2021ApJ...923....5S

We present new Atacama Large Millimeter/submillimeter Array Band 7 observational results of a Lyman-break galaxy at z = 7.15, B14-65666 ("Big Three Dragons"), which is an object detected in [O III] 88 µm, [C II] 158 µm, and dust continuum emission during the epoch of reionization. Our targets are the [N II] 122 µm fine-structure emission line and the underlying 120 µm dust continuum. The dust continuum is detected with a ∼19σ significance. From far-infrared spectral energy distribution sampled at 90, 120, and 160 µm, we obtain a best-fit dust temperature of 40 K (79 K) and an infrared luminosity of log₁₀(L_IR/L_☉)=11.6 (12.1) at the emissivity index β = 2.0 (1.0). The [N II] 122 µm line is not detected. The 3σ upper limit of the [N II] luminosity is 8.1 x 10⁷ L_☉. From the [N II], [O III], and [C II] line luminosities, we use the Cloudy photoionization code to estimate nebular parameters as functions of metallicity. If the metallicity of the galaxy is high (Z > 0.4 Z_☉), the ionization parameter and hydrogen density are log₁₀U≃-2.7±0.1 and n_H ≃ 50-250 cm^–3, respectively, which are comparable to those measured in low-redshift galaxies. The nitrogen-to-oxygen abundance ratio, N/O, is constrained to be subsolar. At Z < 0.4 Z_☉, the allowed U drastically increases as the assumed metallicity decreases. For high ionization parameters, the N/O constraint becomes weak. Finally, our Cloudy models predict the location of B14-65666 on the BPT diagram, thereby allowing a comparison with low-redshift galaxies.