Big Three Dragons: A [N ii] 122 μm Constraint and New Dust-continuum Detection of a z = 7.15 Bright Lyman-break Galaxy with ALMA

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Abstract 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 μ m, [C ii ] 158 μ m, and dust continuum emission during the epoch of reionization. Our targets are the [N ii ] 122 μ m fine-structure emission line and the underlying 120 μ m dust continuum. The dust continuum is detected with a ∼19 σ significance. From far-infrared spectral energy distribution sampled at 90, 120, and 160 μ m, we obtain a best-fit dust temperature of 40 K (79 K) and an infrared luminosity of log 10 ( L IR / L ⊙ ) = 11.6 (12.1) at the emissivity index β = 2.0 (1.0). The [N ii ] 122 μ m line is not detected. The 3 σ upper limit of the [N ii ] luminosity is 8.1 × 10 7 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 log 10 U ≃ − 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.

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