JWST MIRI detections of H$\alpha$ and [O III] and direct metallicity measurement of the $z=10.17$ lensed galaxy MACS0647$-$JD

TL;DR

This paper reports the first direct metallicity measurement of a galaxy at z > 10 using JWST MIRI and NIRSpec spectroscopy, revealing insights into early galaxy evolution and star formation.

Contribution

It presents the first direct metallicity measurement of a z > 10 galaxy through combined JWST MIRI and NIRSpec observations, extending galaxy property studies to earlier cosmic times.

Findings

Detected Hα and [O III] lines at z=10.17 with high significance.

Measured galaxy metallicity at 0.13 Z☉, similar to lower redshift galaxies.

Constrained ionization and star formation parameters in an early universe galaxy.

Abstract

JWST spectroscopy has revolutionized our understanding of galaxies in the early universe. Covering wavelengths up to $5.3\,{\rm \mu m}$, NIRSpec can detect rest-frame optical emission lines H$\alpha$ out to $z = 7$ and [O III] to $z = 9.5$. Observing these lines in more distant galaxies requires longer wavelength spectroscopy with MIRI. Here we present MIRI MRS IFU observations of the lensed galaxy merger MACS0647$-$JD at $z = 10.165$. With exposure times of 4.2 hours in each of two bands, we detect H$\alpha$ at $9\sigma$, [O III]$\,\lambda5008$ at $11\sigma$, and [O III]$\,\lambda4960$ at $3\sigma$. Combined with previously reported NIRSpec spectroscopy that yields seven emission lines including the auroral line [O III]$\,\lambda4363$, we present the first direct metallicity measurement of a $z > 10$ galaxy: $12+{\rm log(O/H)}= 7.79\pm0.09$, or $0.13^{+0.02}_{-0.03}\,Z_{\odot}$. This is similar to galaxies at $z \sim 4 - 9$ with direct metallicity measurements, though higher than expected given the high specific star formation rate ${\rm log(sSFR / yr^{-1})} = -7.4 \pm 0.3$. We further constrain the ionization parameter ${\rm log}(U)$ = $-1.9 \pm 0.1$, ionizing photon production efficiency ${\rm log}(\xi_{\rm ion})$ = $25.3\pm0.1$, and star formation rate $5.0\pm0.6\,M_{\odot}/{\rm yr}$ within the past $10\,{\rm Myr}$. These observations demonstrate the combined power of JWST NIRSpec and MIRI for studying galaxies in the first $500$ million years.