2019ApJ...876....1T

We present the results from Atacama Large Millimeter/submillimeter Array observations of [N II] 205 µm, [C II] 158 µm, and [O III] 88 µm lines in an unlensed submillimeter galaxy at z = 4.3, COSMOS-AzTEC-1, hosting a compact starburst core with an effective radius of ∼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_H^PDR=10^5.5–5.75 cm^–3 and G₀ = 10^3.5–3.75 in the central 1 kpc region and n_H^PDR=10^5.0–5.25 cm^–3 and G₀ = 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_[O III]_/L[N II]_ = 6.4 ± 2.2 indicates a near solar metallicity with Z_gas = 0.7-1.0 Z_☉, suggesting a chemically evolved system at z = 4.3.