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

TL;DR

This paper reports the first detection of specific far-infrared lines in a galaxy at redshift greater than 7, revealing unexpectedly high metallicity and suggesting early galaxy evolution and star formation.

Contribution

It provides the first measurements of [N II] 122 μm and [O III] 52 μm lines in a reionisation-epoch galaxy, indicating higher metallicity than previously observed at similar redshifts.

Findings

Gas-phase metallicity is approximately solar.

Galaxy's star formation began over 250 million years ago.

The galaxy's properties challenge existing metallicity relations.

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, using two different radiative transfer models of the interstellar medium, we estimate an upper limit on electron density of $\lesssim$ 500 cm$^{-3}$ and a gas-phase metallicity $Z/Z_\odot \sim 1.1 \pm 0.2$ 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 $z \gtrsim$ 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.