The stellar mass versus stellar metallicity relation of star-forming galaxies at $1.6\le z\le3.0$ and implications for the evolution of the $\alpha$-enhancement

TL;DR

This study investigates the stellar mass-metallicity relation of star-forming galaxies at redshifts 1.6 to 3.0, revealing lower metallicities compared to local galaxies and providing insights into galaxy evolution and chemical enrichment processes.

Contribution

It presents the first measurement of stellar metallicities for high-redshift galaxies using UV spectra and links these results to galactic chemical evolution models and local stellar populations.

Findings

Stellar metallicities are about six times lower than local counterparts at similar masses.

The average $ m [O/Fe]$ ratio is approximately 0.47, higher than in local galaxies.

Galaxies at $z\, ext{~2}$ show a $ m [O/Fe]$–metallicity relation similar to Milky Way thick-disk stars.

Abstract

We measure the relationship between stellar mass and stellar metallicity, the stellar mass--metallicity relation (MZR), for 1336 star-forming galaxies at $1.6\le z\le3.0$ (<z>=2.2) using rest-frame far-ultraviolet spectra from the zCOSMOS-deep survey. High signal-to-noise composite spectra containing stellar absorption features are fit with population synthesis model spectra of a range of metallicity. We find stellar metallicities, which mostly reflect iron abundances, scaling as $(Z_{Fe,\ast}/Z_{Fe,\odot})=-(0.81\pm0.01)+(0.32+0.03)\log(M_\ast/10^{10}M_\odot)$ across the mass range of $10^9\lesssim M_\ast/M_\odot\lesssim10^{11}$, being $\approx6\times$ lower than seen locally at the same masses. The instantaneous oxygen-to-iron ratio ($\alpha$-enhancement) inferred using the gas-phase oxygen MZRs, is on average found to be [O/Fe]$\approx0.47$, being higher than the local [O/Fe]$\approx0$. The observed changes in [O/Fe] and [Fe/H] are reproduced in simple flow-through gas-regulator models with steady star-formation histories (SFHs) that follow the evolving main sequence. Our models show that the [O/Fe] is determined almost entirely by the instantaneous specific star formation rate alone while being independent of the SFHs, mass, and the gas-regulation characteristics of the systems. We find that the locations of $\sim10^{10}M_\odot$ galaxies at z~2 in the [O/Fe]--metallicity planes are in remarkable agreement with the sequence of low-metallicity thick-disk stars in our Galaxy. This manifests a beautiful concordance between the results of Galactic archaeology and observations of high-redshift Milky Way progenitors. However, there remains a question of how and when the old metal-rich, low-$\alpha$/Fe stars seen in the bulge had formed by z~2 because such a stellar population is not seen in our data and difficult to explain in the context of our models.