The Subaru FMOS Galaxy Redshift Survey ( FastSound ) . III . The mass - metallicity

We present the results from a large near-infrared spectroscopic survey with Subaru/FMOS (FastSound) consisting of ∼ 4,000 galaxies at z ∼ 1.4 with significant Hα detection. We measure the gas-phase metallicity from the [N II]λ6583/Hα emission line ratio of the composite spectra in various stellar mass and star-formation rate bins. The resulting mass-metallicity relation generally agrees with previous studies obtained in a similar redshift range to that of our sample. No clear dependence of the mass-metallicity relation with star-formation rate is found. Our result at z ∼ 1.4 is roughly in agreement with the fundamental metallicity relation at z ∼ 0.1 with fiber aperture corrected star-formation rate. We detect significant [S II]λλ6716,6731 emission lines from the composite spectra. The electron density estimated from the [S II]λλ6716,6731 line ratio ranges from 10 – 500 cm, which generally agrees with that of local galaxies. On the other hand, the distribution of our sample on [N II]λ6583/Hα vs. [S II]λλ6716,6731/Hα is different from that found locally. We estimate the nitrogen-to-oxygen abundance ratio (N/O) from the N2S2 index, and find that the N/O in galaxies at z ∼ 1.4 is significantly higher than the local values at a fixed metallicity and stellar mass. The metallicity at z ∼ 1.4 recalculated with this N/O enhancement taken into account decreases by 0.1 – 0.2 dex. The resulting metallicity is lower than the local fundamental metallicity relation.