The MOSDEF Survey: Detection of [O III]λ4363 and the Direct-method Oxygen Abundance of a Star-forming Galaxy at z = 3.08

Abstract

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}({{O}}/{{H}})={8.00}_-0.14^+0.13. This galaxy is a low-mass ({10}^9.3 {M}_⊙ ), highly star-forming (∼50 {M}_⊙ 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}}\lt 0.14), and a high electron density ({n}_e∼ 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.

Based on data obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA, and was made possible by the generous financial support of the W.M. Keck Foundation.