Abstract
We present the results from a large near-infrared spectroscopic survey with
Subaru/FMOS (FastSound) consisting of $\sim$ 4,000 galaxies at
$z\sim1.4$ with significant H$\alpha$ detection. We measure the gas-phase
metallicity from the N~ii$łambda$6583/H$\alpha$ 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 $z\sim1.4$ is roughly in agreement with the fundamental metallicity relation
at $z\sim0.1$ with fiber aperture corrected star-formation rate. We detect
significant S~ii$łambdałambda$6716,6731 emission lines from the
composite spectra. The electron density estimated from the S~\sc
ii$łambdałambda$6716,6731 line ratio ranges from 10 -- 500 cm$^-3$, which
generally agrees with that of local galaxies. On the other hand, the
distribution of our sample on N~ii$łambda$6583/H$\alpha$ vs. S~\sc
ii$łambdałambda$6716,6731/H$\alpha$ 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 $z\sim1.4$ is significantly higher than
the local values at a fixed metallicity and stellar mass. The metallicity at
$z\sim1.4$ 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.
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