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(O/H) = 8.00-0.14+0.13. This galaxy is a low-mass (109.3 M⊙), highly star-forming (∼50M⊙ 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 (ne ∼ 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 ∼ 1-3 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∼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 ∼ 3, paving the way for robust measurements of the evolution of the mass-metallicity relation to high redshift