The Stellar Mass versus Stellar Metallicity Relation of Star-forming Galaxies at 1.6 ? z ? 3.0 and Implications for the Evolution of the ?-enhancement

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