Sanders, Ryan L., Shapley, Alice E., Reddy, Naveen A., Kriek, Mariska, Siana, Brian, Coil, Alison L., Mobasher, Bahram, Shivaei, Irene, Freeman, William R., Azadi, Mojegan, Price, Sedona H., Leung, Gene, Fetherolf, Tara, de Groot, Laura, Zick, Tom, Fornasini, Francesca M., and Barro, Guillermo. 2020. "The MOSDEF survey: direct-method metallicities and ISM conditions at z ∼ 1.5-3.5." Monthly Notices of the Royal Astronomical Society, 491 1427–1455. https://doi.org/10.1093/mnras/stz3032.
We present detections of [O III] λ4363 and direct-method metallicities for star-forming galaxies at z = 1.7-3.6. We combine new measurements from the MOSFIRE Deep Evolution Field (MOSDEF) survey with literature sources to construct a sample of 18 galaxies with direct-method metallicities at z > 1, spanning 7.5 */M☉) = 7-10. We find that strong-line calibrations based on local analogues of high-redshift galaxies reliably reproduce the metallicity of the z > 1 sample on average. We construct the first mass-metallicity relation at z > 1 based purely on direct-method O/H, finding a slope that is consistent with strong- line results. Direct-method O/H evolves by ≲0.1 dex at fixed M* and star formation rate from z ∼ 0 to 2.2. We employ photoionization models to constrain the ionization parameter and ionizing spectrum in the high-redshift sample. Stellar models with supersolar O/Fe and binary evolution of massive stars are required to reproduce the observed strong-line ratios. We find that the z > 1 sample falls on the z ∼ 0 relation between ionization parameter and O/H, suggesting no evolution of this relation from z ∼ 0 to z ∼ 2. These results suggest that the offset of the strong-line ratios of this sample from local excitation sequences is driven primarily by a harder ionizing spectrum at fixed nebular metallicity compared to what is typical at z ∼ 0, naturally explained by supersolar O/Fe at high redshift caused by rapid formation time-scales. Given the extreme nature of our z > 1 sample, the implications for representative z ∼ 2 galaxy samples at ∼1010 M☉ are unclear, but similarities to z > 6 galaxies suggest that these conclusions can be extended to galaxies in the epoch of reionization.
