We present the results from ALMA observations of [ N ii ] 205 \mu m , [ C ii ] 158 \mu m , and [ O iii ] 88 \mu m lines in an unlensed submillimeter galaxy at z = 4.3 , COSMOS-AzTEC-1 , hosting a compact starburst core with an effective radius of \sim 1 kpc .
The [ C ii ] and [ N ii ] emission are spatially-resolved in 0.3″-resolution ( 1 kpc in radius ) .
The kinematic properties of the [ N ii ] emission are consistent with those of the CO ( 4-3 ) and [ C ii ] emission , suggesting that the ionized gas feels the same gravitational potential as the associated molecular gas and photodissociation regions ( PDRs ) .
On the other hand , the spatial extent is different among the lines and dust continuum : the [ C ii ] emitting gas is the most extended and the dust is the most compact , leading to a difference of the physical conditions in the interstellar medium .
We derive the incident far-ultraviolet flux and the hydrogen gas density through PDR modeling by properly subtracting the contribution of ionized gas to the total [ C ii ] emission .
The observed [ C ii ] emission is likely produced by dense PDRs with n _ { \mathrm { H } } ^ { \mathrm { PDR } } = 10 ^ { 5.5 - 5.75 } cm ^ { -3 } and G _ { 0 } = 10 ^ { 3.5 - 3.75 } in the central 1 kpc region and n _ { \mathrm { H } } ^ { \mathrm { PDR } } = 10 ^ { 5.0 - 5.25 } cm ^ { -3 } and G _ { 0 } = 10 ^ { 3.25 - 3.5 } in the central 3 kpc region .
We have also successfully measured the line ratio of [ O iii ] / [ N ii ] in the central 3 kpc region of COSMOS-AzTEC-1 at z = 4.3 , which is the highest redshift where both nitrogen and oxygen lines are detected .
Under the most likely physical conditions , the measured luminosity ratio of L _ { \mathrm { [ OIII ] } } / L _ { \mathrm { [ NII ] } } = 6.4 \pm 2.2 indicates a near solar metallicity with Z _ { \mathrm { gas } } = 0.7 - 1.0 ~ { } Z _ { \odot } , suggesting a chemically evolved system at z = 4.3 .