the locally defined mass-metallicity-SFR relation, indicating that simply extrapolating such relation to higher redshift may predict an incorrect evolution of MZR. Furthermore, within the uncertainties we find no SFR-metallicity correlation, suggesting a less important role of SFR in controlling the metallicity at high redshift. We finally investigate the redshift evolution of the MZR by using the model by Lilly et al. (2013), finding that the observed evolution from $z=0$ to $z\simeq3.3$ can be accounted for by the model assuming a weak redshift evolution of the star formation efficiency.