PREPRINT

# A solar metallicity galaxy at $z>$ 7? Detection of the [N II] 122 $\mu$m and [O III] 52 $\mu$m lines

Meghana Killi, Darach Watson, Seiji Fujimoto, Hollis Akins, Kirsten Knudsen, Johan Richard, Yuichi Harikane, Dmitra Rigopoulou, Francesca Rizzo, Michele Ginolfi, Gergö Popping, Vasily Kokorev

Submitted on 2 November 2022

## Abstract

We present the first detection of the [N II] 122 $\mu$m and [O III] 52 $\mu$m lines for a reionisation-epoch galaxy. Based on these lines and previous [C II] 158 $\mu$m and [O III] 88 $\mu$m measurements, we estimate an electron density of $\lesssim$ 500 cm${}^{-3}$ and a gas-phase metallicity $Z/{Z}_{\odot }\sim 1.1±0.2$ for A1689-zD1, a gravitationally-lensed, dusty galaxy at $z$ = 7.133. Other measurements or indicators of metallicity so far in galaxy ISMs at $z\gtrsim$ 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 $z\sim$ 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.

## Preprint

Comment: 10 pages, 7 figures; submitted to MNRAS

Subject: Astrophysics - Astrophysics of Galaxies