2023MNRAS.521.2526K

We present the first detection of the [N II] 122 $\mu$m and [O III] 52 $\mu$m lines for a reionization-epoch galaxy. Based on these lines and previous [C II] 158 $\mu$m and [O III] 88 $\mu$m measurements, using two different radiative transfer models of the interstellar medium, we estimate an upper limit on electron density of ≲500 cm^–3 and an approximate gas-phase metallicity of Z/Z_⊙ ∼ 1.1 ± 0.2 for A1689-zD1, a gravitationally lensed dusty galaxy at z = 7.133. Other measurements or indicators of metallicity so far in galaxy interstellar media at z ≳ 6 are typically an order of magnitude lower than this. The unusually high metallicity makes A1689-zD1 inconsistent with the fundamental metallicity relation, although there is likely significant dust obscuration of the stellar mass, which may partly resolve the inconsistency. Given a solar metallicity, the dust-to-metals ratio is a factor of several lower than expected, hinting that galaxies beyond z ∼ 7 may have lower dust formation efficiency. Finally, the inferred nitrogen enrichment compared to oxygen, on which the metallicity measurement depends, indicates that star formation in the system is older than about 250 Myr, pushing the beginnings of this galaxy to z > 10.