2016ApJ...825L..23S

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\gt 2 . We estimate an oxygen abundance of 12+log(Ø/
H)={8.00}_{-0.14}^{+0.13} . This galaxy is a low-mass ( {10}^{9.3} {M}_☉ ), highly star-forming (∼50 {M}_☉/yr) 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}\lt 0.14 ), and a high electron density ( {n}_{e}∼500/cm3). 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.