First [NII] 122 μm line detection in a starburst pair at z = 4.7

We report the first detection of [NII] 122 μm line toward a QSO-SMG pair, BRI 1202-0725, at z = 4.7 using ALMA. Combining with [NII] 205 μm line detection and taking the line ratio of [NII]122/[NII]205, we constrain electron densities of both galaxies. The derived electron densities are ${26_{ - 11}^{ + 12}$ and ${134_{ - 39}^{ + 50}$ cm^-3 for the SMG and the QSO, respectively, which are the first measurements for galaxies at z > 4. The electron density of the SMG is comparable to the Galactic plane and the average of local spiral galaxies, while the value for the QSO is comparable to local starbursts and optical-line based measurements for star-forming galaxies at z ∼ 2-3. Considering the similar star-formation rates (SFRs) of ≈ 1000 M_⊙ yr^-1 for both galaxies, our results suggest a large scatter of electron densities at fixed SFR and caution against using optical lines for dusty starbursts. The details of this report are presented in Lee et al. 2019 (submitted).