The MOSDEF survey: direct-method metallicities and ISM conditions at z ∼ 1.5–3.5. Issue 1 (4th November 2019)

• Record Type: Journal Article
• Authors: 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; Barro, Guillermo
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 491:Issue 1(2020)
• Page Start: 1427
• Page End: 1455
• Publication Date: 2019-11-04
• Subjects: galaxies: abundances -- galaxies: high-redshift; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stz3032
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: 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 < 12+log(O/H) < 8.2 and log( M * /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 ∼10 10 M⊙ are unclear, but similarities to z > 6 galaxies suggest that these conclusions can be extended to galaxies in the epoch of reionization.