The chemical properties of high- z galaxies provide important information for constraining galaxy evolutionary scenarios .
However , widely used metallicity diagnostics based on rest-frame optical emission lines are unusable for heavily dust-enshrouded galaxies ( such as submillimeter galaxies ; SMGs ) , especially at z > 3 .
Here we focus on the flux ratio of the far-infrared fine-structure emission lines [ N ii ] 205 \mu m and [ C ii ] 158 \mu m to assess the metallicity of high- z SMGs .
Through ALMA cycle 0 observations , we have detected the [ N ii ] 205 \mu m emission in a strongly [ C ii ] -emitting SMG , LESS J033229.4–275619 at z = 4.76 .
The velocity-integrated [ N ii ] / [ C ii ] flux ratio is 0.043 \pm 0.008 .
This is the first measurement of the [ N ii ] / [ C ii ] flux ratio in high- z galaxies , and the inferred flux ratio is similar to the ratio observed in the nearby universe ( \sim 0.02 - 0.07 ) .
The velocity-integrated flux ratio and photoionization models suggest that the metallicity in this SMG is consistent with solar , implying that the chemical evolution has progressed very rapidly in this system at z = 4.76 .
We also obtain a tight upper limit on the CO ( 12-11 ) transition , which translates into CO ( 12-11 ) /CO ( 2-1 ) < 3.8 ( 3 \sigma ) .
This suggests that the molecular gas clouds in LESS J033229.4–275619 are not significantly affected by the radiation field emitted by the AGN in this system .