Big Three Dragons: A [N II] 122 $\mu$m Constraint and New Dust-continuum Detection of A $z=7.15$ Bright Lyman Break Galaxy with ALMA

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 [OIII] 88 $\mu \mathrm{m}$, [CII] 158 $\mu \mathrm{m}$, and dust-continuum emission during the epoch of reionization. Our targets are the [NII] 122 $\mu \mathrm{m}$ fine-structure emission line and underlying 120 $\mu \mathrm{m}$ dust continuum. The dust continuum is detected with a $\sim$19$\sigma$ significance. From far-infrared spectral energy distribution sampled at 90, 120, and 160 $\mu \mathrm{m}$, we obtaine a best-fit dust temperature of $40$ K ($79$ K) and an infrared luminosity of ${\log }_{10}(L_{\mathrm{I}\mathrm{R}}\mathrm{/}{\mathrm{L}}_{\odot })=11.6$ ($12.1$) at the emissivity index $\beta =2.0$ (1.0). The [NII] 122 $\mu \mathrm{m}$ line is not detected. The 3$\sigma$ upper limit of the [NII] luminosity is $ 8.1 \times 10^7\ {\rm L}\odot$. From the [NII], [OIII], and [CII] line luminosities, we use the Cloudy photoionization code to estimate nebular parameters as functions of metallicity. If the metallicity of the galaxy is high ($ Z > 0.4\ {\rm Z}\odot$),theionizationparameterandhydrogendensityare$ \log_{10} U \simeq -2.7\pm0.1$and$ n_$\text{H}$ \simeq 50$-$250\ {\rm cm}^{-3}$, respectively, which are comparable to those measured in low-redshift galaxies. The nitrogen-to-oxygen abundance ratio, $\mathrm{N}\mathrm{/}\mathrm{O}$, is constrained to be sub-solar. At $Z<0.4{\mathrm{Z}}_{\odot }$, the allowed $U$ drastically increases as the assumed metallicity decreases. For high ionization parameters, the $\mathrm{N}\mathrm{/}\mathrm{O}$ 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.