The MOSDEF survey: detection of [O _III]λ4363 and the direct-method oxygen abundance of a star-forming galaxy at z = 3.08

We present measurements of the electron-temperature-based oxygen abundance for a highly star-forming galaxy at z = 3.08, COSMOS-1908. This is the highest redshift at which [O iii]λ4363 has been detected and the first time that this line has been measured at $z>2$. We estimate an oxygen abundance of $12+log\left(O/H\right)={8.00}_{-0.14}^{+0.13}$. This galaxy is a low-mass (${10}^{9.3}$ $M_{\odot }$), highly star-forming (∼50 $M_{\odot }$ yr⁻¹) system that hosts a young stellar population (∼160 Myr). We investigate the physical conditions of the ionized gas in COSMOS-1908 and find that this galaxy has a high ionization parameter, little nebular reddening ($E{(B-V)}_{gas}<0.14$), and a high electron density ($n_e\sim 500$ cm⁻³). We compare the ratios of strong oxygen, neon, and hydrogen lines to the direct-method oxygen abundance for COSMOS-1908 and additional star-forming galaxies at $z=0-1.8$ with [O iii]λ4363 measurements and show that galaxies at $z\sim 1\text{--}3$ follow the same strong-line correlations as galaxies in the local universe. This agreement suggests that the relationship between ionization parameter and O/H is similar for $z\sim 0$ and high-redshift galaxies. These results imply that metallicity calibrations based on lines of oxygen, neon, and hydrogen do not strongly evolve with redshift and can reliably estimate abundances out to $z\sim 3$, paving the way for robust measurements of the evolution of the mass–metallicity relation to high redshift.